In the mean time Watt’s attention had been directed to the same subject by the experiments of Priestley, and he was led to the same conclusions as Cavendish, though altogether independent of him, and by means of a different class of experiments. We find him writing to Boulton, then at Cosgarne, as follows, in 1782:—
“You may remember that I have often said that if water could be heated red hot, or something more, it would probably be converted into some kind of air, because steam would in that case have lost all its latent heat, and that it would have been turned wholly into sensible heat, and probably a total change of the nature of the fluid would ensue. Dr. Priestley has proved this by experiment. He took lime and chased out all the fixed air, and made it exceedingly caustic by long-continued and violent heat. He then added to it two ounces of water, and as expeditiously as possible subjected it again to a strong heat, and he obtained two ounces’ weight of air; and, what is most surprising, a balloon which he interposed between the retort and receiver was not sensibly moistened, nor at all heated that could be observed. The air produced was but very little more than common air, and contained scarce any fixed air. So here is a plain account of where the atmospheric air comes from. The Doctor does me justice as to the theory.”[305]
The results of this experiment were by no means conclusive. That water was composed, at least in part, of air or gas of some kind was obvious; but what the gas was, and whether it existed in combination with other gases, was still a matter of conjecture. But Priestley, having proceeded to repeat Cavendish’s experiment[306] of exploding a mixture of oxygen and hydrogen in a glass vessel, which was followed by the usual deposit of water, communicated the fact to Watt, and this at once put him on the track of the true theory. In a letter to Dr. Black, he communicated the result of Dr. Priestley’s experiments, stating that “when quite dry pure inflammable air (hydrogen) and quite dry pure dephlogisticated air (oxygen) are fired by the electric spark in a close vessel, he finds, after the vessel is cold, a quantity of water adhering to the vessel equal, or very nearly equal, to the weight of the whole air.... Are we not then authorised to conclude, that water is composed of dephlogisticated and inflammable air or phlogiston deprived of part of their latent heat; and that dephlogisticated or pure air is composed of water deprived of its phlogiston and united to heat and light; and if light be only a modification of heat, or a component part of phlogiston, then pure air consists of water deprived of its phlogiston or latent heat?”[307] At the same time Watt wrote to Priestley,—who did not himself see the force of the experiments as establishing the true composition of water,—demonstrating the conclusions which they warranted, and which were identical with those already drawn by Cavendish.
Whether Priestley had communicated to Watt the theory of Cavendish does not appear; but it is probable that both arrived at the same conclusions independently of each other; Cavendish from the result of his own experiments, and Watt from those of Priestley. Each was quite competent to have made the discovery; nor is it necessary for the fame of either to strip a leaf of laurel from the brow of the other. Moreover, we are as unwilling to believe that Cavendish would have knowingly appropriated to himself the idea of Watt, as that Watt would have knowingly appropriated the idea of Cavendish. As it was, however, Cavendish and Watt both claimed priority in the discovery; the advocates of Watt’s claim resting their case mainly on the fact of his having first stated his views on the subject in writing, in a letter which he wrote to Dr. Priestley for the purpose of being read to the Royal Society in April, 1783. Before that letter was read, Watt asked that it should be withheld until the results of some new experiments of Dr. Priestley could be ascertained. These proving delusive, Watt sent a revised edition of the letter to his friend De Luc, in November, but the reading of it was delayed until the 29th April, 1784, before which time, on the 15th January, Cavendish’s paper on the same subject had been communicated to the Society. Watt was much annoyed at the circumstance, and alleged that Cavendish had been guilty of “plagiarism.”[308] At a late period of his life, when all bitter feelings on the subject had subsided, Watt declared himself indifferent to the subject of controversy: “After all,” said he, “it matters little whether Cavendish or I discovered the composition of water; the great thing is, that it is discovered.”
Pneumatic chemistry continued to form the principal subject of discussion at the Lunar Society, as we find from numerous references in Boulton and Watt’s letters. “The Lunar Society,” wrote Watt to his partner, “was held yesterday at Mr. Galton’s at Barr. It was rather dull, there having been no philosophical news lately except Mr. Kirwan’s discovery of an air from phosphorus, which takes fire of itself on being mixed with common or dephlogisticated air.”[309] Among Watt’s numerous scientific correspondents was M. Berthollet, the eminent French chemist, who communicated to him the process he had discovered of bleaching by chlorine. Watt proceeded to test the value of the discovery by experiment, after which he recommended his father-in-law, Mr. Macgregor, of Glasgow, to make trial of it on a larger scale. This, however, was postponed until Watt himself could find time to superintend it in person. At the end of 1787, we find him on a visit to Glasgow for the purpose, and writing Boulton that he is making ready for the trial. “I mean,” he writes, “to try it to-morrow, though I am somewhat afraid to attack so fierce and strong a beast. There is almost no bearing the fumes of it. After all, it does not appear that it will prove a cheap way of bleaching, and it weakens the goods more than could be wished, whatever good it may do in the way of expedition.”[310] The experiment succeeded, and we find Mr. Macgregor, in the following February, “engaged in whitening 1500 yards of linen by the process.” The discovery, not being protected by a patent, was immediately made use of by other firms; but the offensive odour of the chlorine was found exceedingly objectionable, until it was discovered that chlorine could be absorbed by slaked lime, the solution of which possessed great bleaching power, and this process in course of time superseded all the old methods of bleaching by chlorine.
It has been recently surmised that the action of light upon nitrate of silver formed the subject of discussion at the Lunar Society, and of experiments by Boulton and Watt; but we find no indications of this in their correspondence. They were so unreserved with each other on all matters of business as well as science that, had any phenomena of so remarkable a character as those which have issued in the art of photography become known to either Boulton or Watt, we feel confident that they must have formed the subject of much personal discussion, and of many written communications. But both correspondents are alike entirely silent on the subject; and we infer that no such experiments were made by them, or, if made, that they led to no results![311]
Among the many foreigners who were attracted by this distinguished circle of scientific men, we find M. Faujas-Saint-Fond, who visited Birmingham in the course of his tour in England in 1785, while the circle was as yet unbroken, and Watt, Boulton, Priestley, and the rest, were in the full tide of business, invention, and inquiry. Saint-Fond had the pleasure of dining one day with Watt when Dr. Priestley was present, and describes in glowing terms the interest of their conversation. “Watt,” he says, “joins to the frankness of a Scotchman the amiability and kindness of a man of the world. Surrounded by charming children, well educated and full of talent, he enjoys in their midst the happiness of regarding them as his friends, while he is almost worshipped by them as the best of fathers.” A subsequent visit which he paid to Dr. Priestley in company with Dr. Withering, leads him to describe the philosopher’s house at Fairhill, then about a mile and a half from Birmingham. “It is,” he says, “a charming residence, with a fine meadow on one side, and a beautiful garden on the other. There was an air of perfect neatness about the place within and without.” He describes the Doctor’s laboratory, in which he conducted his experiments, as “situated at the extremity of a court, and detached from the house to avoid the danger of fire.”
“It consists of several apartments on the ground floor. On entering it, I was struck with the sight of a simple and ingenious apparatus for making experiments on inflammable gas extracted from iron and water reduced to vapour. It consisted of a tube, tolerably long and thick, made out of one piece of copper to avoid soldering. The part exposed to the fire was thicker than the rest. He introduced into the tube cuttings or filings of iron, and instead of letting the water fall into it drop by drop, he preferred introducing it as vapour. The furnace was fired by coke instead of coal, this being the best of combustibles for intensity and equality of heat.... Dr. Priestley kindly allowed me to make a drawing of his apparatus for the purpose of communicating it to the French chemists who are engaged in the same investigations as himself.... The Doctor has embellished his rural retreat with a philosophical cabinet, containing all the instruments necessary for his scientific labours; as well as a library, containing a store of the most valuable books. He employs his time in a variety of studies. History, moral philosophy, and religion, occupy his attention by turns. An active, intelligent mind, and a natural avidity for knowledge, draw him towards the physical sciences; but a soft and impressible heart again leads him to religious and philanthropic inquiries.... I had indeed the greatest pleasure in seeing this amiable savant in the midst of his books, his furnaces, and his philosophical instruments; at his side an educated wife, a lovely daughter, and in a charming residence, where everything bespoke industry, peace, and happiness.”[312]
Only a few years after the date of this visit, while Priestley was still busied with his chemical investigations, his house at Fairhill, thus described by Saint-Fond, was invaded by a brutal mob, who ruthlessly destroyed his library, his apparatus, and his furniture, and forced him to fly from Birmingham, glad to escape with his life.
The Lunar Society continued to exist for some years longer. But one by one the members dropped off. Dr. Priestley emigrated to America; Dr. Withering, Josiah Wedgwood, and Dr. Darwin, died before the close of the century; and, without them, a meeting of the Lunar Society was no longer what it used to be. Instead of an assembly of active, inquiring men, it was more like a meeting of spectres with a Death’s head in the chair. The associations connected with the meeting—reminding the few lingering survivors of the losses of friends—became of too painful a character to be kept alive; and the Lunar Society, like the members of which it was composed, gradually expired. Its spirit, however, did not die. The Society had stimulated inquiry, and quickened the zeal for knowledge of all who had come within the reach of its influence; and this spirit diffused and propagated itself in all directions. Leonard Horner, who visited Soho in 1809, thus referred to the continued moral influence of the association:—“The remnant of the Lunar Society,” he says, “and the fresh remembrance in others of the remarkable men who composed it, are very interesting. The impression which they made is not yet worn out, but shows itself, to the second and third generation, in a spirit of scientific curiosity and free inquiry, which even yet makes some stand against the combined forces of Methodism, Toryism, and the love of gain.”[313]