S4. 



Pi'ecisely the same difficulties had been previously felt and remarked 

 by Priestley*, and he had sought for a partial solution of them in the 

 precipitation of carbonic acid which had occurred in some of these 

 processes, and its absorption by the liquid over which the diminished 

 air was confined. This was the prevailing opinion of the cause of the 

 diminution ; Bergman, Kirwan, and other eminent chemists adopted it, 

 paying little attention to the new views of Lavoisier, who already in 

 1776 had found the key by which these phsenomena were ultimately 

 to be interpreted, and had succeeded in deciphering many of them : 

 but Lavoisier had at the same date deduced from too narrow an in- 

 duction the theory which so long continued to prejudice the chemists 

 of his school, that oxygen is the principle of acidity, and was unprepared 

 in consequence to conceive that it could be one of the constituents of 

 waterf . 



In this state of the subject Cavendish took up those parts of the 

 problem which Lavoisier had not already sufficiently solved : he began, 

 in 1778, by ascertaining that when nitrous gas is mixed with oxygen or 

 common air, no carbonic or vitriolic acid is produced, but, as Lavoisier 

 had said, nitric acid alone ; and he then commenced those eudiometrical 

 researches which were the foundation of all his subsequent discoveries. 



In 1779 he appears to have intermitted his experiments ; but he re- 

 sumed them in 1780, and obtained in this year so complete a mastery 

 over the methods of analysing atmospheric air, as to have determined 

 the proportion of oxygen in it to be l^, at a time when Scheele and 

 Lavoisier supposed it to be ^, and Priestley nearly as much. By the 

 same means he had acquired also the power of detecting the small- 

 est adulteration of oxygen gas, and the amount of the impurity ; and 

 he came therefore, in July 1781, to the question what becomes of the 



* In 1774 Priestley writes, " In what manner air is diminished by phlo- 

 giston, independent of the precipitation of any of its constituent parts, is not 

 easy to conceive ; unless air thus diminished be heavier than air not dimi- 

 nished, which I do not find to be the case. It deserves, however, to be tried 

 with more attention. That phlogiston should communicate absolute levity to 

 the bodies with which it is combined is a supposition that I am not willing to 

 have recourse to, though it would afford an easy solution of the difficulty." 



f " L'analogie, dit il (Lavoisier, Mem. de I'Academie 1781, page 471), 

 ni'avoit parte invinciblement a condure que la combustion de fair inflammable 

 devoit egalement produire un acide. Mais M. Lavoisier a senti que toutes les 

 analogies doivent disparoitre devant des faits positives ; et qu'il y a, entre les 

 analogies les plus fortes et les faits, la difference qui se trouve entre les pro- 

 babilites et la certitude." — Consid. sur les Exper. de M. Priestley, par M. 

 BerthoUet (Annales de Chimie, torn. 3, 1789). 



