434 HERMANN VON HELMHOLTZ 



the diverse forces of Nature have been studied, the more exact 

 has been the confirmation of the law that in none of them has 

 energy been destroyed or increased. It always remains the 

 same immutable amount which changes only in its mode of 

 manifestation. 



' The same law holds even in the most complex and delicate 

 of all the natural processes, in those of organized life. 

 Animals obtain the energy which they develop from the slow 

 combustion of the food they take in with the inspired oxygen 

 of the air, giving out heat and mechanical work in exchange. 

 For plants, on the contrary, the principal source of energy is 

 the sunlight, and in exchange they produce the combustible 

 substances of which they consist, and free oxygen, which they 

 give back to the air. 



' This immense, immutable store of energy, notwithstanding 

 the fact that it participates in the indestructibility of the 

 material substances, and of the luminiferous ether, is yet no 

 substance analogous to them. It is inherent in no particular 

 substances, although from time to time it may permeate and 

 abide in ponderable matter, whether as elastic tension, or as 

 heat, or as chemical affinity. Nor, again, is energy simply 

 bound to any position in space, or divisible with space, like 

 a material substance. There are indeed certain forms which 

 can be shown to have their seat in some definite natural body, 

 when, that is to say, such a body is thrown into agitation by 

 violent motion, or is very hot, or strongly compressed. But 

 along with these we have a whole series of forms of energy, 

 which arise only from the relations of two bodies to one 

 another. The greatest supply of energy known to us is in the 

 gravitation of the celestial bodies to one another, and depends 

 on the magnitude of their reciprocal distances, being smaller 

 the nearer the mutually attracting bodies come to one another. 



' So too for chemical affinities. The energy which appears 

 in the combination of oxygen and carbon exists neither in 

 oxygen alone nor in carbon alone. It exists only in the relation 

 of these to one another, whether this be regarded as direct, or 

 as due to the intervention of electrical forces. Since it is 

 neither contained in the space which is occupied by the 

 oxygen nor in that occupied by the carbon, it is impossible to 

 locate it in any precise spot. 



