330 THE LIFE OF THE PLANT 



e.g. setting in motion our mills. The same energy of the sun 

 heats the atmosphere at certain spots, so as to produce those 

 terrible manifestations of mechanical energy known as whirl- 

 winds, hurricanes, etc. 



So, then, heat transforms itself into mechanical work and 

 vice versa, and during these processes a strict quantitative 

 ratio is maintained. The same is true with regard to other 

 forces of nature, such as light, electricity, chemical affinity. 

 They are all capable of mutual transformation either immedi- 

 ately or by acquiring the latent form of tension, which subse- 

 quently manifests itself in one way or another. It is only as 

 we continually keep in mind this possibility of the mutual 

 transformation of different forces that we come to realise how 

 true is the law of the conservation of energy. 



Let us dwell for a little upon the correlation that exists between 

 heat and chemical affinity, for it will naturally bring us back to 

 the question raised at the beginning of this Lecture. Chemistry 

 teaches that the atoms of the elements are endowed with 

 mutual affinity in various degrees. The atoms of hetero- 

 geneous bodies tend towards each other in much the same way 

 as bodies tend towards the earth, or as these balls tend towards 

 each other by reason of their springs. Our model is meant to 

 illustrate in an obvious way this very fact of chemical affinity. 

 The ball marked by the letter C represents carbon, the ball 

 O 2 oxygen. The atoms of carbon and oxygen tend to combine 

 and to form carbonic acid, in which two atoms of oxygen are 

 combined to one of carbon (CO 2 ). In the same way the atoms 

 of hydrogen (H) tend to combine with the atoms of oxygen and 

 form water, H 2 O, in which two atoms of hydrogen are combined 

 with one of oxygen. On the other hand, the atoms of carbon 

 and hydrogen are endowed with a comparatively much weaker 

 affinity, and therefore, even though combined with each other, 

 tend at the first opportunity to recombine each in its turn with 

 oxygen, thus forming carbonic acid and water. 



At the moment of combination the atoms knock against each 

 other in the same way as these balls do. But whenever bodies 

 knock against each other heat is generated. The same takes 

 place in the case of the blows of atoms. These blows, this collision 

 of the particles of carbon and hydrogen with those of oxygen, 

 is exactly what we mean by combustion. Just as heat and 



