RELATION OF VAPOR AND AIR. 173 



DV the growing importance of tlic steam-engine, in which those law? 

 operated in a practical form. James Watts, the main improver of 

 that machine, was thus a great contributor to speculative knowledge, 

 as well as to practical power. Many of his improvements depended 

 on the laws which regulate the quantity of heat which goes to the 

 formation or condensation of steam ; and the observations which led 

 to these improvements enter into the induction of latent heat. Mea- 

 surements of the force of steam, at all temperatures, were made with 

 the same view. Watts's attention had been drawn to the steam-engine 

 in 1759, by Robison, the former being then an instrument-maker, and 

 the latter a student at the University of Glasgow. 16 In 1 TCI or 1702, 

 he tried some experiments on the force of steam in a Papin's Diges- 

 ter ; 17 and formed a sort of working model of a steam-engine, feeling 

 already his vocation to develops the powers of that invention. His 

 knowledge was at that time principally derived from Desaguliers and 

 Beliclor, but his own experiments added to it rapidly. In 1764 and 

 17G5, he made a more systematical course of experiments, directed to 

 ascertain the force of steam. He tried 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 expe- 

 riments of Lord Charles Cavendish, and some others of Mr. Xairne, to 

 examine the same subject. He made out a table of the correspondence 

 of the elasticity and the temperature of vapor, from thirty-two to two 

 hundred and eighty degrees of Fahrenheit's thermometer. 18 The 

 thing here to be remarked, is the establishment of a law of the pres- 

 sure of steam, down to the freezing-point of water. Ziegler of Basle, 

 in 17G9, and Achard of Berlin, in 1782, made similar experiments. 

 The latter examined also the elasticity of the vapor of alcohol. Be- 

 tancourt, in 1792, published his Memoir on the expansive force of 

 vapors; and his tables were for some time considered the most exact. 



18 Eobison's Works, vol. ii. p. 113. 



1T Denis Papln, who made many of Boyle's experiments for him, had disco- 

 vered that if the vapor be prevented from rising, the water becomes hotter than 

 the usual boiling-poinl ; and had hence invented the instrument called Papin's 

 Digester. It is described in his book, La, manitre cTamolir Ics os et defair* 

 euire toules sorts de viandcs erifort peu de temps et a peu defrais. Paris, 1682. 



18 These were afterwards published in the Encyclopedia Britannica; in the 

 article " Steam," written by Robkon. 



