322 DISCOURSE ON THE STUDY 



the greater compressibility of liquids, and the still 

 greater of gases, strongly induce us to believe that 

 it is heat, and heat alone, which holds the particles 

 of all bodies at that distance from each other which 

 is necessary to allow of compression ; which in fact 

 gives them their elasticity, and acts as the anta- 

 gonist force to their mutual attraction, which would 

 otherwise draw them into actual contact, and retain 

 them in a state of absolute immobility and impene- 

 trability. Thus we learn to regard heat as one of 

 the great maintaining powers of the universe, and to 

 attach to all its laws and relations a degree of im- 

 portance which may justly entitle them to the most 

 assiduous enquiry. 



(360.) It was first ascertained by Dr. Black that 

 when heat produces the liquefaction of a solid, or 

 the conversion of a liquid into vapour, the liquid or 

 the vapour resulting is no hotter than the solid or 

 liquid from which it was produced, though a great 

 deal of heat has been expended in producing this 

 effect, and has actually entered into the substance. 



(361.) Hence he drew the conclusion that it has 

 become latent, and continues to exist in the product, 

 maintaining it in its new state, without increasing 

 its temperature. He further proved, that when the 

 vapour condenses, or the liquid freezes, this latent 

 heat is again given out from it. This great dis- 

 covery, with its natural and hardly less important 

 concomitant, that of the difference of specific heats 

 in different bodies, or the different quantities of heat 

 they require to raise their temperature equally, are 

 the chief reasons for regarding heat as a material 

 substance in a more decided manner than light, 



