“Dr. Darwin will be very cautious who he mentions this affair to, for reasons of delicacy which will have equal weight with us all. I mentioned your generous intention to Dr. P., and that we thought of 20l. each; but that, you will perceive, cannot be, and the Doctor says much less will suffice, as he can go on very well with 100l. per annum.”[298]
Boulton wrote Wedgwood in reply, requesting that the money subscribed should be collected and paid to Dr. Priestley in such a way as not to wound his sensitive feelings. He suggested that in order to avoid this, it might be better if, instead of an annual subscription, a dozen gentlemen were found willing to give a hundred pounds each for the purpose of buying an annuity, or investing the amount in stock for the Doctor’s benefit.
“I have never yet spoken to him on the subject,” he added; “I wish to avoid it, and so doth my neighbour Galton. Therefore I beg you will manage the affair so that we may contribute our mites to so laudable a plan, without the Doctor knowing anything of the matter, and favour us with a line on the subject at your leisure.”[299]
In a subsequent part of the same letter he indicated the subject of Priestley’s experiments at the time:—
“We have long talked,” said he, “of Phlogiston without knowing what we talked about; but now that Dr. Priestley hath brought the matter to light, we can pour that element out of one vessel into another, can take it out of one metal and put it into another, can tell how much of it, by accurate measurement, is necessary to reduce a calx to a metal, which is easily done, and without putting that calx into contact with any visible thing. In short, this goddess of levity can be measured and weighed like other matter. For the rest, I refer you to the Doctor himself.”
The discussions at the Lunar Society were not, however, exclusively chemical, but were varied according to the visitors who from time to time honoured the members with their presence. Thus, in the autumn of 1782, the venerable Smeaton, having occasion to be in Birmingham upon canal business, was invited to attend a meeting of the Society held in Watt’s house at Harper’s Hill. Watt thus described the evening’s proceedings in a letter to Boulton, then in London:—
“He [Smeaton] grows old, and is rather more talkative than he was, but retains in perfection his perspicuity of expression and good sense. He came to the Philosophers’ Meeting at my house on Monday, and we were receiving an account of his experiments on rotatives and some new ones he has made, when unluckily his facts did not agree with Dr. Moyes the blind philosopher’s theories, which made Moyes contradict Smeaton, and brought on a dispute which lost us the information we hoped for, and took away all the pleasure of the meeting, as it lasted two hours without coming half an inch nearer to the point.”[300]
A few days later, we find De Luc paying his first visit to Watt at Birmingham, accompanied by Baron Reden, who desired to inspect the Soho works. “M. De Luc,” wrote Watt, “is a modest ingenious man. On Wednesday, Wilkinson, Reden, and he sent for me to ‘The Castle’ after dinner, and kept me to supper. On the following day De Luc came to breakfast, and spent the whole forenoon, insensing[301] himself with steam and steam-engines. He is making a book, and will mention us in it. Dr. Priestley came also to dinner, and we were all good company till six o’clock, when Wilkinson set off for Broseley, and they for London.”
Meanwhile Priestley continued to pursue his investigations with indefatigable zeal, discovering one gas after another,[302] and immediately proclaiming the facts which he brought to light, so that other minds might be employed on them besides his own. He kept nothing secret. Perhaps, indeed, he was too hasty in publishing the results of experiments still unfinished, as it occasionally led him into contradictions which a more cautious method of procedure would have enabled him to avoid. But he was thoroughly honest, ingenuous, and single-minded in all his proceedings, entertaining the conviction that in the end truth would vindicate itself, and that all that was necessary was to inquire ardently, to experiment incessantly, and to publish freely.
One of the most interesting speculations to which Priestley’s experiments gave rise was the composition of water. The merit of discovering the true theory has been variously attributed to Watt, to Cavendish, and to Lavoisier; and perhaps no scientific question has been the subject of more protracted controversy. It had been known for some years that a certain mixture of inflammable and dephlogisticated air (hydrogen and oxygen), or common air and hydrogen, could be fired by the electric spark. The experiment had been made by Volta and Macquer in 1776–7; and in the spring of 1781 Priestley made what he called a “random experiment” of the same kind, to entertain some philosophical friends. He exploded a mixture of common air and hydrogen in a glass globe by sending an electric spark through it, and when the explosion had taken place it was observed that the sides of the glass were bedewed with moisture. Mr. Warltire, a lecturer on Natural Philosophy at Birmingham,[303] was present at the experiment, and afterwards repeated it in a copper flask for the purpose of trying “whether heat is heavy or not.” In the mean time, Mr. Cavendish, who had for some years been occupied in the special study of pneumatic chemistry, and satisfactorily solved the question of the true composition of atmospheric air, having had his attention directed to Mr. Warltire’s experiment, repeated it in London, in the summer of 1781, employing a glass vessel instead of a copper one; and again the deposit of dew was observed on the sides of the glass. This phenomenon, which Priestley had disregarded, appeared to him to be of considerable importance, and “likely to throw great light” upon the subject of the disappearance of oxygen during combustion, which he had been pursuing experimentally by means of his well-known eudiometer. “The liquid which resulted from the detonations was very carefully analysed, and proved in all the experiments with hydrogen and air, and in some of those with hydrogen and oxygen, to be pure water; but in certain of the latter it contained a sensible quantity of nitric acid. Till the source of this was ascertained, it would have been premature to conclude that hydrogen and oxygen could be turned into pure water.”[304] These experiments, however, were not published, being still regarded as inconclusive. But with the communicativeness which distinguishes the true man of science, Cavendish made them known to Priestley, and, through his friend Dr. Blagden, to Lavoisier. It was not until January, 1784, that he communicated the results of his long series of experiments on the subject to the Royal Society.