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210 H, Helmholtz on the Interaction of Natural Forces^ 



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 o _ 



the perpetual motion? 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 combina- 

 tions ; and these rays, which for the most part are blue or violet, 

 ^re 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 accu- 

 mulate 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. Inas- 

 much as the light coming from them does not contain the chem- 

 ' ical 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 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 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 j 



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

 share the same etherial 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 me- 

 teorological, climatic, geological, and organic processes of our 

 earth are almost wholly preserved in action by the light and 



heat-o'ivinD^ ravs nf rhp cnn. r.r^A ^ri'^^^ on^o in +>tiG o Tpmarkable 



