He was shy and bashful to a degree bordering on disease; he could not bear to have any person introduced to him, or to be pointed out in any way as a remarkable man. One Sunday evening he was standing at Sir Joseph Banks’s in a crowded room, conversing with Mr. Hatchett, when Dr. Ingenhousz, who had a good deal of pomposity of manner, came up with an Austrian gentleman in his hand, and introduced him formally to Mr. Cavendish. He mentioned the titles and qualifications of his friend at great length, and said that he had been peculiarly anxious to be introduced to a philosopher so profound and so universally known and celebrated as Mr. Cavendish. As soon as Dr. Ingenhousz had finished, the Austrian gentleman began, and assured Mr. Cavendish that his principal reason for coming to London was to see and converse with one of the greatest ornaments of the age, and one of the most illustrious philosophers that ever existed. To all these high-flown speeches Mr. Cavendish answered not a word, but stood with his eyes cast down quite abashed and confounded. At last, spying an opening in the crowd, he darted through it with all the speed of which he was master; nor did he stop till he reached his carriage, which drove him directly home.
Of a man, whose habits were so retired, and whose intercourse with society was so small, there is nothing else to relate except his scientific labours: the current of his life passed on with the utmost regularity; the description of a single day would convey a correct idea of his whole existence. At one time he was in the habit of keeping an individual to assist him in his experiments. This place was for some time filled by Sir Charles Blagden; but they did not agree well together, and after some time Sir Charles left him. Mr. Cavendish died on the 4th of February, 1810, aged seventy-eight years, four months, and six days. When he found himself dying, he gave directions to his servant to leave him alone, and not to return till a certain time which he specified, and by which period he expected to be no longer alive. The servant, however, who was aware of the state of his master, and was anxious about him, opened the door of the room before the time specified, and approached the bed to take a look at the dying man. Mr. Cavendish, who was still sensible, was offended at the intrusion, and ordered him out of the room with a voice of displeasure, commanding him not by any means to return till the time specified. When he did come back at that time, he found his master dead. What a contrast between the characters of Mr. Cavendish and Dr. Black!
The appearance of Mr. Cavendish did not much prepossess strangers in his favour; he was somewhat above the middle size, his body rather thick, and his neck rather short. He stuttered a little in his speech, which gave him an air of awkwardness: his countenance was not strongly marked, so as to indicate the profound abilities which he possessed. This was probably owing to the total absence of all the violent passions. His education seems to have been very complete; he was an excellent mathematician, a profound electrician, and a most acute and ingenious chemist. He never ventured to give an opinion on any subject, unless he had studied it to the bottom. He appeared before the world first as a chemist, and afterwards as an electrician. The whole of his literary labours consist of eighteen papers, published in the Philosophical Transactions, which, though they occupy only a few pages, are full of the most important discoveries and the most profound investigations. Of these papers, there are ten which treat of chemical subjects, two treat of electricity, two of meteorology, three are connected with astronomy, and there is one, the last which he wrote, which gives his method of dividing astronomical instruments. Of the papers in question, those alone which treat of Chemistry can be analyzed in a work like this.
1. His first paper, entitled, “Experiments on fictitious Air,” was published in the year 1766, when Mr. Cavendish was thirty-five years of age. Dr. Hales had demonstrated (as had previously been done by Van Helmont and Glauber) that air is given out by a vast number of bodies in peculiar circumstances. But he never suspected that any of the airs which he obtained differed from common air. Indeed common air had always been considered as an elementary substance to which every elastic fluid was referred. Dr. Black had shown that the mild alkalies and limestone, and carbonate of magnesia, were combinations of these bodies with a gaseous substance, to which he had given the name of fixed air; and he had pointed out various methods of collecting this fixed air; though he himself had not made much progress in investigating its properties. This paper of Mr. Cavendish may be considered as a continuation of the investigations begun by Dr. Black. He shows that there exist two species of air quite different in their properties from common air: and he calls them inflammable air and fixed air.
Inflammable air (hydrogen gas) is evolved when iron, zinc, or tin, are dissolved in dilute sulphuric or muriatic acid. Iron yielded about 1-22d part of its weight, of inflammable air, zinc about 1-23d or 1-24th of its weight, and tin about 1-44th of its weight. The properties of the inflammable air were the same, whichever of the three metals was used to procure it, and whether they were dissolved in sulphuric or muriatic acids. When the sulphuric acid was concentrated, iron and zinc dissolved in it with difficulty and only by the assistance of heat. The air given out was not inflammable, but consisted of sulphurous acid. These facts induced Mr. Cavendish to conclude that the inflammable air evolved in the first case was the unaltered phlogiston of the metals, while the sulphurous acid evolved in the second case, was a compound of the same phlogiston and a portion of the acid, which deprived it of its inflammability. This opinion was very different from that of Stahl, who considered combustible bodies as compounds of phlogiston with acids or calces.
Cavendish found the specific gravity of his inflammable air about eleven times less than that of common air. This determination is under the truth; but the error is, at least in part, owing to the quantity of water held in solution by the air, and which, as Mr. Cavendish showed, amounted to about 1-9th of the weight of the air. He tried the combustibility of the inflammable air, when mixed with various proportions of common air, and found that it exploded with the greatest violence when mixed with rather more than its bulk of common air.
Copper he found, when dissolved in muriatic acid by the assistance of heat, yielded no inflammable air, but an air which lost its elasticity when it came in contact with water. This air, the nature of which Mr. Cavendish did not examine, was muriatic acid gas, the properties of which were afterwards investigated by Dr. Priestley.
The fixed air (carbonic acid gas) on which Mr. Cavendish made his experiments was obtained by dissolving marble in muriatic acid. He found that it might be kept over mercury for any length of time without undergoing any alteration; that it was gradually absorbed by cold water; and that 100 measures of water of the temperature 55° absorbed 103·8 measures of fixed air. The whole of the air thus absorbed was separated again by exposing the water to a boiling heat, or by leaving it for sometime in an open vessel. Alcohol (the specific gravity not mentioned) absorbed 2¼ times its bulk of this air, and olive-oil about 1-3d of its bulk.
The specific gravity of fixed air he found 1·57, that of common air being 1.[186] Fixed air is incapable of supporting combustion, and common air, when mixed with it, supports combustion a much shorter time than when pure. A small wax taper burnt eighty seconds in a receiver which held 180 ounce measures, when filled with common air only. The same taper burnt fifty-one seconds in the same receiver when filled with a mixture of one volume fixed air, and nineteen volumes of common air. When the fixed air was 3-40ths of the whole volume the taper burnt twenty-three seconds. When the fixed air was 1-10th, the taper burnt eleven seconds. When it was 6-55ths or 1-9·16 of the whole mixture, the taper would not burn at all.
Mr. Cavendish was of opinion that more than one kind of fixed air was given out by marble; in other words, that the elastic fluid emitted, consisted of two different airs, one more absorbable by water than the other. He drew his conclusion from the circumstance that after a solution of potash had been exposed to a quantity of fixed air for some time, it ceased to absorb any more; yet, if the residual portion of air were thrown away and new fixed air substituted in its place, it began to absorb again; but Mr. Dalton has since given a satisfactory explanation of this seeming anomaly by showing that the absorbability of fixed air in water is proportional to its purity, and that when mixed with a great quantity of common air or any other gas not soluble in water, it ceases to be sensibly absorbed.