T&E COPLEY MEDALIST OF 1871. 333 



The italics in this memorable passage, written, it is to be 

 remembered, in 1843, are Mr. Joule's own, 



The concluding paragraph of this British association 

 paper equally illustrates his insight and precision, regarding 

 the nature of chemical and latent heat. " I had," he 

 writes, "endeavored to prove that when two atoms com- 

 bine together, the heat evolved is exactly that which would 

 have been evolved by the electrical current due to the 

 chemical action taking place, and is therefore proportional 

 to the intensity of the chemical force causing the atoms to 

 combine. I now venture to state more explicitly, that it is 

 not precisely the attraction of affinity, but rather the me- 

 chanical force expended by the atoms in falling toward one 

 another, which determines the intensity of the current, 

 and, consequently, the quantity of heat evolved; so that we 

 have a simple hypothesis by which we may explain why 

 heat is evolved so freely in the combination of gases, and 

 by which indeed we may account ' latent heat 7 as a me- 

 chanical power, prepared for action, as a watch-spring is 

 when wound up. Suppose, for the sake of illustration, that 

 8 Ibs. of oxygen and 1 Ib. of hydrogen were presented to 

 one another in the gaseous state, and then exploded; the 

 heat evolved would be about 1 degree Fahr. in 60,000 Ibs. 

 of water, indicating a mechanical force, expended in the 

 combination, equal to a weight of about 50,000,000 Ibs. 

 raised to the height of one foot. Now if the oxygen and 

 hydrogen could be presented to each other in a liquid state, 

 the heat of combination would be less than before, because 

 the atoms in combining would fall through less space/' No 

 words of mine are needed to point out the commanding 

 grasp of molecular physics, in their relation to the mechan- 

 ical theory of heat, implied by this statement. 



Perfectly assured of the importance of the principle 

 which his experiments aimed at establishing, Mr. Joule did 

 not rest content with results presenting such discrepancies 

 as those above referred to. He resorted in 1844 to entirely 

 new methods, and made elaborate experiments on the 

 thermal changes produced in air during its expansion: 

 firstly against a pressure, and therefore performing work; 

 secondly, against no pressure, and therefore performing no 

 work. He thus established anew the relation between the 

 heat consumed and the work done. From five different 

 series of experiments he deduced five different mechanical 



