Matter and Energy. 19 



C. INDIRECTLY AVAILABLE SOURCES OF ENERGY 

 (KINETIC). 



1. Gravity. The law of gravity has been already stated 

 at page 7, and it is in virtue of that attractive force that 

 we are able to make use of such directly available sources 

 of energy as some of those already mentioned, e.g. tides, 

 ordinary water-power, falling bodies, &c. 



2. Solar radiation. This is the first of the two great 

 sources of energy looked at from a biological point of view. 

 Its relation to plant and animal life will be discussed in 

 detail afterwards. At present we need only direct attention 

 to the irregular heating of the atmosphere by the sun's rays 

 in relation to the formation of winds and of ocean currents. 

 Its influence on the evaporation of water, &c. may also be 

 specially noted. 



3. The rotation of the earth on its axis causes changes 

 in the direction of the atmospheric and oceanic currents, 

 and thus, in conjunction with solar radiation, may be said 

 to be the cause of them. 



4. The earth's internal heat In accordance with the 

 nebular hypothesis, now almost universally accepted in some 

 form by physicists, the earth's internal heat is the residuum 

 of the heat it possessed in its originally molten condition. It 

 manifests itself chiefly in the forms referred to under the 

 fifth heading of the class of kinetic energies entitled ' directly 

 available.' It is a source of energy which must manifestly 

 become gradually exhausted as the age of the earth in- 

 creases, 



5. Chemical affinity. This is to be considered as the 

 second of the two great sources of the energy of living 

 things, and on that account merits fuller treatment 



We have already defined a chemical element as a sub- 

 stance homogeneous in itself, i.e. whose atoms are all of one 



c 2 



