254 Dr. Scpiuster 07i 



himself, in the Reports of the British Association for 1861. 

 In this report he summarises the results of his own experi- 

 ments as follows. 



1. The radiating power of thin polished plates of dif- 

 ferent substances was found to vary as their absorptive 

 power, so that the radiation of a plate of rock salt was only 

 15 per cent of the total lampblack radiation for the same 

 temperature. 



2. It was shown that the radiation from thick plates of 

 diathermanous substance is greater than that from thin 

 plates, no such difference being manifested when the sub- 

 stances are athermanous. 



3. It was found that heat, radiated by a thin diather- 

 manous plate, is less transmissible through a screen of the 

 same material as the heated plate than ordinary or lampblack 

 heat, this difference being very marked in the case of rock 

 salt. 



4. Lastly, heat from a thick diathermanous plate is more 

 easily transmitted through a screen of the same nature as 

 the source of heat than that from a thin plate. 



The conclusions drawn from these experiments are now 

 generally recognised. Kirchhoff, somewhat later than 

 Stewart, fully proved them from thermodynamical princi- 

 ples, and gave a more definite meaning to the terms radia- 

 tion and absorption. In view of the great importance, which 

 now attaches to the science of Spectrum Analysis, a few 

 words are necessary to explain the difference between 

 Stewart's and Kirchhoff 's treatment. Stewart proved that 

 the streams of radiant heat crossing any point of an 

 enclosure of uniform temperature, are not altered by the 

 interposition of a body whether opaque or transparent ; and 

 this, as the author shows, is easily explained on the theory 

 that, when equilibrium of temperature is established, any 

 body or part of a body radiates as much heat as it gains. 

 The radiation crossing any point is made up of heat which 



