CHAPTER VII. 



HEAT WHAT IT IS THEORY OF HEAT THE THERMOMETER EXPAN- 

 SION BY HEAT EBULLITION AND DISTILLATION LATENT HEAT 



SPECIFIC HEAT. 



WHAT is Heat ? We will consider this question, and endeavour to- 

 explain it before we speak of its effects on water and other matter. 



Heat is now believed to be the effects of the rapid motion of all the 

 particles of a body. It is quite certain that a heated body is no heavier 

 than the same body before it was made " hot," so the heat could not have 

 gone into it, nor does the " heat " leave it when it has become what we call 

 " cold," which is a relative term. Heat is therefore believed to be a 

 vibratory motion, or the effects of very rapid motion of matter. 



The Science of Heat, as we may term it, is only in its infancy, or 

 certainly has scarcely come of age. Formerly heat was considered a 

 chemical agent, and was termed caloric, but now heat is found to be 

 motion, which affects our nerves of feeling and sight ; and, as Professor 

 Stewart tells us, " a heated body gives a series of blows to the medium 

 around it ; and although these blows do not affect the ear, they affect the 

 eye, and give us a sense of light." 



Although^it is only within a comparatively few years that heat has 

 been really looked upon as other than matter, many ancient philosophers- 

 regarded it as merely a quality of matter. They thought it the active 

 principle of the universe. Epicurus declared that heat was an effluxion of 

 minute spherical particles possessing rapid motion, and Lucretius maintained 

 that the sun's light and heat are the result of motion of primary particles. 

 Fire was worshipped as the active agent of the universe, and Prometheus- 

 was fabled to have stolen fire from heaven to vivify mankind. The views 

 of the ancients were more or less adopted in the Middle Ages ; but John 

 Locke recognized the theory of heat being a motion of matter. He says : 

 " What in our sensation is heat, in the object is nothing but motion" 



Gradually two theories arose concerning heat ; ont, the Material 

 theory the theory of Caloric or Phlogiston ; the other, the Kinetic theory. 

 Before the beginning of the present century the former theory was generally 

 accepted, and the development of heat was accounted for by asserting that 

 the friction or percussion altered the capacity for heat of the substances 

 acted upon. The heat was squeezed out by the hammer, and the amount 



