ABSORPTION AND EMISSION. 55 



of the leaves of plants) in ether, produces phenomena of the 

 same kind, the spectrum exhibiting dark bands in the red, 

 yellow, green, and violet. The absorption bands of human 

 blood in the yellow and green portions of the spectrum are 

 very marked, and are visible even when the blood contained 

 in the solution is so small as to affect but slightly the 

 colour. 



The colours of natural bodies are, probably, due to ab- 

 sorption, the incident light being reflected, after having 

 penetrated the medium to some minute depth, which is of 

 course twice traversed. 



(70) The rays of the Sun produce calorific and chemical 

 effects, as well as those of light and colour. The effect of 

 the solar rays on the thermometer are obvious. The chemical 

 effects consist in the power of decomposing certain chemical 

 compounds ; and it is to them that we owe all the beautiful 

 and important results of photography. These three effects, 

 the luminous, the calorific, and the chemical, all vary with 

 the refrangibility of the rays, but in different proportions ; 

 and accordingly, when the Sun's rays are dispersed by re- 

 fraction, the maxima of light, of heat, and of decomposing 

 power, occupy different parts of the spectrum. In fact, the 

 thermic and the chemical effects are found to be greatest 

 beyond the limits of the visible spectrum, the maximum of 

 the former being beyond the red rays, and that of the latter 

 beyond the violet. The power of the rays to produce these 

 different effects, therefore, varies with the refrangibility of 

 the rays, or with the rapidity of the vibrations, the slower 

 vibrations producing the effect of heat, the more rapid those 

 of light, and the most rapid the chemical effects. 



It appears from the experiments of MM. Jamin and 

 Becquerel that, in a given substance, and for a ray of given 

 refrangibility, the luminous, the thermic, and the chemical 



