Manchester Memoirs, Vol. Ixv. (1921), No. 3 5 



ture ; or, the absolute values of two temperatures are to one 

 another in proportion of the heat taken in to the heat 

 rejected in a perfect thermodynamic engine, working with 

 a source and refrigerator at the higher and lower tempera- 

 tures respectively." 



This definition led to an absolute scale of temperature 

 which was independent of the substance operated on, and the 

 scale practically coincided with the ordinary air thermometer 

 scale with small deviations due to certain properties of air 

 which were jointly investigated by Joule and Thomson in 

 later years. Thomson, by this method, also determined the 

 absolute zero of temperature as — 273°C, and he justified by 

 an accurate deduction Joule's suggestion made in a letter 

 written to Thomson in 1848, that the probable value of 

 Carnot's function is the reciprocal of the absolute temperature 

 as measured on a perfect gas thermometer. 



I have taken you through the various steps which the 

 powerful mind of Sir William Thomson had to< pass before 

 he gave up the material theory and accepted Joule's dyna- 

 mical theory with its various deductions, in order to show 

 how great was the genius of Joule in proving the truth step 

 by step by elaborate experiments continued from 1840 to 

 1849, and how difficult it was even for men of great ability to 

 shake off the errors of the old and accept the truth of the new 

 ideas. If a man of Thomson's great powers of intellect found 

 it difficult to accept Joule's work as late as 1848, how little 

 likely was it that the somewhat vague ideas of Davy and 

 Rumford would ever establish the definite ideas of to-day for 

 which we are indebted mainly to Joule, and also to Carnot, 

 to Macquorn Rankine, Thomson, Clausius, and, to a much 

 smaller extent, to Mayer. 



Sir William Thomson recognised this fully in an article 

 on ■" Heat," written by him and published in the " Encyclo- 

 paedia Britannica " of 1880, in which he says : — 



" Joule's great experiments from 1840 to 1849, creating 

 new provinces of science in the thermodynamics of elec- 

 tricity and magnetism, and electro-chemistry, recalled 

 attention to Davy and Rumford's doctrine regarding the 

 nature of heat, and supplied several fresh proofs each, like 

 Davy's, absolutely in itself complete and cogent, that heat 

 is not a material substance, and each advancing with exact 

 dynamical measurement on the way pointed out by Rum- 

 ford in his measurements of the quantitv of heat generated 



