“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. Keir to Priestley. “The more we discover of Nature, the further we are removed from the conceit of our being able to understand the operations.