HEAT. 



327 



The principal properties of heat are so nearly identical with those of light, 

 that the supposition that heat is obscure light is countenanced by strong proba- 

 bilities. Heat proceeds in straight lines from the point whence it emanates, 

 diverging in every direction. These lines are called rays of heat, and the 

 process is called radiation. Heat radiates through certain bodies which are 

 transparent to it, as glass is to light. It passes freely through air or gas ; it 

 also passes through a vacuum, and therefore its propagation by radiation does 

 not depend on the presence of matter. Indeed, the great velocity with which 

 it is propagated by radiation proves that it does not proceed by transmission 

 from particle to particle. 



The rays of heat are reflected and refracted according to the same laws as 

 those of light. They are collected in foci by concave mirrors and convex 

 lenses. These undergo polarization, both Tjy reflection and refraction, in the 

 same manner as rays of light. They are subject to all the complicated phe- 

 nomena of double refraction by certain crystals, in the same manner exactly as 

 rays of light. 



Certain bodies possess imperfect transparency to heat : such bodies transmit 

 a portion of the heat which impinges on them, and absorb the remainder, the 

 portions which they absorb raising their temperature. 



Surfaces also possess the power of reflecting heat in different degrees. They 

 reflect a greater or less portion of the heat incident on them, absorbing the re- 

 mainder. The power of transmission, absorption, and reflection, vary accord- 

 ing to the nature of the body and state of its surface, with respect to smooth- 

 ness, roughness, and color. 



Rays of heat, like those of light, are differently refrangible, and the average 

 refrangibility of calorific rays is less than that of luminous rays. 



When a body at a high temperature, as the flame of a lamp or fire, is placed 

 in contact with the surface of a solid, the particles immediately in contact with 

 the source of heat receive an elevated temperature. These communicate heat 

 to the contiguous particles, and these again to particles more remote. Thus 

 the increased temperature is gradually transmitted through the dimensions of 

 the body, until the whole mass in contact with the source of heat has attained 

 the temperature of the body in contact with it. 



Different substances exhibit different degrees of facility in transmitting heat 

 through their dimensions in this manner. In some the temperature spreads 

 with rapidity, and an equilibrium is soon established between the body receiv- 

 ing heat and the body imparting it. Such substances are said to be good con- 

 ductors of heat. Metals in general are instances of this ; earths and woods are 

 bad conductors ; and soft, porous, or spongy substances still worse. 



When the temperature of a body has been raised to a certain extent by the 

 application of any source of heat, it is observed to become luminous, so as to 

 be visible in the absence of other light, and to render objects around it visible. 

 Thus, a piece of iron, by the application of heat, will at first emit a dull, red 

 light, and will become more luminous as the temperature is raised, until the 

 red light is converted to a clear, white one, and the iron is said to be white hot. 

 This process, by which a body becomes luminous by the increase of its tem- 

 perature, is called incandescence. There is reason to believe that all solid 

 bodies begin to be luminous when heated at the same temperature. 



The degree of heat of incandescent bodies is distinguished by their color ; 

 the lowest incandescent heat is a red heat, next the orange neat, the yellow 

 heat, and the greatest a white heat. 



The heating powers of rays of light vary with their color, in general those 

 of the lightest color having the most heating power. Thus yellow light has a 

 greater calorific power then green, and green than blue. 



