Absorbing Powers of different Bodies for Light and Heat, 13 



ultimate success, since the form of the function in question is no 

 doubt simple, as is the case with all functions hitherto discovered 

 that do not depend on the properties of individual bodies. 

 Whenever this problem is solved, the full fertility of the law 

 above demonstrated will be apparent ; even at present, however, 

 important consequences may be deduced from it. 



§ 12. If a body (a platinum wire, for example) be gradually 

 heated up to a certain temperature, it only emits rays consisting 

 of waves longer than those of the visible rays. Beyond that 

 point, waves of the length of the extreme red begin to appear ; 

 and as the temperature rises, shorter and shorter waves are added ; 

 so that, for every temperature, rays of a corresponding length of 

 wave are originated, while the intensity of the rays of greater 

 wave-length is increased. If the law we have established be 

 applied to this case, it will be seen that the function I, for waves 

 of any given length, must vanish for all temperatures below that 

 answering to the wave-length in question, and that, for tempera- 

 tures above this, it must increase with the temperature. 



Whence, applying the same proposition to other bodies, it 

 follows that all bodies, when their temperature is gradually 

 raised, begin to emit waves of the same length at the same 

 temperature, and therefore become red-hot at the same tempera- 

 ture, emit yellow rays at the same temperature, &c* The 

 intensity of rays consisting of waves of a given length, which 

 different bodies emit at the same temperature, may, however, be 

 very different, since it is proportional to the power of absorption 

 of the body for waves of that particular length. At the same 

 temperature, accordingly, metal glows more brightly than glass, 

 and glass more brightly than a gas. A body that remains per- 

 fectly transparent at the highest temperature never becomes red- 

 hot. In a platinum ring of about 5 millims. diameter, I placed a 

 small portion of phosphate of soda, and heated it in the dull 

 flame of Bunsen's lamp. The salt melted, formed a fluid lens, 

 and remained perfectly transparent ; it, however, emitted no light, 

 while the platinum ring, with which it was in contact, glowed 

 brilliantly. 



§ 13. For the same temperatnre the magnitude I is a con- 

 tinuous function of the wave-length, except for such values of the 

 latter as render I evanescent. The truth of this assertion may 

 be concluded from the continuity of the spectrum of a red-hot 

 platinum-wire, provided it be admitted that the power of absorp- 

 tion of such a body is a continuous function of the length of the 

 waves of the incident rays. It may also be affirmed, with the 

 highest degree of probability, that while the temperature remains 



* Draper, Phil. Mag. vol. xxx. p. 345; Berl. Ber. 1847. 



