Prof. Helmholtz on the Interaction of Natural Forces. 511 



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. Inas- 

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

 mical 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 ai'e 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 hereto- 

 fore 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 sethereal origin, as also the whole vegetable world, and 

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

 the youngest ofi'spring 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 

 meteorological, 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 ex- 

 ternal conditions, may exhibit itself in nature. Besides these, the 

 earth experiences an action of another kind from its central lumi- 

 nary, as well as from its satellite the moon, which exhibits itself 

 iu the remarkable phsenomenon of the ebb and flow of the tide. 



Each of these bodies excites, by its attraction upon the waters 

 of the sea, two gigantic waves, which flow in the same direction 

 round the world, as the attracting bodies themselves apparently 

 do. The two waves of the moon, on account of her greater near- 

 ness, are about 3^ times as large as those excited by the sun. 



