RELATION OF VAPOUR AND AIR. .~>7o 



this force, however, only at temperatures above the 

 boiling-point; and inferred it at lower degrees from 

 the supposed continuity of the law thus obtained. 

 His friend Robison, also, was soon after led, by 

 reading the account of some experiments of Lord 

 Charles Cavendish, and some others of Mr. Nairne, 

 to examine the same subject. He made out a table 

 of the correspondence of the elasticity and the tem- 

 perature of vapour, from thirty-two to two hundred 

 and eighty degrees of Fahrenheit's thermometer 16 . 

 The thing here to be remarked, is the establishment 

 of a law of the pressure of steam, down to the 

 freezing-point of water. Ziegler of Basle, in 1769, 

 and Achard of Berlin, in 1782, made similar expe- 

 riments. The latter examined also the elasticity 

 of the vapour of alcohol. Betancourt, in 1792, 

 published his Memoir on the expansive force of 

 vapours ; and his tables were for some time consi- 

 dered the most exact. Prony in his Architecture 

 Hydraulique (1796), established a mathematical 

 formula 17 , on the experiments of Betancourt, who 

 began his researches in the belief that he was first 

 in the field, although he afterwards found that he 

 had been anticipated by Ziegler. Gren compared 

 the experiments of Betancourt and De Luc with 

 his own. He ascertained an important fact, that 

 when water boils, the elasticity of the steam is 



6 These were afterwards published in the Encyclopaedia 

 Britannica ; in the article " Steam" written by Robison. 

 17 Architecture Hydraulique, Seconde Partie, p. 163. 



