478 FRAGMENTS OF SCIENCE. 



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 " as 

 a mechanical 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 

 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 combina- 

 tion 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 com- 

 manding grasp of molecular physics, in their relation 

 to the mechanical theory of heat, implied by this state- 

 ment. 



Perfectly assured of the importance of the principle 

 which his experiments aimed at establishing, Mr. Joule 

 did not rest content with results presenting such discre- 

 pancies as those above referred to. He resorted in 1 844 

 to entirely new methods, and made elaborate experi- 

 ments 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 equivalents ; the 

 agreement between them being far greater than that 



