XXX INTRODUCTION. 



into the equivalent amount of kinetic energy (heat, work, electrical 

 phenomena). 



But how do plants form these complex food-stuffs so rich in potential 

 energy? It is plain, that the potential energy of plants must be 

 obtained from some other form of energy. This potential energy 

 is supplied to plants by the rays of the sun, whose chemical light-rays 

 are absorbed by plants. Without the rays of the sun there could be no 

 plants. Plants absorb from the air and the soil, C0 2 , H 2 0, NH 3 , and N, 

 of which carbonic acid, water, and ammonia (from urea), are also pro- 

 duced by the excreta of animals. Plants absorb the kinetic energy of 

 light from the suns rays and transform it into potential energy, which is 

 accumulated during the growth of the plant in its tissues, and in the 

 food-stuffs produced in them during their growth. This formation of 

 complex chemical compounds is accompanied by the simultaneous 

 excretion of 0. 



Occasionally, kinetic energy, such as we universally meet with in animals, is 

 liberated in plants. Many plants develop considerable quantities of heat in their 

 flowers e.g., the arum tribe. We must also remember that, during the forma- 

 tion of the solid parts of plants, when fluid juices are changed into solid masses, 

 heat is set free. In plants, under certain circumstances, is absorbed, and C0 2 

 is excreted, but these processes are so trivial as compared with the typical condi- 

 tion in the vegetable kingdom, that they may be regarded as of small moment. 



Plants, therefore, are organisms which, by a reduction process, trans- 

 form simple stable combinations into complex compounds, whereby 

 potential solar energy is transformed into the chemically-potential 

 energy of vegetable tissues. Animals are living beings, which, by 

 oxidation, decompose or break up the complex grouping of atoms 

 manufactured by plants, whereby potential is transformed into kinetic 

 energy. Thus, there is a constant circulation of matter and a constant 

 exchange of energy between plants and animals. All the energy of 

 animals is derived from plants. All the energy of plants arises from 

 the sun. Thus the sun is the cause, the original source of all energy 

 in the organism, i.e., of the whole of life. 



As the formation of solar heat and solar light is explicable by the 

 gravitation of masses, gravity is perhaps the original form of energy of 

 all life. 



We may thus represent the formation of kinetic energy in the animal 

 body from the potential energy of plants. Let us suppose the atoms of 

 the substances formed in organisms, as simple small bodies, balls, or 

 blocks. As long as these lie in a single layer, or in a few layers, upon 

 the surface, there is a stable arrangement, and they continue to 

 remain at rest. If, however, an artificial tower be built of these 

 blocks, so that an unstable erection is produced, and the same tower 

 be afterwards knocked down, then for this purpose we require (1) 



