mct. ii] DISSERTATION SECOND. 81 



actually escapes, being present in the regulus, but not in 

 the calx of the metal. This substance, to which they gave 

 the name of phlogiston, was probably that which, by its 

 escape, rendered the metal heavier, and must, therefore, 

 be itself endued with absolute levity. 



The in.stantia cruris which extricated philosophers from 

 this difficulty, was furnished by an experiment of the cele- 

 brated Lavoisier. That excellent chemist included a 

 quantity of tin in a glass retort, hermetically sealed, and ac- 

 curately weighed together with its contents ; he then ap- 

 plied the necessary heat, and when the calcination of the 

 tin was finished, he found the weight of the whole precise- 

 ly the same as before. This proved, that no substance, 

 which was either light or heavy, in a sensible degree, had 

 made its way through the glass. The experiment went 

 still farther. When the retort was cooled and opened, the 

 air rushed in, so that it was evident that a part of the air 

 had^disappeared, or had lost its elasticity. On weighing 

 the whole apparatus, it was now found that its weight was 

 increased by ten grains ; so that ten grains of air had enter- 

 ed into the retort when it was opened. The calx was 

 next taken out, and weighed separately, and it was found 

 to have become heavier by ten grains precisely. The ten 

 grains of air then which had disappeared, and which had 

 made way for the ten grains that rushed into the retort, 

 had combined with the metal during the process of cal- 

 cination. The farther prosecution of this very decisive 

 experiment led to the knowledge of that species of air 

 which combines with metals when they are calcined. 

 The doctrine of phlogiston was of course exploded and 

 a creature of the imagination replaced by a real exist- 

 ence. 



The principle which conducts to the contrivance of an 



experimentum cruris is not difficult to be understood. 



