164 ON THE INTERACTION OF NATURAL FORCES. 



the animal. They take the consumed carbon given off in respi- 

 ration, as carbonic acid, from the air, the consumed hydrogen as 

 water, the nitrogen in its simplest and closest combination as 

 ammonia ; and from these materials, with the assistance of small 

 ingredients which they take from the soil, they generate anew 

 the compound combustible substances, albumen, sugar, oil, on 

 which the animal subsists. Here, therefore, is a circuit which 

 appears to be a perpetual store of force. Plants prepare fuel 

 and nutriment, animals consume these, burn them slowly in 

 their lungs, and from the products of combustion the plants 

 again derive their nutriment. The latter is an eternal source of 

 chemical, the former of mechanical forces. Would not the 

 combination of both organic kingdoms produce the perpetual 

 motion 1 We must not conclude hastily : further inquiry shows, 

 that plants are capable of producing combustible substances 

 only when they are under the influence of the sun. A portion 

 of the sun's rays exhibits a remarkable relation to chemical 

 forces, it can produce and destroy chemical combinations ; and 

 these rays, which for the most part are blue or violet, are called 

 therefore chemical rays. We make use of their action in the 

 production of photographs. Here compounds of silver are 

 decomposed at the place where the sun's rays strike them. The 

 same rays overpower in the green leaves of plants the strong 

 chemical affinity of the carbon of the carbonic acid for oxygen, 

 give back the latter free to the atmosphere, and accumulate the 

 other, in combination with other bodies, as woody fibre, starch, 

 oil, or resin. These chemically active rays of the sun disappear 

 completely as soon as they encounter the green portions of the 

 plants, and hence it is that in Daguerreotype images the green 

 leaves of plants appear uniformly black. Inasmuch as the 

 light coming from them does not contain the chemical rays, it is 

 unable to act upon the silver compounds. But besides the 

 blue and violet, the yellow rays play an important part in the 

 growth of plants. They also are comparatively strongly ab- 

 sorbed by the leaves. 



Hence a certain portion of force disappears from the sun- 

 light, while combustible substances are generated and accumu- 



