190 



EELATION OF HEAT AND LIGHT. 



to render solids luminous. If, therefore, they be susceptible of incandescence, 

 their point of incandescence must be far above the point of incandescence of 

 bodies in the solid or liquid form. Mr. Wedgwood constructed a spiral tube of 

 porcelain, which was carried through a crucible surrounded with sand. To 

 one end of it was attached a pair of bellows, and the air thus driven through 

 it was received from the other extremity into a globular vessel, furnished with 

 a valve by which air was allowed to escape, but none to enter. In the side of 

 this globular vessel was an opening, in which was inserted a piece of glass, 

 through which the interior could be viewed. The sand in the crucible being 

 then rendered red-hot, air was blown through the earthern tube, and made to 

 pass into the glass vessel at the other end of the tube. When viewed through 

 the glass in the side of the vessel, it was observed not to be luminous ; but 

 a piece of gold wire introduced into that part of the vessel near the mouth 

 of the spiral tube, was immediately rendered red-hot by the blast of hot air 

 which issued from it. The air, therefore, had a temperature at least equal to 

 the temperature of the incandescence of gold. 



Such experiments render it manifest that gases are incapable of attaining 

 incandescence at the same temperature as that at which solids become luminous ; 

 but it appears to me that we cannot hence infer that the matter of the gas is 

 not susceptible of incandescence, even at the temperature at which other bodies 

 pass into that state ; for if a gas were liquified, and confined by pressure so as 

 to prevent it from dilating again into the form of gas, it is probable that in that 

 state a quantity of heat would render it incandescent which would be altogether 

 incapable of producing the same effect on it in the form of gas. 



Established facts and analogy founded on them, therefore, lead to the con- 

 clusion, that if a sufficient quantity of heat be supplied to any body, that body 

 will at length become luminous ; and, therefore, that light is invariably a con- 

 sequence of heat, when that heat attains a certain degree of intensity ; the 

 quantity of heat necessary for the production of light differing according to the 

 nature of the body which contains that heat, those having a less specific' heat 

 requiring a less supply of heat to render them luminous. 



Let us now inquire how far the presence of heat is a necessary consequence 

 of the presence of light. 



It has been proved that the least refrangible rays of solar light are those which 

 possess the quality of heat in the highest degree ; the most refrangible luminous 

 rays, though still indicating the presence of the calorific principle, exhibit that 

 in a very slight degree ; while the invisible chemical rays, still more refrangi- 

 ble than these, produce no susceptible effect on the thermometer. We are, 

 therefore, led to infer, that, in solar light, the heating qualities of the rays in- 

 crease as their refrangibility diminishes. 



When light falls on an opaque body, it is either wholly or partially absorbed. 

 If it be generally absorbed, that portion which is not absorbed is reflected, or 

 driven back into the space from which the light came. Now it is clear that, 

 so far as light is the means of communicating heat to any opaque body under 

 these circumstances, this heat must proceed altogether from the light which is 

 absorbed. 



It has been explained, that the solar light is composed of lights of several 

 different colors. When this light falls on an opaque body, it happens that lights 

 of certain colors are absorbed by the surface of the body, and the remainder of 

 the solar light is reflected. On this fact depend all the phenomena of the col- 

 ors of natural bodies. When a body appears to be of a red color, it reflects 

 from its surface that portion of the sun's light which is red, and it absorbs all 

 the other colors. Again, if a body appear green, it absorbs all the sun's light 

 which strikes upon it except the green light, and that alone is reflected, and so 



