448 FRAGMENTS OF SCIENCE 



rent, 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 lbs. of oxygen and 1 lb. 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 lbs. of water, indicating a mechanical force, 

 expended in the combination, equal to a weight of about 

 60,000,000 lbs. 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 mechanical 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 discrep- 

 ancies 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 expan- 

 sion: first, against a pressure, and therefore performing 

 work; secondly, against no pressure, and therefore per- 

 forming 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 differ- 

 ent mechanical equivalents; the agreement between them 

 being far greater than that attained in his first experi- 



