g6 ABSOLUTE ZERO OF TEMPERATURE. 



by Davy's hypothesis, the latter was subsequently abandoned 

 after it had done excellent service. 



In the first instance while still using " momentum " Joule 

 was led by this hypothesis to the solution of one of the oldest 

 fundamental questions in science by his discovery of the 

 absolute zero of temperature. Since the pressure of gas in 

 a closed vessel is proportional to the temperature, and as 

 both of these quantities are by Joule's definition of Davy's 

 hypothesis proportional to the heat in the gas, it followed as 

 a simple deduction that the absolute zero of temperature 

 corresponded with the temperature of zero of pressure as 

 indicated on the gaseous thermometer. This value Joule 

 gave in the first statement of his theory, made in the appen- 

 dix referred to, as 480 degrees Fahrenheit below freezing 

 point. And although, on account of a small error in the 

 determination of the co-efficient of expansion of air, on the 

 publication of Regnault's determination of this co-efficient 

 in 1848, this value had to be corrected by Joule from 480 to- 

 493 below freezing point, to Joule belongs the credit of 

 having discovered the existence and accurately determined 

 the position of the absolute zero of temperature. 



Nor was this all Joule learned by means of his hypo- 

 thesis. According to this the specific heat of a body 

 is proportional to the number of atoms in combination 

 divided by the atomic weight. It thus directed his attention 

 to the laws connecting the specific heats of gases and solids,, 

 and the weights and composition of the molecules ; leading 

 him first to study and discuss all the work then done on this 

 subject from Haycraft to Regnault, and then to compare 

 the results with his theory. Which comparison, showing'a 

 remarkable agreement, he read before the British Asso- 

 ciation, September, 1844. 



