36 



their weight remained in the vessel: the conclusion therefore which 

 Cavendish drew was infallible, (being a necessary consequence of the 

 indestruction of matter) — " that when mixed in these proportions and 

 exploded, almost all the inflammable air and about ^ of the com- 

 mon air lose their elasticity, and are condensed into the dew which 

 lines the glass*." 



The subsequent laborious comparisons instituted by the French phi- 

 losophers f between the absolute weight of the gas consumed, and the 

 weight of the fluid produced, added nothing to the certainty of this 

 proof of its composition, nor even to the accuracy of our knowledge of 

 the proportion of its constituent parts ; they took the only means from 

 which in such a method any accuracy can be expected ; operating with 

 immense quantities of gas, at great cost, with much expense of time 

 and toil, and taking all imaginable precautions, they yet never obtained 

 a weight of water exactly equal to that of the gases, though near 

 enough undoubtedly to satisfy those to whom such a mode of proof 

 was more familiar than that devised by Cavendish ; but every chemist 

 knows that the method of volumes, first introduced on this occasion by 

 him, is the proper method of pneumatic determinations. 



Thus far had he advanced in the early part of July 1 781 (MSS.p. 1 13). 

 One more experiment, so contrived as to enable him to consume a large 

 quantity of the gases, sufficed to prove that the fluid condensed was 

 pure water ; and thus, on one of the latter Sundays of that month, 

 (MSS. p. 127, foot note,) the general fact of the composition of water 

 was completely established. 



In August (MSS. p. 120) he examined by the test of similar expe- 

 riments whether there was any difference in the hydrogen furnished by 

 zinc, or iron, and found none ; he contrived also an instrument for mea- 

 suring the force of the explosions (MSS. p. 130), and appears to have 

 used it as a test of the identity and purity of the gases, as well as of 

 their combining proportions, though its indications were not precise 

 enough to induce him to mention it in his paper. In these experi- 

 ments on the force of explosion he employed oxygen as well as atmo- 

 spheric air ; and proceeding in September to collect the fluid produced 

 by the combustion, found on the 28th of that month, 1781, (MSS. p. 

 146,) that the fluid produced by exploding two volumes of hydrogen with 

 one of oxygen contained nitric acid. 



* Phil. Trans. 1784, p. 128. 



t The first proof which Lavoisier gave of the composition of water was the 

 same as that given by Cavendish ; and he deduced the equality of the weights 

 as a consequence or corollary from this proof. 



