It was not until Priestley, when dining with him in the autumn of 1774 (being in Paris with Lord Shelburne at the time), had informed Lavoisier of his discovery of “dephlogisticated” air, that the ideas of the latter upon the subject became precise. Priestley’s own words are:--“Having made the discovery some time before I was in Paris, in the year 1774, I mentioned it at the table of Mr. Lavoisier, when most of the philosophical people of the city were present, saying that it was a kind of air in which a candle burned much better than in common air, but I had not then given it any name. At this all the company, and Mr. and Mrs. Lavoisier as much as any, expressed great surprise. I told them I had gotten it from _precipitate per se_, and also from _red-lead_. Speaking French very imperfectly, and being little acquainted with the terms of chemistry, I said _plombe rouge_, which was not understood till Mr. Macquer said I must mean _minium_.” Shortly after this, Lavoisier repeated Priestley’s experiments and confirmed their truth; and this led to the true explanation of experiments of which an account is given in the _Memoirs_ of the French Academy for 1774, and which were fundamental in their character. They referred to the calcination of tin in hermetically-sealed retorts. The tin was placed in a retort which was heated on a sand-bath until the metal had melted. The beak of the retort, previously drawn out into a capillary, was then sealed, the air expelled having been collected and its weight noted. The retort was then cooled and weighed. It was again heated, and the temperature was maintained until the calcination of the tin stopped. With a large retort the calcination was more complete than when a smaller one was employed, this implying that the degree to which the calcination proceeded was dependent upon the amount of air present. After cooling the retort a second time, it was again weighed, when it was found to have undergone no change of weight. The beak was then broken, and air entered with a hissing noise. The gain in weight was now about 10 grains with a large retort. The tin and its calx were next weighed, and it was found that the gain in weight of the tin was always equal to the loss of weight of the air in the retort, measured by the quantity of air which entered on breaking the beak of the retort, less the air driven out of the retort before hermetically sealing it. From this Lavoisier concluded that calx of tin is a compound of tin and air.