ENERGY. 13 



heat that was present in the skin. The raising of water 

 from the surface of the earth in forming clouds is an every- 

 day instance of work done by latent heat. 



The food of those plants which are not parasites consists 

 of carbonic acid (CO 2 X water (H 2 O), mineral salts (such 

 as phosphates, nitrates and sulphates of lime, potash and 

 magnesia), and small quantities of oxygen and ammonia. 

 Tubercles on the roots of plants appear to exert a strong 

 influence in facilitating the absorption of nitrogen either 

 simple or combined. By means of the energy received in 

 the form of heat and light from the sun, plants are able 

 to carry on their vital functions and to store up sub- 

 stances which are possessed of energy in the form of chemical 

 affinity for oxygen. In the manufacture of these substances 

 (e.g. woody fibre, starch, oil, and vegetable albumen) by a 

 plant, the oxygen-containing compounds which are ab- 

 sorbed by the plant (carbonic acid and water, for example) 

 are deprived more or less of their oxygen, which the plant 

 gives off into the atmosphere. Thus, in order to manufacture 

 10 parts by weight of woody fibre, the plant requires 16.3 

 parts of carbonic acid gas and 5.5 parts of water, the loss 

 being n.8 parts of oxygen. Hence, when woody fibre is 

 burnt, its potential (stored-up) energy, in the form of chemical 

 affinity for oxygen, is productive of a considerable amount of 

 heat. As starch has the same chemical composition as woody 

 fibre, it yields to the animal which eats and digests it the 

 same amount of energy a? woody fibre produces in a fire ; for 

 in both we have the act of combustion, which is oxidation. 

 The chemical reaction in both cases is expressed as follows : 



6 C0 2 + 5 H 2 - C 6 H 10 5 + 120 



(Carbonic (Water.) (Starch or (Oxygen.) 



acid. ) woody fibre. ) 



In vegetable oils and fats the proportion of oxygen is still 

 less, and consequently they possess more stored-up energy. 



