LIGHT AND ITS ARTIFICIAL PRODUCTION. 289 



between the bulb of the thermometer and the platinum, the level of 

 the liquid returns to its original position. This shows that glass and 

 water absorb the whole of the heat emitted by the glowing platinum. 

 Our method is not sensitive enough to detect the light radiations 

 transmitted through the water when the platinum is heated to white- 

 ness, but this can, however, be very easily measured by a sensitive 

 bolometer. As soon as the vessel is removed, the level of the liquid 

 changes rapidly. 



The property of absorbing heat rays and of freely transmitting light 

 rays is utilized in the construction of glass windows. Although long 

 unknown to man, they separate, for his comfort, warmth and light, 

 preventing the escape of the heat of the room from within and pre- 

 A^enting the penetration of cold from without, while they transmit 

 sunlight almost undiminished. 



On the same property depends the action of clouds in retaining the 

 heat of the earth. Impenetrable for the long heat waves, the clouds 

 prevent the earth from radiating its warmth received from the sun 

 during the day into infinite space and thus protect it from loss of heat. 

 Without this moist blanket the earth would lose enormous quantities 

 of heat by radiation during the long, starry, winter nights, and thus a 

 strong cooling would take place. The fact that our eye is insensitive 

 to waves of great wave length is due to the absorptive action of 

 liquids. The infra-red rays are simply absorbed by the lens and 

 aqueous humor of the eye before they reach the retina. According 

 to Darwin, this indicates that our eye is modeled after that of the 

 Amphibia. For these, the absorption of the infra-red rays by the 

 eye is of small importance, for they have already been absorbed by 

 the upper layers of the water, in whose depths their eyes must still 

 be able to see. 



i!^rom the experiment you have just seen we recognize that the tran- 

 sition from a nonluminous to a luminous body, or better, from one 

 radiating heat to one radiating light and actinic rays, is in reality 

 determioed by a sufficient increase of its temperature. It matters not 

 how its temperature be increased, whether by the electric current or by 

 the direct application of heat, a material heated electrically or in a 

 furnace begins to glow at the same temperature. 



Moreover, the luminosity of a hot body is a secondary property — one 

 might almost say an accidental property. If it were not for the eye 

 the luminous state could only be distinguished from the nonluminous 

 in that the energy of the different waves is not the same. This is not 

 surprising when we consider that the heat residing in a body, and on 

 which its temperature depends, consists solely in the energy of motion 

 of its molecules; that is, of its smallest mechanically conceivable parts. 



I] EAT IS MOTION. 



It was long supposed that a heated body emitted a subtle material 

 in the same way as it was supposed that light was a material substance. 

 SM 97 19 



