Popular Science Monthly/Volume 59/September 1901/Henry Cavendish
| HENRY CAVENDISH. |
By C. K. EDMUNDS,
JOHNS HOPKINS UNIVERSITY.
PERHAPS the most remarkable character in the history of science is Henry Cavendish. One of the few men of science who have possessed great fortunes, he yet ignored the power of his wealth, allowing himself but few and simple wants. Highest born of the distinguished chemists of Great Britain, he cared nothing for the external advantages of birth, preferring to house himself till forty years of age in his father's stables, where unmolested he might devote his days to the pursuit of truth. Outside the monk's cell and the prisoner's dungeon, few men have lived and held so little communication with their fellows or made so few friendships as he. Yet his fame could not be kept from proclaiming itself even during his lifetime, while to-day he is called the 'Newton of Chemistry' and the 'Father of Quantitative Physics,' being declared by Biot to be 'the richest of scientists, and the most scientific of the rich.'
Of a family, tracing its pedigree to the Lord Chief Justice under Edward III., he was the son of Lord Charles Cavendish, the third son of the Duke of Devonshire, and of Lady Anne Grey, daughter of Henry, Duke of Kent, and was born October 10, 1731, at Nice, Italy, whither his mother had gone on account of ill-health. His mother died two years later, after giving birth to a second son, Frederick; and Cavendish, residing with his father in London till eleven years of age and spending the next eleven years away at school, was deprived at the most critical period of his life of the salutary influences of a happy home, that might have tempered the peculiarities of his character, which in the last analysis, however, are to be referred chiefly to original conformation.
Having entered Peterhouse College, Cambridge, in 17-19, he left in 1753 without taking his degree, a step scarcely due to fear of the examinations themselves, but rather in keeping with his very pronounced shyness, which he was said to possess to a degree bordering on disease. His personal history is a blank for the next ten years, but his subsequent writings show that they were spent in mathematical, chemical and physical research. He was elected a fellow of the Royal Society in 1760.
In 1783, the death of his father left him free to follow his own tastes. During his father's lifetime he was kept on an annuity of £500, and some regard this fact as explaining in part the peculiarities of his character; for during this period he acquired those habits of economy and those singular oddities of character which he ever afterward exhibited in so striking a manner. For some years Cavendish was allowed by his father to attend the Royal Society Club regularly, but was given the exact five shillings for the dinner, not a penny more. There is reason to believe that his father's parsimony has been misapprehended, for while Cuvier, Biot and Lord Brougham make dissatisfaction with Henry for not entering on public or political life the ground for his illiberality towards him, yet others assert that Lord Charles Cavendish was not a rich man and allowed his son all he could afford. There is no certainty as to when or from what source Cavendish inherited the riches which ultimately came into his possession, though they were probably a legacy from a rich uncle. All the testimony, however, is at one on two cardinal facts: that Cavendish was for the first forty years of his life a poor man, and for the last thirty-nine an exceedingly wealthy one.
The possession of several hundred thousand pounds did not alter his life in the least; he simply did not know what to do with it, and hence let it alone, allowing it to accumulate till at his death his estate was worth £1,175,000. Cavendish's indifference to pecuniary affairs was so great that when his banker called on him with regard to the investment of a portion of the vast sum that had grown on his hands, he was rudely ordered to be gone and not to come there to plague him, or he would lose the control of the funds. It may seem strange that none of this large fortune was devoted to scientific or charitable purposes, but we must remember that Cavendish never thought of himself, much less of others. Sir Humphry Davy was indebted to him for 'some bits of platinum,' but tacitly appealed in vain for financial aid in his electrical researches. Just before the subscription for the enlarged voltaic battery was taken. Cavendish was in Davy's apartments at the Royal Institution, and upon Davy expressing fear that he should fail to secure the necessary amount. Cavendish joined heartily in deploring the lack of liberality in the patrons of science, but did not seem to consider himself at all called upon actively to forward the desired object. Yet had he been directly asked to sign a cheque in Sir Humphry's name for £500 he would probably have done so at once. When reminded of some needy charitable object, he gave liberally, but he never himself saw a need. Whether from original or acquired indifference, he exhibited a passive selfishness in all his dealings.
To his town residence, close to the British Museum, few visitors were admitted, and these have reported it to contain only books and apparatus. For the former he also set aside a separate mansion on Bradford Square, and here collected a large and carefully chosen library of science, which he threw open to all engaged in research, and from which he himself never took a book without leaving a formal receipt. His favorite residence was a beautiful suburban villa at Clapham, which now, as well as a street in the neighborhood, bears his name. The whole house was occupied with workshops and laboratory, only a small part being set aside for personal comfort. He needed nothing more for himself, and he did not wish others to visit him. When occasionally he had guests, they were always feasted on a leg of mutton. On one occasion his housekeeper suggested that a leg of mutton would not be enough. Well, then get two, was the reply.
The more prominent of Cavendish's contemporaries have left graphic estimates of his remarkable and interesting peculiarities of character. The most striking was a singular love of being alone. He held aloof from social intercourse, even with members of his own family, and only once a year saw the one he had made his heir. To the great objects of common regard which excite the fancy, the emotions and the higher affections, he was equally indifferent. The beautiful, the sublime and the spiritual seem to have lain entirely beyond his horizon. Although he is thought to have held Unitarian views, he is also understood never to have attended a place of worship. In the words of one of his contemporaries, 'he was the coldest and most indifferent of mortals.' He never married and was reputed to have a positive dislike for women. Lord Brougham tells us that Cavendish ordered his dinner daily by a note, left on the hall table, where the housekeeper could afterwards get it. Another authority relates that Cavendish 'one day met a maid servant on the stairs with a broom and pail, and was so annoyed that he immediately ordered a back staircase to be built.' His dress was that of the gentleman of the preceding half century. The frilled shirtwaist, the greatcoat of a greenish color, and the cocked hat, made a picture no one was likely to mistake. But gleams of genius often broke through this unpromising exterior. He never spoke except to the point, and always supplied excellent information or drew some important conclusion from his own very extensive and accurate knowledge. So that while Sir Humphry Davy said of him, "His voice was squeaking, his manner nervous, he was afraid of strangers, and seemed, when embarrassed, even to articulate with difficulty," he also said, "He was acute, sagacious, and profound, and, I think, the most accomplished British philosopher of his time."
But two additonal dates remain to be given in reference to his personal history: The first, March 25, 1803, when he was elected one of the eight foreign associates of the French Institute; the second, February 24, 1810, when he died, in his seventy-ninth year. As he lived so he died by rule, predicting his death as if it had been the eclipse of some great luminary (as indeed it was), and counting the moment when the shadow of the unseen world should enshroud him in its darkness. After an illness of only three days, the only one he ever had, he called his servant, told him he was going to die, and commanded him to stay away and to keep everyone else away until the event was over. The servant obeyed, and, when he returned. Cavendish had breathed his last.
An intellectual head thinking, a pair of wonderfully acute eyes observing, and a pair of very skilful hands experimenting and recording, are all that we realize in reading his memorials. His theory of the universe seems to have been that it consisted of a multitude of objects to be weighed, numbered and measured; and the vocation to which he thought himself called was to weigh, number and measure as many of these objects as his threescore years would allow. Whenever we catch sight of him, we find him with his measuring rod and balance, his graduated jar, thermometer, barometer and table of logarithms. Most of his researches were avowedly quantitative; he weighed the earth, he analyzed the air, he discovered the compound nature of water, and he noted with numerical precision the actions of the ancient element fire. Everything pertaining to each, to which a quantitative value could be attached, was set down in figures, before it went out to the scientific world with its passport signed and sealed. In all his researches he displayed the greatest caution, not from hesitation or timidity, but from his recognition of the difficulties which attend the investigation of nature. Cavendo tutus was the motto of his family, and seems ever to have been before him, so that he well deserves the title—'Father of Quantitative Physics.'
His first recorded scientific work was 'Experiments on Arsenic' (1764). In 1765 'Experiments in Heat' were performed which, though written out for a friend, were not made public till nineteen years later, but which, had they been published in 1764, would have given Cavendish precedence to Black in some of his discoveries as to 'latent heat' and 'specific heat,' and equal merit in others. In his first public contribution to science, 'Experiments on Factitious Airs,' sent to the Royal Society in 1766, he defines 'factitious air' as air which is driven off when compounds are heated or treated with acids, and the questions of the permanent elasticity of 'factitious airs,' their solubility in different liquids, their power to support combustion, their specific gravity, and likewise their combining equivalents, were all carefully considered. 'Fixed air' (CO2) was only a particular kind of factitious air driven off from the alkalis (carbonates), and he found that when it was mixed with common air in the proportion of one part to nine, it rendered the air unfit for respiration. Cavendish first isolated and experimented with hydrogen, though he cannot be called its discoverer, for Paracelsus, about 1540, obtained it by acting on metals with sulphuric acid, the result of which he described as the 'rising of the wind'; and many of Cavendish's predecessors, Boyle among others, had encountered it; but Cavendish, who called the gas 'inflammable air,' was the first to examine its properties carefully and to describe them.[1] Cavendish is also entitled to be called the discoverer of the constant composition of the atmosphere, and its first accurate analyst, for in 'An Account of a New Eudiometer' (1783) he showed the atmosphere to be of constant composition and to consist chiefly of 'phlogisticated' and 'dephlogisticated air' (nitrogen and oxygen), and he observed that when the electric spark passing through his eudiometer caused the 'phlogisticated' and 'dephlogisticated air' to unite, there was always left a small bubble which he could not get rid of in any way. This small bubble we now know to have been 'argon' In his celebrated paper read before the Royal Society in 1784, on 'Experiments on Air,' he gave an account of the discovery of the composition of water and of nitric acid. He showed that nitric acid, which had been known by Geber probably in the eighth century, was produced when nitrogen mixed in small quantity with hydrogen was exploded by the electric spark in the presence of an excess of oxygen. But strictly speaking we cannot assign to him the merit of the discovery of the composition of nitric acid, for he regarded nitric acid as a simple, or at least an undecompounded body, while nitrogen, according to him, was a compound. He was thus not the direct asserter of the modern doctrine of the composition of nitric acid, and to Lavoisier belongs the merit of the true interpretation of Cavendish's results.
Wilson's presentation of 'A Critical Inquiry into the Claims of All the Alleged Authors of the Discovery of the Composition of Water'[2] makes it certain that Cavendish was the first consciously to convert hydrogen and oxygen into water, and to teach that it consisted of them. In his own words 'water consists of dephlogisticated air united with phlogiston,' and as dephlogisticated air was his term for oxygen and phlogiston his term for hydrogen, this statement corresponds closely with the modern view of the nature of water. His inheritance of the prejudices of the early phlogiston school led him to the erroneous conclusion that every combustible contains hydrogen, and that the deoxidation of air and the oxidation of combustibles are invariably accompanied by the production of water. The discoverer of so great a truth as the composition of water may be forgiven for overestimating its importance.
While his experiments on the composition of water were made in the summer of 1781, his paper, 'Experiments on Air/ was not read to the Eoyal Society till January, 1784; and this delay, resulting from his desire to investigate the nature of the acid (nitric) formed on the passage of the electric spark through a mixture of hydrogen and oxygen, containing, as was afterwards found, a little nitrogen, caused his claim to the discovery of the composition of water to be contested by no less rivals than the celebrated James Watt and the great French chemist, Lavoisier. The modest, retiring and almost inordinately cautious man, whose personal history has just been detailed, has been accused of both incapacity and dishonesty, by men distinguished in letters and science, whose connection with the vexed question gives it an interest apart from its intrinsic merit.
Though Cuvier, in 1812, as secretary of the French Academy in reading an éloge on Cavendish could say that "his demeanor and the modest tone of his writings procured him the uncommon distinction of never having his repose disturbed either by jealousy or by criticism," Cuvier's distinguished, successor, Arago, in writing the éloge on Watt in 1839, charged that Cavendish learned the composition of water, not by experiments of his own, but by obtaining sight of a letter from Watt to Priestley. The French Academy heard the one side argued, and the British Association in the same year heard Cavendish's vindication delivered by the Rev. W. Vernon Harcourt, the president for that year.
At the very threshold of the water controversy we encounter a perplexing dilemma. Two unusually modest and unambitious men, universally respected for their integrity, famous for their discoveries and inventions, and possessed of rare intelligence, are suddenly found standing in a hostile relation to each other, and, although declining to publish their own unquestioned achievements, are seen contending for a single discovery, which the one believes the other to have learned at second hand from the revelations made to a common friend, and which that other accuses his rival of having gathered from a letter that he was allowed to peruse. A misunderstanding such as this would never have occurred had Watt and Cavendish been intimate in 1783. As yet, however, the friendly intercourse which afterwards subsisted between them had not commenced. The one was resident in London, the other in Birmingham, and each was informed of the other's doings by third parties, upon whom mainly though not equally, rests the blame of having occasioned the water controversy. Those in question are: Dr. Priestley, J. A. DeLuc and Sir Charles Blagden, all eminent men of unblemished character. Through the first, knowledge of Cavendish's experiments passed to Watt, and a knowledge of Watt's conclusions to Cavendish; by the second. Watt was informed that Cavendish had deliberately pilfered his theory; and the third, who was Cavendish's assistant, reported the latter's conclusions as well as those of Watt, to Lavoisier, whom he accused of appropriating the ideas of both English philosophers. Blagden also made certain changes in the manuscript of Cavendish's 'Experiments on Air,' and while superintending, in his capacity of secretary of the Royal Society, the printing of the paper, and of Watt's rival essay, suffered certain typographical errors to occur, which involved himself and his principle in accusations of unfairness, in which, however, Wilson shows that with the exception of a carelessness in correcting printer's proof, Blagden was guiltless of any wrong toward Watt or of unfairness toward Lavoisier, and that to DeLuc belongs the unenviable distinction of deliberately provoking the water controversy, doing Cavendish and also Watt a great wrong by hastily deciding against the former, and filling Watt's mind with suspicions that Cavendish had borrowed from Watt's letter to Priestley the views which he published as his own, because he had in truth discovered them for himself, and that too at an earlier date (1781) than Watt (1784:) or Lavoisier (1784-86).
Remarkable as Cavendish's achievements in chemistry were, his greatest fame as a scientist will ever be based on his researches in physics; his single experiment in dynamics would place him in the first rank, and a very large part of the modern development of electrical science is based upon his work in electrostatics and electro-dynamics.
Cavendish verified the law of inverse squares for gravitational attraction, and determined the mean density of the Earth. The apparatus, devised by the Rev. John Michell, consisted of a horizontal lever six feet long, suspended by a wire forty inches in length, carrying at each end a lead ball two inches in diameter. Two large masses of lead (each about 317 lbs.) were placed near the ends of the lever and on opposite sides, so that, their attraction would produce turning moments in the same direction. The angle of rotation having been measured by means of telescopes with verniers, and the torsion of the suspending wire determined by observing the time of vibration of the rod, the force was calculated which would have been exerted by a globe of water the size of the earth on the same body on its surface, and from this the density of the earth was obtained as the mean of seventeen terminations to be 5.48 ± 0.38.[3]
Cavendish cared more for investigation than for publication. He would undertake the most laborious researches in order to clear up a difficulty which none but himself could apprehend, or was even aware of, and we cannot doubt that the result of his enquiries gave him a certain degree of satisfaction. But it did not excite in him the desire to communicate the discovery to others which, in the case of ordinary men of science, generally ensures the publication of their results. How completely these researches of Cavendish remained unknown to other men of science is shown by the subsequent external history of electricity.
Cavendish's work as a member of the committee appointed by the Royal Society to investigate protection from lightning shows him cooperating with Franklin and others in an investigation on behalf of the nation. But most of his work was a private matter and in electrical science, in which he was by far the authority of his day, he published only two papers, 'Of the Electrical Property of the Torpedo' (1776) and 'An Attempt to explain some of the principal Phenomena of Electricity by means of an Elastic Fluid' (1771-72). Yet he left behind him some twenty packets of manuscript on mathematical and experimental electricity, which were but little known till Maxwell edited them in 1879, for they were only alluded to in his celebrated paper on the Torpedo. They anticipated, however, many of the facts subsequently made known by Coulomb and other celebrated physicists, and contained some of the results of experiments of a refined kind instituted at a much later day.
Cavendish proved the law of inverse squares for electric charges not by actually measuring the forces as in the case of gravitational attraction, but by showing that the entire charge resides on the surface of a charged body and that there is no charge at all communicated to a sphere within a sphere when electrically connected and a positive charge is given to the outer one. He then established the theorem that the force must vary inversely as the second power of the distance between charges in order to explain this result of experiment, showing that if it varied according to any higher power, the inner globe would receive a part of the positive charge, if according to any lower power, the inner globe would be negatively charged.
These experiments on the law of inverse squares were performed in December, 1772, and in fact all of his work in electrostatics was completed before 1774, while it was not till 1785 that Coulomb published the first of his seven memoirs, on the data of which the mathematical theory of electricity as we now know it was founded by Poisson; and as Cavendish never published his at all, it is plain that each worked in ignorance of the other's results. The method of each was distinctly his own. Coulomb made direct measurements of the electric force at different distances and compared the density of the surface charge on different parts of conductors. On the other hand, the very idea of the capacity of a conductor as a subject for investigation is due entirely to Cavendish, and nothing equivalent to it occurs in the memoirs of Coulomb. The method that Cavendish adhered to throughout his experimental work was the comparison of capacities, and the formation of a graduated series of condensers, such as is now regarded as the most important apparatus in electrostatic measurements.
Great difficulty is very often encountered in interpreting the work of former experimenters in terms of modern units, yet Cavendish had such a clear insight and worked so quantitatively that we can readily express his results in terms of modern nomenclature and units. His 'inches of electricity' for instance, can be directly compared with modern measurements, for while his 'inches' express the diameter of a sphere of equal capacity, modern measurements express capacity as the radius of the same sphere in centimeters. When we consider the crudeness of some of Cavendish's apparatus, we are amazed at the accuracy of the results he obtained. The capacity of a circular disc, for example, was determined experimentally by him to be 1/1.57 of that of a sphere of the same radius, while the most modern calculation gives 1/1.571 for the same ratio.
Cavendish also entertained exceedingly clear views on what we now know as 'potential' and 'resistance,' and, besides Coulomb's law of inverse squares, his papers contain anticipations of Faraday's 'specific inductive capacity' and 'electric absorption,' and Ohm's 'law of electrical resistance.' In observing that the charges of coated plates were always several times greater than the charges computed from their thickness and the area of the coatings, Cavendish not only anticipated Faraday's discovery of the specific inductive capacity of different substances, but actually measured its numerical value in some cases. He also considered the question, of fundamental importance in the theory of dielectrics, whether the electric induction is strictly proportional to the electromotive force which produces it, or in other words is the capacity of a condenser the same for high as for low potentials. He regarded his results as not decisive, but, in observing that the apparent capacity of a Florence flask was greater when it continued charged a good while than when it was charged and discharged immediately, Cavendish discovered the phenomenon called by Faraday 'electric absorption,' which was fully studied later for different kinds of glass by Dr. Hopkinson, and connected with the long-known phenomenon of 'residual charge.'
But besides this series of experiments on electric capacity, another course of experiments on electric resistance was going on between 1773 and 1781, the knowledge of which seems never to have been communicated to the world till Maxwell edited Cavendish's electrical researches in 1879. We learn from the manuscripts thus published that Cavendish was his own galvanometer, comparing the intensity of currents by the intensity of the sensations he felt in his wrist and elbows when they passed through his body. The accuracy of his discriminations of the intensity of shocks is truly marvelous, whether we judge by the consistency of his results among themselves or by comparing them with the latest results obtained with a galvanometer, using all the precautions suggested by experience in measuring the resistance of electrolytes. Indeed, such was the reputation of Cavendish for scientific accuracy, that his bare results were accepted at once and readily became a part of general knowledge, although no one conjectured by what method he had obtained them, more than forty years before the invention of the galvanometer, the only instrument by which any one else has ever been able to compare electrical resistances. In carrying out this work Cavendish not only arrived at the result, which Kohlrausch has since shown to be nearly accurate, that for weak solutions the product of the resistance by the percentage of salt is nearly constant, and also stated accurately the laws of multiple and divided currents, but he even anticipated, in January, 1781, the law of electrical resistance, discovered independently by Ohm and published by him in 1827. Moreover, in a very remarkable set of experiments on a series of salts and acids in order to determine their relative electric resistance, Cavendish tells us, 'that the quantity of acid in each should be equivalent to that in a solution of salt in twenty-nine of water,' and it is difficult to account for agreement not only of the ratios, but also for the absolute numbers given by Cavendish with those of the modern system, in which the equivalent weight of hydrogen is taken as unity. They must have been derived from his own work, for Wenzel's 'Lehre von der Verwandschaften' was published in 1777, and also gives values greater than those used by Cavendish, and Richter's 'Anfangsgrunde der Stochyometrie' was not published till 1792, while Cavendish's experiments were made in 1777. It is only by comparing the dates of these researches with the dates of the principal discoveries in chemistry that we become aware that in the incidental mention of these numbers we have the sole record of one of those secret and solitary researches, the value of which to other men of science Cavendish does not seem to have taken into account after he had satisfied his own mind as to the facts. He dealt with his discoveries as with his great wealth, allowing the larger part of them to lie unused.
- ↑ Lavoisier named the gas 'hydrogen,' i. e., water-former.
- ↑ In the 'Life and Works of Cavendish,' by Dr. G. Wilson, published for the Cavendish Society, London, in 1851.
- ↑ This work has recently been edited, together with that of Newton, Bouguer and others, by Professor A. S. Mackenzie in 'The Laws of Gravitation,' Scientific Memoirs, American Book Company.