Popular Science Monthly/Volume 5/May 1874/Synthetic Chemistry
| SYNTHETIC CHEMISTRY. |
By Prof. JOHN W. LANGLEY,
OF THE WESTERN UNIVERSITY, PENNSYLVANIA.
CHEMISTRY has been called the analytical science, and undoubtedly with justice in the past, since the most exact processes with which it deals are still those which go technically by the name of analysis; but, of recent years, the arts have been enriched by many perfumes, colors, and drugs, which are the results of careful and laborious construction on the part of the manufacturer, operating under chemical laws. Gradually there has arisen a new branch of the science, whose aim is to produce artificially new compounds out of old material; and, since the living vegetable and animal offer familiar though mysterious examples of the same process, it is only natural that the philosopher should have tried to obtain, by known methods, some of those results which are so silently and wonderfully wrought out by the substance of living tissue. It will be the object of this paper to indicate, in a non-technical way, some of the steps which have been taken in the effort to compete with vitality.
The word synthesis, in its broad or general acceptance, signifies the union of any two or more substances to form a physically-homogeneous mass, and into a product which cannot be mechanically divided into dissimilar parts. Under this definition nearly every operation of chemistry would be synthetical; for, even in the case where an element is isolated, this is done only at the expense of some other bodies which pass into the condition of compounds; thus, when gold is precipitated in a metallic state from solution, we must use iron, zinc, or some other substance, which shall become oxidized and dissolved in the place of the precipitated gold.
Most cases of synthesis take place with great facility, automatically in fact, for, when coal is burned, we start the fire, and after that the oxygen and carbon unite to form carbonic acid, without any further effort on our part.
Now, in the world there are vast numbers of distinct compounds, some of which have a simple and others a complex structure; it is found, by long experience, that there is a general disposition on the part of these substances to pass from the state of feebly-united units to that of great fixity; thus, wood, which is composed of carbon, oxygen, and hydrogen principally, burns in the air into two stable and incombustible bodies, carbonic acid and water; and, in so doing, evolves a large amount of heat. Neither of these final products, so formed, can in any way be caused to evolve a further amount of energy, without bringing in the aid of external matter; dynamically, they are dead, as physiologically an animal is, when no longer capable of movement.
To raise a body into a state so that it contains within itself force in a potential form, and so as to be able to liberate this force spontaneously, when certain conditions are fulfilled, is an operation opposed to the general tendencies of the material world, and directly contrary to what may be called the habits of non-vitalized matter. Now, it is precisely this thing, this act of elevating some limited portion of matter to a higher plane, where its potentiality and complexity are both increased, that is meant by chemical synthesis, and it will be seen how much this use of the term restricts the meaning of the words as ordinarily used.
To raise a weight from the ground, to make water run up-hill, or to generate heat by friction, are all processes which require the expenditure of effort and material substance, if we would perform them ourselves. So in Nature, this form of synthesis is never accomplished without the expenditure of energy from some source. In the vegetable, the power comes directly from the sun, and the world in summer is colder and less brilliant (by an infinitesimal amount, it is true), because of that growth of leaves and wood which in the winter we reconvert into heat and light in our stoves; the animal gets the force for its synthesis by eating and consuming the products of vegetable life; and the chemist in his laboratory obtains his power mainly by converting a large amount of some complex body into simpler ones, in order to raise a smaller portion of some other mass to a more loosely aggregated and heterogeneous condition.
Starting with the elements and simple mineral bodies, the construction of such substances as salts, by the union of an acid and a base; of a suffocating gas, by the combustion of sulphur; of the conversion of iron into rust, by the action of water, air, etc., is very easy up to a certain point—so easy, that probably the very earliest beginnings of chemistry lay in observations of these facts. Certainly, the historic origin of the science in alchemy leaves no doubt of it as regards the middle ages, and, at the present day, most of the technical applications of the science are examples of the building up of compounds. So long as the experimenter's efforts were confined to mineral matter, he met with but little trouble; but, the moment he tried to reproduce any organic body, any compound which was the direct product of life, either animal or vegetable, he met a barrier which seemed to be insurmountable, and which bade fair never to be crossed. It was easy enough to analyze any of these vital products, and to determine the exact number and amount of their ingredients; but, once separated, the chemist vainly endeavored to make the elements reunite as they were before.
This was the state of things up to forty or fifty years ago. The power of the chemist had grown to be very great; he could either bind or loose, as pleased him, and he thought be had a tolerably complete knowledge of the elements and forces he was dealing with. Is it so much wonder, then, that he fell back on the assumption of the existence of a mysterious force outside of his domain, and that his defeat, coupled with the known impossibility of restoring life to the dead animal, should have led to the assumption that these organic bodies were the result of chemical processes which had been aided and controlled by a special entity, denominated vital force?
It is not the object of this paper to take sides on a question which is still a matter of debate; and, in regard to vital force, it has only to chronicle here some of the steps by which bodies, hitherto solely evolved by the action of living matter, may now, under the guidance of the human intellect, and by the instrumentality of physical forces, be formed artificially at will.
In 1828, Wöhler, a German chemist, discovered that cyanate of ammonia, a purely mineral compound, under certain circumstances became changed into urea, without either loss or gain of any foreign body, the elements rearranging themselves to form a more complex structure. But here it was claimed, by the vitalists, that urea, being an excrementitious substance, and one of the waste products of the animal system, must be considered as in reality a mineral, and only accidentally, as it were, akin to animal tissue.
There exists in the bodies of ants a secretion which apparently serves them as a weapon; it is called formic acid, and, for some years after its discovery, was universally prepared by pounding up sundry ounces of ants in a mortar, and distilling them with water, when the bodies of the insects were left in the retort, and a diluted acid was found in the receiver. Analysis showed this substance to be formed of but three elements, and which, moreover, did not appear to be united in a complex manner. From certain analogies it was inferred that the acid might be obtained by some method of gradual oxidation; at last, the right substance was found, and, by duly acting on starch by oxygen, formic acid was produced. This was considered a very great step in advance, for an animal product had been at last formed from a vegetable one; and though it is true the body in question had a simpler constitution than the starch, still the plane of possible chemical processes had been elevated into the animal kingdom.
Quite recently a method has been discovered by which formic acid may be generated directly from its elements. To do this, carbon, say a bit of charcoal or coke, is heated in a limited supply of air, and the result is carbonic oxide; this gas, if exposed for a long time over caustic potash, combines with it, and this product, if distilled with oil of vitriol, yields formic acid. Thus the body has been formed without any thing having been used which is the product of life.
From this point progress was rapid, though at first apparently it rather tended away from the matter at issue. Previous study in the department of mineral chemistry had gradually forced the conception that the position of the elements in a compound had as much to do with its properties as did their number and amount, and it also had developed the fact that certain elements might be withdrawn and their places filled by something else, without changing the general character of the substance. Indeed, a compound body was called a chemical structure, and likened to a real edifice, in which the elementary atoms were the bricks of the house, and the resulting properties constituted the shape of the building. Now, by replacing one element by another, the same kind of change was produced as would be caused by the substitution of marble for bricks, or iron for stone, in the real house. Its appearance and habitability might be greatly altered, but its general shape and character remained.
As an example of this class of syntheses, the alcohols may be cited. Ordinary or wine alcohol is one of a large class of bodies which have similar features, and to which the same general name has been given; they constitute a series: thus, we have methylic alcohol, vinic alcohol, tetrylic alcohol, and so on. Now, in most of these we can substitute certain metals for hydrogen; for instance, metallic zinc can be thus inserted, and hydrogen removed, yielding zinc-ethyl, and, with the addition of oxygen, zinc-alcohol, and we get a colorless, fragrant liquid, in which, singularly enough, the zinc has so far lost its usual characteristics as to be both invisible and volatile. By carrying on the steps of this process still further, several metals may be introduced leading to the production of bodies of great resultant complexity, but which, through all their metamorphoses, are yet true members of the alcohol family.
In this sense the artificial processes may be said to surpass the natural ones; for man is able to add many individuals to a series of which Nature presents us with only scattered terms; and, in addition, in this particular group, he is able to form some of the natural members, such as wine alcohol and glycerine, by a direct process of construction, starting with the free elements, carbon, hydrogen, and oxygen. The method is briefly as follows: Carbon, in the form of black-lead, and therefore strictly a mineral substance, is heated intensely between the poles of a galvanic battery; when it is brilliantly incandescent, hydrogen gas is made to pass over its surface, in a suitable apparatus, the sides of which are kept comparatively cool, and the result is the formation of an invisible but extremely irritating gas, known as acetylene. Now, if acetylene is brought into a solution of copper, it combines with it, forming a dark-red explosive compound, and, if we act upon this body by hydrogen, the copper will be expelled, and olefiant gas, a sweetish ethereal substance, is obtained; and, finally, by distilling this last with sulphuric acid, alcohol is one of the products. Thus, in the several steps leading to this result, only mineral matter and ordinary chemical forces have been employed.
It must not be inferred, from the above meagre examples, that the number of syntheses is equally limited. Already there have been formed several natural vegetable acids, many of the alcohol family, some of the sugars, a whole host of ureas, a multitude of bodies analogous to the vegetable alkaloids, as well as many of the natural flavoring and coloring agents; these last, indeed, on an extended commercial scale.
So far as the evidence of experience goes, there seems no limit to the possible production of organic bodies which possess a definite chemical structure, at least of those which have the power of crystallizing, for the number, even now beyond the powers of an ordinary memory, is constantly increasing in an accelerating ratio, and already, as has been referred to, in some instances exceeds the range of Nature herself.
But are we entitled from these facts to indulge in self-glorification, and to assert that ultimately the exact reproduction of all the tissues found in living organisms will be possible? Certainly not, so far as the chemist is concerned, for other methods than those which properly belong to his science must obviously be sought to give the specialized forms and functions of living substance. The gulf which separates the artificial processes of synthetic chemistry from those really employed by the plant or animal is a wide one, across which lies as yet no solid bridge of fact and theory. It is known that all vegetable matter is derived primarily from carbonic acid, water, and ammonia, under the influence of the sun's rays, but the method by which this is done is still a mystery; a mystery, though, which may be cleared away, for very recently most interesting experiments have been made on the chemical action of light. All we know about vegetable chemistry with certainty is, that, whatever are the processes occurring in living tissue, most of them, in their earlier stages, are radically different from those of art. In the laboratory, synthesis starts with the elements, and from them, by exceedingly wasteful means, builds laboriously, from platform to platform, up to the desired height. On the other hand, the smallest speck of green vegetable tissue, if living, elaborates its substance, not from elementary bodies, but from compound ones, and those too which are among the most stable and most highly-oxidized known; each new step, then, in the artificial method, tends to carry us away, rather than to approximate us to the natural agent.
To return to the metaphor of the house. The chemist starts from the ground, and completes the edifice by piling up one by one, the elementary bricks, and binding them together by the natural cement of atomic attraction. The living organism begins its labors at the top, and chiefly from the three firmly-knit compounds, water, carbonic acid, and ammonia, builds, by infinite gradation, down to an elementary foundation; and, at this day, science is scarcely more able to tell how this is done, than can the mason inform us how to start a block of houses by commencing with the roof.
This distinction as to methods includes obviously a vast domain of facts, that which has been referred to as the region lying between the natural and artificial synthetic methods. It is a sort of debatable land, for, though it is yet unknown, there is not the slightest proof that we shall not cross it some day, and possibly soon; and already the space is inhabited by speculations and embryonic theories, those shadowy precursors of substantial knowledge.
But the limit where the chemist must stop is the dividing line where the naked individuality of a chemical compound becomes clothed by the definite outline of an organ, a cell, or a speck of vitalized protoplasm; as soon as form other than crystalline appears, the sharpness of atomic characteristics is merged in the idiosyncrasies of the cellular unit, and the chemist unaided can trace the work no further. A new set of forces apparently comes into play, and whether we say this new agent is vital force, or prefer to hold fast to scientific accumulations of facts and methods of thought, and regard the new mystery as but the result of new or changed conditions, it is equally evident that the province of pure chemistry ceases, for this limit was long ago self-assigned by the science, and indeed exists in a latent form in all its definitions.
Doubtless, either biology, or some as yet unspecified portion of science, drawn from the provinces of chemistry, physiology, anatomy, and physics, will take hold of this problem and solve it; to a certain extent is doing so now, and thus far the little light which has been gained seems to indicate the unbroken operation of uniform law acting through the known physical forces. Perhaps under this heading common gun-cotton offers one of the most striking examples. The cotton-fibre is an elongated cell, it is of course both vegetable and a special functional organ, and is therefore outside of what was above spoken of as the limit of pure synthesis; nevertheless, by the simple operation of immersing it in strong nitric acid, the properties of the tissue are remarkably changed. It has become highly combustible and explosive, and also soluble in a mixture of alcohol and ether. Now, the chemical change has consisted in a bodily removal of hydrogen, and the insertion into its place of a heavy red compound gas, and yet the shape, color, and texture of the cotton-fibre are so little altered that none but an expert can perceive any external change. Moreover, by a reverse process, gun-cotton can be changed back again into the ordinary article. It is, then, possible to most profoundly alter the chemical structure and properties of one of these organic cells without visibly changing its individual shape.
On the other hand, when gun-cotton is dissolved in ether, it becomes collodion, and when this solution is evaporated the vegetable tissue is left, not in its original fibrous form, but as an amorphous film; so here we have the cellular characteristics utterly destroyed by an agent which is not regarded as exerting any chemical action at all; and, by analogy, we may infer that the specialized forms of organized bodies are not therefore the necessary results of their atomic structure only.
Another example of the same kind is the facility by which certain crystals may be made to take on either an amorphous or an apparently cellular form. It is only necessary to add gum, or some other mucilaginous material, to the water in which they have been dissolved, to have them appear, on solidifying in this anomalous way, all their beautiful sharp angles and edges lost in a formless mass or in rounded nodules, like many of the renal and vesical calculi.
But this paper has already reached the limit of facts, and has perhaps entered too far into the region of speculation. In the controversy now going on, as to the spontaneous generation of life, some of the contestants have appealed triumphantly to the results of synthetic chemistry, as indicating clearly that the "arcana of life had been entered and the mysterious divinity, vital force, overthrown." Does it not rather appear that chemistry, as yet, had not cut the Gordian knot, but was rather compelled to look to other sets of forces than those known as chemical for the chief agencies concerned in this work?
