Priestley’s influence and position in the Society may be inferred from the circumstance that almost immediately after he joined it Pneumatic Chemistry became one of the chief topics of discussion. This is amply demonstrated in the correspondence of its various members, which has been preserved to us in the biographies of Watt, Wedgwood and others, and in the scientific letters of Priestley, which have been collected and edited by Mr H. Carrington Bolton. One direct outcome of this interest is seen in Watt’s connection with the History of the Discovery of the Composition of Water. It is reasonably certain that if Watt and Priestley had not foregathered round the festive board of the Lunar Society, Watt would not have been stimulated to theorise on the meaning and true significance of Priestley’s experiments, and as to their bearing upon the fact that Priestley’s dephlogisticated air (oxygen) and inflammable air (hydrogen) enter into the composition of water. Watt’s claim to be considered as the discoverer of the composition of water rests upon his interpretation of the experimental phenomena made known to him by Priestley shortly after his arrival in Birmingham. The Water Controversy—a controversy which keenly excited the entire scientific world a generation or so ago—may be said to have arisen from the accident of Priestley’s removal to Birmingham and to his association with the Lunar Society.
Priestley’s connection with the Society influenced the progress of chemistry in this country both directly and indirectly. As already stated, he himself was greatly stimulated to accumulate chemical facts by his association with men like Boulton, Watt, Wedgwood, Keir, Darwin, who loved knowledge for its own sake, but who were at the same time quite alive to the material benefits which they and their fellow-men might derive from the pursuit of scientific inquiry. The measure of their interest may be gauged by the extent of their support, and by the readiness with which they furnished Priestley with the means to carry on his investigations. Priestley not only freely communicated to them the results of his labours, but he incidentally fixed their attention on a class of phenomena which, more than any other, were calculated to afford an insight into the real nature of chemical change, and to lead to a rational explanation of chemical phenomena.
Priestley was not consciously a casuist, but there can be no question that the interpretation which his active and ingenious mind occasionally led him to place upon his work not only served to blind himself, but was the means of obscuring the truth for a time from others. We have only to read the correspondence, already more than once alluded to, to find ample proof that such was the case. In a letter to Wedgwood, of March 30, 1781, Boulton writes:—
“We have long talked 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 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.”
In the following year (March 21, 1782) we find Priestley also writing to Wedgwood:—
“Before my late experiments, phlogiston was indeed almost given up by the Lunar Society, but now it seems to be re-established.”
How difficult it was to convince Priestley may be seen from the following extract from a letter to his friend Franklin, who was then in Paris, written at about the same time:—
“Birmingham, June 24, 1782.
“Please to inform the Duc de Rochefoucauld, whose civilities to me I remember with pleasure, that my experiments are certainly inconsistent with Mr Lavoisier’s supposition of there being no such thing as phlogiston, and that it is the addition of air, and not the loss of anything, that converts a metal into a calx. In their usual state calces of metals do not contain air, but that may be expelled by heat, and after this I reduce them to a perfect metallic state by nothing but inflammable air, which they imbibe in toto, without any decomposition. I lately reduced 101 ounce measures of this air to two by calx of lead, and that small remainder was still inflammable. I explain Mr Lavoisier’s experiments by supposing that precipitate per se [mercuric oxide] contains all the phlogiston of the metal mercury, but in a different state; but I can show other calces which also contain more phlogiston than the metals themselves. That mercury in its metallic state does contain phlogiston or inflammable air is evident from the production of nitrous air by the solution of it in spirits of nitre, and I make nitrous air from nothing but nitrous vapour and inflammable air; so that it indisputably consists of these two ingredients. I have already ascertained the proportion of inflammable air that enters into the composition of lead, tin, copper and silver, and am proceeding with the other metals as fast as I can. When the whole is completed I shall give you a further account of it.
“I am exceedingly concerned to find that it is so difficult a thing to make peace; but I hope before the campaign is over all parties will have had enough of war, and be sensible that they will gain nothing by continuing it. If I had any voice in the business, the prospect of seeing you in this country would be a strong additional motive to accelerate the negotiations.
“With the greatest respect and every good wish.—I am, dear sir, yours sincerely, J. Priestley.”
There were already many indications prior to 1780 that men were beginning to be troubled as to the sufficiency of Stahl’s generalisation to account for the rapidly-accumulating mass of facts which the application of quantitative chemistry to the study of natural phenomena was bringing to light. Priestley’s advent in Birmingham certainly retarded by the weight of his authority the growth in heterodoxy in that particular among the members of the Lunar Society, and indirectly therefore all whom they could influence.
The following letter from Keir is typical of many which passed between the members of the Society in reference to Priestley’s work and of the discussions which it occasioned.