Aromatics and the Soul: A Study of Smells/Chapter 8
CHAPTER VIII
THEORIES OF OLFACTION
(The Piéce de Résistance)
The anatomical structure of the olfactory end- organ in the nose is, as we saw in Chapter II., simple.
Contrast it with the eye. Here we have what is obviously an optical instrument, with lens, iris diaphragm, dark walls, and sensitive plate complete—a photographic camera, in a word.
Contrast it also with the ear, which is an acoustic apparatus reminding us in its detail of a recording gramophone leading to a closed box in which are what look like a series of resonators, like the wires of a piano.
In the antechamber of each of those organs the physical vibrations to which they respond undergo considerable modification before they reach the sensory cells.
In the antechamber of the olfactory organ, on the other hand, the amount of modification necessary is evidently but slight, as the olfactory region of the nasal chamber is merely a narrow, open passage. As far as we know, all that takes place is that the incoming stimulus, the odorous molecule, is warmed and received by the nasal mucus.
Thus the very complexity of the structure both of the eye and of the ear helps us to comprehend their function.
But what can we deduce from a flat surface in which all we can see is a collection of cells with minute protoplasmic hairs projecting from their distal ends ? Obviously, little or nothing. We are, in fact, confounded by simplicity. It may be that we are here dealing with one of the essential properties of all living matter, little, if at all, altered from its primitive condition.
To the physiologist, then, olfaction is the most mysterious of all the senses. It still retains its secrets, and therein lies the fascination of its study.
Of late years, the exploration of this dark region of physiology has been, and is still being, vigorously pushed, and we shall now proceed to give what, however, can only be a brief and superficial account of the progress made and of the opinions held. Even so we shall be compelled to make an incursion into the high and dry realms of modern chemical and physical theory. That may not be good hearing, but what is still worse is that almost every single point we shall be discussing is a matter of controversy.
Let us commence with a few of the details, mostly unimportant, upon which there is general agreement.
Consider, first of all, the variety, the almost infinite variety, of odours. We have, for example, all the odours of the world of Nature, the emanations of inorganic matter, of the earth itself, its soil and its minerals ; to these we must add the multitudinous perfumes of the vegetable kingdom, of barks, roots, leaves, flowers and fruits, including those of growing herbaceous plants, which differ so widely from one another that it is said of Rousseau, whose myopia was compensated for by an unusually acute sense of smell, and who was, moreover, no mean botanist, that he could have classified the plants according to their smell had there been a sufficiency of olfactory terms for the purpose ; then we have the thousand effluvia, some pleasant and others not so pleasant, of living animals, including the various races of mankind ; next come the—mostly repulsive—odours of decaying vegetable and putrefying animal matter ; and finally the products of man's own proud ingenuity and skill, such as the artificial perfumes and flavours on the one hand and on the other coal-gas, acetylene, carbon disulphide, and the like.
Parker notes it as worthy of remark that man has created, both accidentally and intentionally, many new odours—smells, that is to say, which have no fellow in the world of Nature—and he emphasises the fact that the nose is nevertheless capable of appreciating such novel sensations.
In this connection we may mention that the art of modern perfumery can imitate closely many of the natural perfumes, and more particularly the natural flavours, by mixing together essences, or components, which in no way resemble the final product.
Thus the flavour of peaches can be compounded artificially of aldehyde, acetate, formate, butyrate, valerianate, œnanthylate, and sebate of ethyl, and salicylate of methyl, with glycerine, glycerine being added to the fruit essences, as it is to wines, in order to restrain the evaporation of the volatile bodies. (The fruit essences are used only in the making of flavours. They cannot be employed as perfumes, as they are too irritating to the nose.)
The union of components to form a product different from any one of them is found also in vision, When the colours of the spectrum, for example, are commingled, the resultant white light is devoid of any colour.
Thus the potential responsiveness of the olfactory organ seems to be practically inexhaustible. So far, at all events, it has not yet reached the limits of its capacity.
The number and variety of recognised smells being so great, then, one can readily understand how difficult it is to construct a classification of odours. Many attempts have, in fact, been made, but, depending as they do more or less upon subjective sensation, no two classifiers give us the same classification. Indeed, a division of all smells into “nice,” “neutral,” and “nasty” would be about as good as many much more ambitious efforts.
Zwaardemaker’s is the classification most usually followed at present, and as it is to him we owe most of our knowledge of scientific olfaction, we shall detail it here :
(1) Ethereal or fruity odours; (2) aromatic, including as sub-classes camphrous, herbaceous, anisic and thymic, citronous, and the bitter almond group; (3) balsamic, with sub-groups floral, liliaceous, and vanillar; (4) ambrosial or muscous ; (5) garlicky (including garlic), oniony, fishy, and the bromine type of odour; (6) empyreumatic (guaiacol) ; (7) caprylic (valerianic acid) ; (8) disgusting ; and (9) nauseating.
The subjective character of these classes is obvious, especially in the last two groups, but, apart from that objection, most people will be inclined to protest when they learn that chloroform and iodoform arc put into the first, the ethereal or fruity, group, while it is suggested, though to be sure with a query, that coffee, bread, and burnt sugar may belong to the “repulsive” (pyridine) group !
The fact is that Zwaardemaker's classification is based upon a chemical foundation, that is to say, upon properties which, as we shall see later on, do not necessarily correspond with the odours as we smell them. That, no doubt, explains his inclusion of iodoform among the “fruity” odours.—lodoform fruity !—Shades of George Saintsbury and his “Cellar Book” !
A shorter classification is that of Heyninx, who, aiming at objectivity, bases his arrangement, to some extent at all events, upon the spectrum analysis of odorous molecules in the atmospheric medium, of which more anon. His list is : acrid, rotten, fetid, burning, spicy, vanillar or ethereal, and garlicky. But here, also, the coupling of vanillar with ethereal odours seems a little inappropriate.
We stand, perhaps, on rather firmer ground when we turn to the manufacturer’s classification, founded as it is frankly upon subjective sensation, and therefore devoid of any surprises to the logical faculty. Here is Rimmel's arrangement : rose, Jasmine, orange, tuberose, violet, balsam, spice, clove, camphor, sandal-wood, lemon, lavender, mint, anise, almond, musk, ambergris, fruit (pear).
It may be objected, perhaps, that this is a catalogue merely, not a scientific classification. That is quite true. But what is also true is that the others we have quoted are little, if any, better. The fact is that we do not yet possess the knowledge necessary to enable us to arrange odours in classes.
The manufacturers, of course, concern themselves with agreeable and attractive odours only. To the great and growing company of the stinks they pay no attention whatever. For that reason their contribution to our knowledge is necessarily but partial and limited.
In their own proper domain, however, they can point to several great successes. They recognise, for practical purposes, about eighty primitive scents. Many natural (to say nothing of many unnatural) perfumes can now be prepared artificially, and some 80 prepared are said to be even more powerful than the natural productions. Artificial musk, for example, is one thousand times stronger than natural musk, Parker tells us. Deite, on the other hand, says that the smell of artificial musk is not equal to that of the natural ! Indeed, according to this authority, although synthetic perfumes play an important part in the concocting of scents, there are only a few of them which can be used instead of the natural product. What happens is that the artificial and the natural are generally used in combination. Thus the “mignonette” of the shops is prepared by passing geraniol, an artificial odorivector made from citronella oil, over the natural mignonette flowers, the resulting product being an essence smelling strongly of mignonette, and not at all of geraniol.
One or two, as we said, are purely artificial imitations ; coumarin, for example, the “new-mown hay” of sentimental memory, which used to be obtained from the tonka bean, is now entirely made up by the synthetic chemist, But for all the more subtle essences we have still to rely upon Nature’s laboratory. The manufacturer steps in and distils the precious essential oil certainly, but it is from flowers that he obtains it. Attar of roses, for instance, contains, in addition to natural geraniol, a number of other ingredients which have so far escaped analysis, a hundred thousand roses supplying only an ounce of it, In like manner a ton of orange blossom yields but thirty to forty ounces of the odorous essential oil.
Many of the costly plant perfumes come from tropical or semi-tropical countries, such as Ceylon, Mexico, and Peru. But tropical perfumes, though strong, lack the delicacy of those found in temperate climates. Cannes, on the Riviera, gives us roscs, acacias, jasmine and neroli ; from Nimes come thyme, rosemary, and lavender oil ; from Nizza, on the Italian Riviera, we get violets ; from Sicily, oranges and lemons ; from Italy, iris and bergamot. English lavender, until quite recently the most highly esteemed, came from the towns of Hitchin and Mitcham. But I am informed that the growing of lavender in England is no longer pursued with the same success as formerly, and we have to regret the disappearance of this old and truly English industry.
The natural musk, curiously enough, which comes from the musk-deer of Tibet, is not used in making musk perfume. It is, however, widely employed in the perfumer's art, as it has the curious property of enhancing the strength of other perfumes and of rendering them permanent. Civet, also an animal product, being “the very uncleanly flux” of the civet cat, has similar properties. It is added to other perfumes to strengthen them (“to set them off,” as it were) and to render them more stable.
But the most curious, and also one of the most ancient of perfumes is ambergris, which is a fatty, wax-like substance found floating in the sea or washed ashore. It comes from places as far apart as the west coast of Ireland, China, and South America. The origin of this substance was for long a mystery. But we know now that it consists of the undigested remnants of cephalopads (squids and octopuses) swallowed by the spermaceti whale. Ambergris is used, like musk and civet, to render other scents durable.
But while the victory of the chemist is by no means so complete as it is in the matter of the dye-stuffs, research is steadily going on, and the next few years will almost certainly witness an evergrowing conquest over this department of natural chemistry.
In the meantime chemists are applying themselves to the creation of new varieties of perfume, and, if we may judge from those disseminated by certain ladies in public places, with a success that startles and even irritates us. Compared with them, the love-philtres of olden days must have been but feeble things.
“How d’you know you're in the right ’bus ?” asked the 'bus conductor of the blind man who was confidently boarding his vehicle.
“”This is the Maida Vale ’bus,” was the contemptuous reply. “I knows it by the smell o’ musk.”
The inexhaustible capacity of the olfactory organ, to which we alluded above, is by no means its only marvel. It is also of the most wonderful delicacy, equalling, even if it does not surpass, in this respect, the sensitiveness of the eye to light.
This property of the smell-organ has been scientifically estimated. There are many ways of doing so, that by means of Zwaardemaker’s olfactometer being perhaps the most popular :
“This consists of two tubes that slide one within the other, and so shaped that one end of the inner tube may be applied to the nostril. The odorous material is carried on the inner surface of the outer tube. When the inner tube, which is graduated, is slipped into the outer one so as to cover completely its inner face, and air is drawn into the nostril through the tube, the odorous surface, being covered, gives out no particles, and no odour is perceived. By adjusting the inner tube in relation to the outer one, whereby more or less of the oderous surface is exposed, a point can be found where minimum stimulation occurs. The amount of odorous substance delivered under these circumstances to the air current has been designated by Zwaardemaker as an olfactie, the unit of olfactory stimulation. Having determined for a given substance the area necessary for the delivery of one olfactic, doubling that surface by an appropriatc movement of the inner tube will produce a stimulus of two olfacties, and so forth. Thus a graded series of measured olfactory stimuli can easily be obtained. Further, by using outer tubes carrying ditferent odorous substances various comparisons can be instituted as measured in olfacties”(Parker).
Instruments more elaborate and of greater accuracy have, as a matter of fact, been devised and used, but they need not detain us.
The results obtained by these and other methods of determining the minimum stimulus of olfaction are certainly astonishing, and reveal as nothing else can the delicate acuteness of the sense.
Fischer and Penzoldt found that they could plainly smell one milligram of chlorphenol evaporated in a room of 230 cubic metres capacity. This is equivalent to 1⁄230,000,000 of a milligram to each cubic centimetre of air, or, assuming 50 cubic centimetres of air as the minimum needed for olfaction, the amount of chlorphenol capable of exciting sensation is 1⁄4,600,000 of the thousandth part of a gram—approximately 1⁄276,000,000 of a grain !
Many other odours have been similarly tested, and although there is much numerical discrepancy in the records made by different observers, all agree as to the extreme delicacy of the sense, (For vanillin and mercaptan, see p. 39.)
Those experiments and estimations explain how it comes about that many odours (musk, for example) may go on giving off their scent until they part with the whole of it without undergoing any appreciable loss of weight.
Thus there is no chemical test known to us so delicate as olfaction.
It has been found, for example, that over-assiduous efforts at filtering and purifying the air used for ventilation so as to remove all noxious chemical and bacterial ingredients defeat their own end. Such air, although to our artificial tests absolutcly clean and pure, seems to the sense of smell to lack freshness. And the nose is right. The tests are wrong. For sojourn in such an atmosphere induces lassitude and torpor of mind, as members of the Houses of Parliament, where this method has been tried, know to their cost—and ours.
But albeit so highly sensitive to minute traces, the sense occasionally fails to perceive a highly concentrated odour.
For example, every one is aware that a bunch of violets which is filling a room with its fragrance seems when held to the nose to have no smell at all, or at the most to have but a vague, indefinable sort of odour.
The effect, as a matter of fact, varies with the perfume employed. Some, like violets, have no smell at all. Others give a different smell when concentrated from what they give when dilute. Muskone, for one, the essential constituent of musk, has an odour of pines when concentrated ; and storax, a delightful perfume when dilute, is disagreeable when too powerful, and so on.
It is to be noted that the disagreeable character of these last is not due to the mental “cloying” or “sickening” of excessive sweetness; it is a definite odour. Nor is the anosmia for concentrated violets due to the exhaustion of the sense.
Heyninx, comparing, as we shall see, olfaction with vision, believes the indefinite odour of concentrated violets to be akin to the absence of colour in white light. But this explanation seems to me to be improbable, since the effect is due not to the combination of a number of odours, as white light is the combination of all the colours of the spectrum, but to the overpowering influence of a single odour.
Indeed, none of the other senses shows the same phenomenon. If we happen to catch a momentary glimpse of the noonday sun, we plainly see a disc of intense light (it is pale blue in colour to my eye), surrounded by a fiery halo, before it blinds us. In the same way, when a gun is fired close to the ear, we hear the sound before we are deafened by it.
It is for such reasons that perfumers never sniff at a bottle of scent ; they take a little, rub it on the back of the hand, and then wait until the spirit has evaporated before they proceed to smell it.
The exquisite delicacy of the sense might lead us to suppose that the olfactory organ must be quick at responding to its proper stimulus. But such is not the case. It is, on the other hand, relatively “slow in the uptake.”
Gleg has estimated that the reaction time for auditory sensation is from 0’12 to 0’15 of a second, whereas the reaction time for smell is as much as 0’5 of a second, only one sensory stimulus being slower, that of pain, namely, which occupies 0’9 of a second.
Odours are conveyed to the olfactory end-organ in the air we breathe. Before they can rise into the air from the odorivector (the odorous body) and be transported they must, it is clear, pass into the vaporous or gaseous state. (In the case of fish, of course, the odour must undergo solution, that is pass into the liquid state.) Many of the natural properties manifested by smells have been related to this transformation into vapour.
Everybody knows how rich garden scents become after a shower. It has been claimed that this results from the lightening of the atmosphere by the storm, in consequence of which the diffusion of odorous vapours, following the law that governs the diffusibility of gases, is facilitated. But some of the effect must be due, one would think, partly to the impact of the raindrops breaking up and dispersing the halo of perfumed air that surrounds each flower, and partly also to the evaporation of the rain-water that has absorbed these floral emanations.
We are told also that during the night and in the chill of early morning the air is less charged with odours because cold checks the diffusion of gases. This may be true enough for some odours, but I am inclined to think that the fact is not stated with perfect accuracy, as there are certain perfumes, that of the tobacco-plant for one and that of the night-scented stock for another, which are most prevalent after nightfall. And it has always seemed to me that Mother Earth is never so nicely perfumed as on a cool September morning, although I should never be inclined to call any morning “incense-breathing,” like Gray, for anything less like incense could scarcely be imagined.
There is no doubt, however, that frost seals up all odorivectors and renders the air quite odourless.
A physical law appertaining to gases is also invoked to explain the “clinging” of odours. Many, if not all, solids and liquids when exposed to air and other gases adsorb (cause to adhere) to their surfaces a thin, dense layer or film of the gas. If now that gas happens to contain an odour, or is itself odorous, the odour must also be adsorbed, and so in the case of porous materials, such as fabrics, permeated by the odour, it lingers tenaciously in their depths.
Odorous bodies in the solid or powdered form are known to retain their perfume for prolonged periods, Look how long a sandal-wood box remains aromatic. This property is supposed to depend upon the lowered vapour tension of the odorous molecules in the depths of the solid or powder, in virtue of which they rise into the air, or evaporate, but slowly.
It would seem to bc natural to suppose that, as vaporisation plays such an important part in the dissemination of odours, the volatile bodies and liquids would be more odorous than the non-volatile. But, as Zwaardemaker has pointed out, this is by no means always the case. Many substances of low volatility are nevertheless highly odorous, and vice versâ.
We turn now for a moment to consider the behaviour of the odorous vapour in the nose.
As it passes through the nose the current of inspired air sweeps along the lower and middle regions only ; the upper or olfactory region is not directly traversed. But almost certainly some of the air is diverted up into the olfactory region in light eddies, and the act of sniffing, which is a short inspiration abruptly begun and ended, and which we instinctively resort to when trying to detect a faint odour, is obviously of a nature to propel side-streams or eddies up into the olfactory zone. One is reminded of the production of smoke rings from a box.
We smell not only during inspiration, however, but also during expiration, the latter conveying to the olfactory region the flavours of food and drink.
Flavours, that is to say the olfactory elements of so-called “taste,” are not appreciated to the full until after deglutition. To most of us, although experts and connoisseurs can determine it by smelling the wine in the glass, the bouquet of port has really no meaning until after it is drunk, simply because the expiratory current of air as it ascends through the throat into the nose receives the concentrated vapours of the warmed volatile higher alcohols which are clinging about the fauces.
We may here remark that although we are usually able to perceive that the odour and the flavour of a sapid food or drink are akin to each other, the sensation of the odour anticipating that of the flavour, yet they are by no means always identical. They may strike us as do a plain and a coloured version of the same print. Sometimes the flavour seems to be the more powerful, sometimes the odour. Nearly all bouillons, for example, possess a flavour more rich and full than the odour they give off with their steam. On the other hand, valerian has a strong, objectionable smell, which, strange to say, becomes subdued and relatively tolerable when that medicine is being swallowed.
It is a curious fact, well known to expert “tasters,” that if the cyes are kept closed during the test, the delicacy of appreciation of flavours, and also of the smell of the wine in the glass, is entirely lost. I cannot suggest any explanation for this curious phenomenon.
Anosmia, absence of smell, which is the next topic for our consideration, is a not uncommon defect. It is generally the result of some form of nasal obstruction, such as a bad “cold in the head,” as Æsop’s fox was clever enough to remember, This type is temporary and remediable. But there are other forms that are due to nerve-disease, and for these nothing can be done.
A congenital anosmia is occasionally met with, and a curious partial anosmia, reminding us of colour-blindness or tone-deafness. I myself know people who cannot smell coal-gas unless it is very strong, and I once knew a cook,—a cook who couldn’t smell a bad egg !
Albinos are said to be congenitally anosmic, and there was recorded many years ago by Hutchison the case of a negro who, gradually losing all his pigment, became anosmic in consequence (cited by Ogle). As the sustentacular cells of the olfactory area contain granules of pigment (see Chapter II.), we are forced to conclude that it must exercise a highly important function in the perception of odours. We shall see later on that its presence is supposed by some to support the theory that odour is a specific ethereal vibration similar to light.
We turn now to discuss the real nature of odour, a section of our subject which is still theoretical and highly problematical.
Having accomplished so much in the art of perfumery, the chemist ought, one would think, to be able to tell us whether or not there is any relationship or correspondence between odour and chemical constitution.
When investigation of this point was begun, a hopeful fact came to light, as it was pointed out that certain bodies of similar chemical composition had all the same kind of smell. These were the compounds of arsenic, bismuth, and phosphorus, all of which smell of garlic. But it was soon realised that this fact was of little or no significance, as the oxides of many of the metals, although quite different from the former group, also smell of garlic. To these we may add the instance of water and sulphuretted hydrogen, two substances which are related chemically, as their formule show (H2O and H2S), and yet one of them is odourless, while the other has a strong, unpleasant smell. Finally, according to Deite, natural and artificial musk have nothing in common but their smell. Chemically they are quite different.
The property of odour, then, does not depend upon the chemical constitution of bodies.
The next question that arises is : Do bodies exhaling the 8ame kind of odour resemble each other in the structure of their molecules ? In other words, can odour be related to molecular structure ?
To the chemist all matter is made up of atoms and molecules. The elements, bodies which cannot be broken up by chemical action into any simpler form, are composed of atoms. On the other hand, when elements combine to form a compound, the unit of the new body, composed as it is of two or more atoms of different elements linked together, is known as a molecule. (Probably the elements also exist in the molecular state, the atoms of which they are composed being linked together in groups.) Both atoms and molecules are, of course, very minute in size.
For reasons we need not enter into here, the molecule is held to have a certain structural form, which form is indicated by what is known as a graphic formula. The graphic formula of water, one of the simplest, may be written as H—O—H, and we may regard it as having a linear form. (Modern views indicate that it is not a simple line, but in two planes.)
Many molecules, however, particularly those of the organic compounds, are highly complex, and their structural form must be very different from that of water.
The question, then, now before us is: Does odour bear any relationship to the molecular structure of bodies? And again it has been maintained that a clue to the problem of the real nature of odour lies here.
There is a well-known series of chemical bodies known as the “aromatics,” by reason of the fact that they possess strong smells more or less similar in quality. With regard to this series, which is made up of groups of what are known as radicles which occupy definite positions on a molecule shaped like a ring—the benzene ring, as it is called—Henning, a German observer, has expressed the opinion that the odour depends, not upon the radicles as such, but upon the position they occupy on the ring.
Transferring his argument to odorous bodies in general, and taking six groups as embracing all (spicy, flowery, fruity, resinous, burnt, and foul), he associates each of these types with some feature in the constitution of the molecule which is common to all the members of each group.
To enter more fully into this branch of the subject would carry us too deeply into chemistry. I shall content myself therefore with saying that Henning’s views have received considerable support from scientific chemists and have led to several interesting and suggestive developments.
Heyninx, however, criticising this theory, points out that hydrocyanic (or prussic) acid and nitrobenzol, two substances with the same smell, have each a molecular structure in no way resembling
the other.
The graphic formulae of these bodies, which I give here, plainly show the difference between them :
H−C≡N (hydrocyanic acid) and
(nitrobenzol).
(T. H. Fairbrother, to whom I am indebted for much information on the chemistry of olfaction, would dispose of this criticism of Heyninx’s by denying that the odours of those two substances are identical. See later, p. 132.)
Chemistry, then, having, according to the critics, failed us, we turn to the allied science of physics. Physics deals with matter in its ultimate state, beginning, so to speak, where chemistry, with its work of changes and combinations, ceases, and taking us deep into the heart of matter independent of its chemical properties and behaviour.
We have seen that, chemically speaking, elements and their compounds exist as molecules made up of atoms, Now molecules may be minute, and atoms even more minute, but in “electrons,” the name given to the last divisible particle of matter known to the physicist, we are dealing with minuteness inconceivable. Sir Oliver Lodge has said that if an atom could be expanded to fill a space equal to that of the entire solar system, the electrons composing it would each be the size of an orange ! There is supposed, indeed, to be an atomic “system” composed of a central nucleus like the sun, with electrons revolving round it, the nucleus having a positive, and the revolving particles a negative, electric charge. Further (whether in virtue of these moving electrons or otherwise is not quite clear), the molecule is supposed to be in a state of constant vibration.
The physical theory of odour, then, refers that quality to the vibration of the molecule. It suggests that the molecules of an odorous body passing in the gaseous or, in fishes, the liquid state into the olfactory region of the nose, are there received by the film of mucus in which the olfactory hairs lie, and stimulate these hairs by their molecular vibration. No chemical change is supposed to take place, only, as it were, a mechanical stimulation, comparable to the mechanical stimulation of the retina by the waves of light.
A recent development of the theory which we owe to Heyninx, a Belgian scientist, brings the process very closely into harmony with what occurs in the eye. According to this authority, olfaction is in reality a perception of ethereal undulations of the same character as the undulations of light, these undulations being provoked by the intra-molecular vibrations of the odorous vapour in the nasal mucus and transmitted to the olfactory hairs not by immediate contact, but through the medium of the ether.
We owe this last suggestion to the curious fact, but recently discovered, that many odorous substances (in their gaseous form in the air) absorb the rays of ultra-violet light.
In order to make clear what this means, we must say a preliminary word regarding the spectrum and spectrum analysis.
The passage of a beam of white light through a glass prism breaks it up into its component parts, beginning with red, then orange, yellow, green, blue, and ending with violet. Beyond the violet end of the spectrum we know there are rays invisible to us, but capable of acting on a photographic plate. These are called the ultra-violet rays.
In like manner, beyond the red end of the spectrum we know there are also rays, likewise invisible to us, but perceptible by our tactile sense as heat. These are called the infra-red rays.
Now, the rate of vibration of all these different rays, visible and invisible, has been estimated, and they increase in frequency from the infra-red, which are the slowest, to the ultra-violet, which are the most rapid.
As we have already said, it has recently been shown that the odorous vapours absorb certain ultra-violet rays. That is to say, when the beam of light is directed through a chamber containing the odorous vapour before entering the prism, what are known as absorption-bands—vertical blacklines in the white—appear in the photograph of the spectrum.
Similar lines are seen, as a matter of fact, in the visible spectrum of sunlight, and as these correspond in position with the spectrum given by chemical elements in an incandescent gaseous state, it is supposed that they are produced by the absorption of the corresponding light-rays by these gases in the solar atmosphere.
The physical explanation given of this phenomenon is that the molecules of the gas in the sun absorb such light-rays as are equal in rate of vibration to the rate of their own vibrating molecule.
In the same way, Heyninx and others argue that the odorous vapour is composed of molecules which are vibrating with a period equal to that of the light-rays they absorb.
Moreover, since the position of the absorption-band in the photograph varies, lying in some cases nearer to the visible violet and in others further away from it, and since this position varies with the particular fundamental odour employed, it is suggested that not only do the molecules vibrate with a period equal to that of the ultra-violet rays they absorb, but as this vibration varies in rate, so it is to this variation that we must ascribe the differences in odours, This is analogous, of course, to the appreciation of colour by the eye. One odorous molecule, that is to say, like the colour red, having a slower ratc of vibration, will give rise to one kind of smell ; another, like the colour yellow, with a more rapid rate, will give rise to another kind of smell, and 80 on for all the fundamental odours. Heyninx, indeed, goes so far as to fix the position in the olfactory gamut of all fundamental odours, and to base upon it the classification we have already considered.
It is supposed, that is to say, that the vibrations of the odorous molecule set up undulations in the ether, and that it is those ethereal undulations that stimulate the olfactory hairs, just as ethereal undulations emanating from a luminous source stimulate the retina.
There is one great difference, however, between light and odour, a difference admitted, we may mention, by the supporters of the undulatory theory, but not emphasised by them. The difference is this: in the case of visible light the ethereal undulations emanate from a source at a distance (it may be like starlight at an enormous distance) from the sensory end-organ, whereas in the case of odour the undulation is supposed to be generated by the odorous molecule in close proximity to the end-organ.
The theory makes no attempt to explain how the olfactory hairs respond to these hypothetical ethereal waves.
Finally, we have the question of the olfactory pigment to consider, and in this matter we cannot do better than follow the exposition of William Ogle, an English physician who wrote as long ago as 1870, As will be seen, he forestalls the modern undulatory theory of olfaction in a remarkable manner.
Ogle contends that the presence of pigment must be of great importance in the function for the following reasons :
First, the epithelium of the olfactory region is pigmented, while that of the rest of the nasal chamber and sinuses is devoid of colouring matter.
Secondly, there seems to be some correspondence between the degree of pigmentation and the acuteness of smell, as the following facts suggest :—
In macrosmatic animals, such as the dog, cat, fox, sheep, and rabbit, pigmentation extends over a larger space and is darker in tint than in man. In these animals also the mucus covering the olfactory area of the nose is itself pigmented.
We have seen that human albinos are anosmic, and the same is probably true of animal albinos. But care is necessary in making observations on suspected albinos in animals, as even when they are altogether white a certain amount of black pigment remains about the face and nose.
The following reports, however, would lead us to conclude that as with man, so with the animals, a relative deficiency of pigment is associated with a dull olfactory sense.
It is by smell that the herbivora detect and avoid plants which are poisonous, and when poisoning does occur, it is usually a white animal that suffers. In some parts of Virginia the farmers will only rear black pigs, because, they say, the white ones eat and are poisoned by the roots of Lachtanthus tinctoria. For the same reason in the Tarentino only black sheep are reared.
Thirdly, the dark-skinned human races have a keener sense of smell than the lighter races.
Fourthly, the sense grows more acute as we get older, as we have already seen, and nasal pigmentation, it is said, also increases with age.
As to the function of the olfactory pigment, Ogle remarks first of all that odours are absorbed more readily by dark than by light materials.
Pigment is also present in the labyrinth of the ear as well as in the eye, and its presence in these organs seems to be essential to their activity.
It is to be noted that the pigment does not occur on the nerve structure in any of those end-organs, but external, though contiguous to it. In the eye, it lies in contact with the rods and cones of the retina ; in the nose, with the olfactory hairs ; in the ear, with the terminal bodies of the auditory nerve.
Hence the pigment, he supposes, must be associated with the reception of the sensory impressions.
In the eye and the ear those impressions are undulatory in character. That being so, he holds that the undulatory theory of olfaction also is probably the correct one.
Ogle finishes with the remark that the theory would be strengthened if it could be shown that pigment was specially suited for the absorption and modification of undulations.
It is interesting to us to learn that claims are now being made that pigment does possess the power necessitated by Ogle’s theory. Atall events, there is a theory of vision (Castelli’s) which claims for the ocular pigment the power of absorbing and modifying light waves, and Heyninx holds that the olfactory pigment possesses a similar property.
Summing the whole matter up, then, we may say that the undulatory theory of olfaction is, that an odorivector gives off in the form of vapour (in the aerial medium) extremely attenuated portions of its substance, too minute to be weighed, and that this vapour, disseminated through the air, enters the nose in respiration, and, being wafted up into the olfactory region, is received by the mucus bathing the olfactory hairs, where, in virtue
of the ultra-violet radiations which proceed from its molecules and are modified by the olfactory pigment, it acts on the hairs, setting up changes (it may be also undulatory in nature) in them and in their cells, which changes are transmitted thence by the olfactory nerves to the neurones or nerve-cells of the olfactory bulb (or lobe) of the brain.
The undulatory theory of olfaction, then, as will be evident to the reader, has a good deal in its favour. And in addition to what we have already said of it as accounting for the absorption by odorous vapours of ultra-violet rays, and as giving a hint regarding the function of pigment in the olfactory area, there are also a number of other phenomena which it seems to explain. We have seen, for example, how one odorivector, such as musk or civet, may have the property of enhancing the power of another, and this is a property which is characteristic also of certain luminous conditions (fluorescence, lumino-luminescence).
Again, there is a harmony existing between certain of the manufacturers’ primitive odours ; “they go well together,” and are employed for that reason in the art of perfumery. This resembles the harmony existing in another class of undulations, the sound waves.
On the other hand, just as one sound may silence another by the clashing of their waves, so one odour may “kill” or neutralise another odour (iodoform and coffee, e.g.).
There are several other minor phenomena which are in agreement with this theory. They need not detain us.
We turn now to the criticism of the undulatory theory of odour.
First of all, we shall dispose of an objection which, at first sight, has a very serious aspect.
It may seem difficult to understand how vibrations which appear to us when of a certain rate to be light should when they are of another rate become to us smell. How can one and the same physical condition produce sensations so different ?
The same difference, however, is encountered when we pass to the rays at the other end of the spectrum, the reds and infra-reds. On one side of the dividing line we only perceive these as heat ; on the other side they also become light.
Obviously, the difference can only be due to the different character of the sensory end-organ, the receptor of these vibrations. As Head says : “Each peripheral end-organ is a specific resonator attuned to some particular kind of physical vibration”—reminding us not only of sound-resonators, but also of wireless receivers, which are “tuned” or accommodated to particular wave-lengths.
Thus, if red rays encounter certain tactile end-organs in the skin, they are perceived by the mind as heat, and if they pass into the eye and stimulate the retina, they are perceived as red light. In other words, in whatsoever manner an end-organ is stimulated, it only induces its own particular sensation.
How it comes about that the various end-organs induce such different sensations is not yet known.
The ultra-violet theory of olfaction, however, has to run the gauntlet of much more serious criticism than the difficulty we have just disposed of.
One great objection to it (to my mind) is that it fails to account for another absorption phenomenon of which I have not yet made any mention. It was first observed by Tyndall nearly fifty years ago.
On submitting odorous vapours to examination Tyndall found, not that they absorbed ultra-violet rays, as this method is of quite recent usage, but that they absorbed heat-rays, or the infra-red rays of the spectrum. So that, if it be correct to say that odours set up ultra-violet rays in the other, we must be equally ready to credit them with setting up infra-red rays also !
But there is another, and perhaps a stronger, objection to the ultra-violet theory.
In the interesting and highly instructive schema drawn up by Heyninx of the wave-lengths of ultra-violet absorbed by odours, we find one or two discrepancies of a serious character.
For example, iodoform and cinnamic aldehyde show absorption-bands occupying nearly the same position on the spectrum ; and presumably, there- fore, these substances have the same molecular vibration-rate. Yet their odours are not at all alike !
Again, acetone-methylnonic and butyric acids have precisely the same absorption bands, and yet they also exhale totally different odours.
But the most serious discrepancy remains. The absorption bands of hydrocyanic acid and watery vapour (steam) have precisely the same position in the spectrum, yet one of these has a highly characteristic odour, and the other has none at all !
It is rather difficult, in view of these findings, to believe that this absorption phenomenon can have anything to do with the quality of odour.
My friend Mr. T. H. Fairbrother writes regarding this controversy —
“Whilst I do not for one moment suggest that the whole phenomena of smell can be explained entirely in terms of chemical constitution, I do maintain that it has much to do with it, and T certainly think that more valuable information about the cause of various odours has been obtained from considerations of chemical constitution than from the many extravagant physical theories which do not lead us very far. In my view the physicists arc begging the question, because they usually postulate something which we cannot prove, and whilst it is possible that the vibration of electrons causes smell, how much wiser does that statement make us ? One might easily say that it was possible that the bombardment of electrons caused smell, etc. On the chemical side, however, we are bound down to experimental facts, and we do know that csterification of carboxylic acids does bring about a fruity odour invariably, etc. Chemical constitution cannot explain fully all these phenomena, because chemical formulz themselves are only approximations, but the effect of groups in a nucleus has done much to help synthetic production of odorous bodies. When the physicist can control the vibrations of his electrons and make them rotate in accordance with his will, then he may be able to synthesise new odours—till then we have no means of testing his theories.”
The older view of olfaction—and many modern scientists, as we see, still adhere to it—is that the odorous molecule acts as a chemical reagent upon the olfactory hairs. And there is something to be said for this opinion.
To begin with, no one doubts nowadays that odours are material. They pass through the air as vapours, and they are known to travel miles on the wind. That is to say, apart from those hypothetical varietics of odour (if we can call them odour at all) discussed by Fabre earlier in this book, odours do not emanate from a point and disperse in all directions as light and sound do, Why then drag in the ether? Is it not more probable that the odorous molecule acts on the olfactory hairs by direct material contact, and that it sets up chemical changes in them ?
We are asked to believe that the ultra-violet rays of odour stimulate the olfactory hairs as visible light-rays stimulate the retina, But it must not be forgotten that in the eye those rays may induce first of all chemical changes in the retina, just as they would act on the silver salt of a photographic plate, and that it may be by these changes that the retina is stimulated.
In the phenomenon of olfactory exhaustion, as we said in our first chapter, we have a circumstance which suggests the presence of some chemical reagent in the olfactory area.
It may be, of course, that in the nose as well as in the eye the process is a combination of chemical and physical changes. And in any cage we are here dealing with that obscure region where chemistry and physics meet and mingle.
We have now come to the end of our discourse upon the theories of odour, and it must be confessed that we are still very much in the dark as to the nature of the odorous, and as to the manner in which it excites the olfactory organ to activity.
Still more mysterious, however, is the process by which the physical quality of odour becomes the sensation of the mind we call smell.
The transmutation of a physical quality into a sensation is indeed the great mystery of all our senses. Olfaction is not the only one before which we throw up our hands, and this in spite of the detailed and voluminous information which modern physiology, neurology, and psychology place at our disposal, perhaps less in spite of this information than because of it, seeing that the further our knowledge extends the wider seems the unknown realm beyond. Our science is an ever-expanding sphere, no doubt, but it is expanding into the infinite.
How is it that the rhythmic vibration of matter becomes what we call “sound,” or the rhythmic vibration of the ether “light” ?
Tow does the physical pass into and become part of the psychic ?
According to recent teaching, the physical can be followed as such from the sensory end-organ itself as far as the first synapse, or junction with the neurone. But there something happens ; … then it reappears in a new guise, vibration becomes sensation, the physical psychic, the objective subjective, the real ideal, the dead alive ! In that brief tumble of time what a miraculous transformation !
Modern science has cleared up much of the mystery of the objective world, and although it may be far from the end of its search, although, indeed, the search, one must think, can never entirely elucidate the dense obscurity that envelops us on every side, dark as a starless night around a candle, yet we already know this much, that the real world is very different from the world depicted for us by our senses.
Only a little imagination is needed to convey us out of the magic circle into which we have been born, and what a strange universe do we then find ourselves in! Entangled in a meshwork of space-time and permeated by whirling maelstroms of varied and innumerable oscillations, we lose all hold on reality in the very act of grasping it.
But although we do possess some sort of vague notion as to the constitution of the outer universe, before the inner we stand ignorant and speechless.
Regarded as a machine, the brain, it is true, like the world without, is reluctantly yielding up its secrets one by one. We are learning how it works as a chemical factory, as a physical power-house, so that already we can surmise that here also we have probably to deal with a multiplicity of vibrations, of exquisitely minute transformations of energy, of involved intercommunications, of deft though intricatc associations, of rapid yet permanent recordings and registrations.
We are now able to follow the undulations we term light, not only into the eye, but into the brain itself, locating their central station in the occipital lobe, whence their effects radiate all over the organism. And in the case of olfaction Pawlow has taught us that its chief vegetative function, the result of radiations from the olfactory central station in the brain, is the arousing of the digestive glands to activity. The first act of digestion is olfaction, But the routes which the olfactory stimuli follow in the central nervous system and their communications with other sensory paths are not yet known.
The secrets of the brain which have been dis- closed to us, however wonderful they may be, concern only, we must remember, the machinery of the nervous system, that part, namely, which is of the same nature and order as the objective world, of which indeed it is a member. Hitherto have we come, but no further :
“The traveller hails. The echoing walls respond.
And there the matter ends. The wilds beyond
Are broken rock and desert where no foot
Can venture on to trace a further route,
For none hath trodden or shall ever tread
This hither limbus of the outer dread.
Cloven abrupt, the absolute abyss
Falls sheer beneath us, fathoms fathomless,
And still high o'er us heaves the unclimbed hill,
And the unanswered questions front us still.”
The “thought” escapes us. Somewhere beyond the boundary of the physical flits this elusive, this tantalising ghost. How it is acted upon and how it reacts we know to some extent. But what the nature of its action may be is more than we can determine.
Nay! A moment ago we lightly spoke of passing out of the magic circle into which we have been born, and we forthwith proceeded to talk as if we had in reality escaped from this our prison. But there is no escape for us, of course. No man can jump out of his skin. There undoubtedly arc such things as “waves,” or “undulations,” or “oscillations,” or “vibrations,” or whatever we like to call them. But they are not what we imagine them to be. There is, we may suppose, a four-dimensioned universe of “space-time.” But it is beyond our conception. There is “objective reality,” in a word. But it is no reality to us. Those very expressions, glibly used though they be, are but metaphors—“pretendings” a child would call them—attempts to bring the remote a little nearer to us, to clothe the uncouth in the garments we ourselves wear ; all of which is nothing but Maya—illusion—shadow-play.
Let us not deceive ourselves. Along with the recent revelations of physical science there comes, say certain modern philosophers, the suspicion that the universe is irrational. At every point we are brought up short by the unknowable.
For example, Einstein tells us that what we call the “ether” has no existence. It is merely a void.”—But how can we call that void which contains something—undulations, to wit ?
“Nay!” you argue; “the undulations traverse the ether, but they are not it. The ether is a non-entity. It has no existence. It is nothing.”
To which I reply: “But ‘nothing’ is an absolute term. It means ‘no thing’, How, then, can undulations, or anything else for that matter, pass through nothing ? ”
“What nonsense !” you cry ; “this kind of verbal poser is just the silly old metaphysicians’ parlour game of playing with words.”
I know it is. But the word-play has its uses. It demonstrates to us that words, language, logic, all alike, fail our thought, not so much because those instruments are limited in power as because the thought itself is lacking in precision and comprehensiveness.
It is when our word-play probes the expression that the vagueness of the idea is made manifest. Our foil, even with the button on, goes clean through the phantom.
The mind, in short, has not absorbed, nor can it absorb, the fact. We seize a glass of water to drain it, and presently, like Alice, we find ourselves swimming about in an ocean! Obviously the universe is beyond our comprehension, a conclusion desperate if you like, yet undeniable.
But how very annoying it is, after all our heavy labour, to hear the ancient scoff of Zophar the Naamathite still ringing triumphant :
“Canst thou by searching find out God ? Canst thou find out the Almighty unto perfection ?”
(Still we mean to go on trying !)
Yet of all the senses none surely is so mysterious as that of smell. For, as we have shown, the nature of the emanations that stir it to activity is still unknown ; the simple structure of its end-organ confronts us, like a sphinx, with silence ; and after the reception of the stimulus in the olfactory lobe of the brain its further connections and communications still remain unsurveyed, albeit, as I haye already so amply displayed, its effects upon the psyche are both wide and deep, at once obvious and subtle.