36 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



an hour and a quarter, until the surface of the molten metal 

 ceased to tarnish and a considerable quantity of a black 1 

 calx had collected. In spite of the calcination that had 

 taken place the sealed retort had only changed in weight 

 by the loss of a quarter of a grain. If, therefore, the 

 metal had gained in weight "it was necessary to look 

 for the cause in the interior of the retort" (Works, 



II. 112). 



The retort was then broken into halves by cracking it 

 with a hot coal, and the whole was weighed again. A 

 comparison with the first weighing of the tin and" retort 

 showed a gain of 3-13 grains; further weighings showed 

 that the tin in the retort had gained 3-12 grains, whilst the 

 retort itself had not changed in weight. Lavoisier was 

 able to calculate that the weight of air which he had 

 sealed up in the retort was 15 \ grains, and therefore 

 concluded that one-fifth of this air had been absorbed by 

 the tin. 



In a second experiment 20 ounces of tin were heated 

 during two and a-half hours in a large retort of 250 cubic 

 inches capacity. After allowing air to enter through a small 

 crack, the retort and its contents were found to be io'o6 

 grains heavier than when they were first weighed : the tin 

 had gained in weight by 10*00 grains or \ to \ of the 

 weight of air in the retort. The agreement between these 

 two figures could not be predicted, as the cracked retort 

 did not contain ordinary air ; but it is referred to below 

 as evidence that the ordinary air entering the retort had 

 almost the same density as the portion which had been 

 absorbed by the tin. 



Lavoisier's figures are given on p. 37. 



Lavoisier concluded : 



" i. That only a limited quantity of tin can be calcined 

 in a given quantity of air. 



1 When tin is heated in an excess of air, a white calx is produced. 



