ON THE INTERACTION OF NATURAL FORCES. 185 



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 com- 

 pletely 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. In- 

 asmuch 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 absorbed by the 

 leaves. 



Hence a certain portion of force disappears from the 

 sunlight, while combustible substances are generated and 

 accumulated in plants ; and we can assume it as very 

 probable, that the former is the cause of the latter. I 

 must indeed remark, that we are in possession of no ex- 

 periments from which we might determine whether the 

 vis viva of the sun's rays which have disappeared corre- 

 sponds to the chemical forces accumulated during the 

 same time ; and as long as these experiments are wanting, 

 we cannot regard the stated relation as a certainty. If 

 this view should prove correct, we derive from it the 

 flattering result, that all force, by means of which our 

 bodies live and move, finds its source in the piuest sun- 



