HERMANN VON HELMHOLTZ 297 



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 ac- 

 cumulate 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. 



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 

 experiments from which we might determine whether the vis viva of 

 the sun's rays which have disappeared, corresponds to the chemical 

 forces accumulated during the same time ; and as long as these ex- 

 periments are wanting, we cannot regard the stated relation as a 

 certainty. H 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 purest sunlight ; and hence we are all, 

 in point of nobility, not behind the race of the great monarch of China, 

 who heretofore alone called himself Son of the Sun. But it must 

 also be conceded that our lower fellow-beings, the frog and leech, 

 share the same ethereal origin, as also the whole vegetable world, 

 and even the fuel which comes to us from the ages past, as well 

 as the youngest offspring of the forest with which we heat our stoves 

 and set our machines in motion. 



You see, then, that the immense wealth of ever-changing mete- 

 orological, climatic, geological, and organic processes of our earth are 

 almost wholly preserved in action by the light and heat-giving rays of 

 the sun ; and you see in this a remarkable example, how Proteus-like 

 the effects of a single cause, under altered external conditions, may 

 exhibit itself in nature. Besides these, the earth experiences an action 



