ABSORPTION AND EMISSION, 57 



less refrangible end. Yellow and red glasses absorb the 

 rays of higher refrangibility, and, along with them, the 

 chemical rays. Yellow glass, which transmits light abun- 

 dantly, is therefore employed with much advantage in photo- 

 graphic workshops. On the other hand, blue glasses transmit 

 the chemical rays freely, but absorb the thermic rays beyond 

 the red. 



(72) The phenomena of the absorption of light by the 

 gases are very different from those presented by solids and 

 liquids. The rays stopped by them are few and well-defined ; 

 so that when the light is analysed by a prism, the spectrum 

 exhibits a small number of fine dark lines, wholly separated 

 from one another, and variously distributed. Accordingly 

 the gases allow all the light to pass, with the exception of 

 a few rays of definite refrangibility. It will be seen from 

 the foregoing account, that the spectra of light transmitted 

 through the gases resemble the solar spectrum, only that the 

 dark lines are fewer, and the whole phenomenon less com- 

 plex. And this resemblance has led to the inference that 

 the dark lines in the solar spectrum are due to gaseous ab- 

 sorption. We have only to assume that, in place of a 

 single gas absorbing a small number of definite rays, the 

 light of the Sun traverses a mixture of many gases, each of 

 which produces a different set of lines by absorption. 



(73) All solid and liquid bodies, when sufficiently heated, 

 emit heat and light, the emitted rays increasing in refrangi-, 

 bility as the temperature is augmented. Thus, at the tem- 

 perature of boiling water, the non-luminous heat-rays alone 

 are emitted. "When the temperature is raised to 525 C., 

 according to Dr. Draper, the less refrangible red rays 

 begin to appear. The more refrangible rays are developed 

 in succession, as the temperature is further increased, until, 



