Manchester Memoirs^ Vol. xliii. ( 1 899), No. 4. 3 



tion of the small parts of bodies, wherein the nature of 

 heat seems chiefly, if not solely, to consist." 



In the year 1841, a great advance was made by Mayer 

 and by Joule. Mayer regarded the heat evolved by the 

 compression of a gas as equivalent to the work necessary 

 to compress it. At the same time, he indicated a method 

 by which the work equivalent to the heat could be 

 calculated. And Joule determined with great accuracy 

 the amount of heat produced by friction caused by a 

 known quantity of work ; a quantity now known as the 

 " mechanical equivalent of heat." 



These considerations led to a mechanical conception 

 of heat in its relation to gases. The gases were treated as 

 if they were composed of small particles in motion. The 

 pressure of the gas is caused by the impact of these small 

 particles, or molecules, on the walls of the containing 

 vessel ; and the motion is believed to be more rapid when 

 the temperature is raised, according to the kinetic theory 

 of gases. This conception of heat, as due to the motion of 

 the molecules, was developed by Joule, Rankine, Clerk- 

 Maxwell, Sir William Thomson (Lord Kelvin), and by 

 the German physicist Clausius. 



Now, when heat is communicated to a gas, and the 

 gas is confined in a tight vessel, so that its volume cannot 

 alter, the pressure on the walls of the vessel rises. As the 

 number of molecules of the gas undergoes no alteration, 

 this increased pressure must be due to the increased rate 

 of motion of the molecules. The increase of pressure can 

 be measured ; and it corresponds to an increased power 

 of performing work. The gas, in other words, has gained 

 energy ; and it is possible to calculate, from the known 

 rise of pressure, the amount of energy gained by a known 

 quantity of gas, which has been raised through a known 

 temperature (say I'^'C), and which has acquired a certain 

 amount of heat thus to raise its temperature. If we take 



