COAL AS A RESERVOIR OF POWER. 
157 
vary in their physical conditions, and their orbits would be 
changed. There is no evidence that either the one or the 
other has resulted. Let us see if we can guess at any process 
by which this stability of the solar system is maintained. 
It was first shown by Faraday, in a series of experimental 
investigations which may be regarded as the most beautiful 
example of inductive science with which the world has been 
favoured since Bacon promulgated his new philosophy, that the 
quantity of electricity contained in a body, was exactly the 
quantity which was necessary to decompose that body. For 
example, in a voltaic battery — of zinc and copper-plates — a 
certain fixed quantity of electricity is eliminated by the oxyda- 
tion of a portion of the zinc. If, to produce this effect, the 
oxygen of a given measure of water — say a drop — is necessary, 
the electricity developed will be exactly that which is required 
to separate the gaseous elements of a drop of water from each 
other. An equivalent of electricity is developed by the oxyda- 
tion of an equivalent of zinc, and that electricity is required 
for the decomposition of an equivalent of water, or the same 
quantity of electricity would be equal to the power of effecting 
the recombination of oxygen and hydrogen, into an equivalent 
of water. The law which has been so perfectly established for 
electricity is found to be true of the other physical forces. By 
the combustion — which is a condition of oxydation — of an equiva- 
lent of carbon, or of any body susceptible of this change of state, 
exact volumes of light and heat are liberated. It is theoretically 
certain that these equivalents of light and heat are exactly the 
quantities necessary for the formation of the substance from 
which those energies have been derived. That which takes place 
in terrestrial phenomena is, it is highly probable, constantly 
taking place in solar phenomena. Chemical changes, or distur- 
bances analogous to them, of vast energy are constantly progres- 
sing in the sun, and thus is maintained that unceasing outpour 
of sunshine, which gladdens the earth, and illumines all the 
planets of our system. Every solar ray is a bundle of powerful 
forces : light, the luminous life-maintaining energy, giving 
colour to all things ; heat, the calorific power which determines 
the conditions of all terrestrial matter ; actinism, peculiarly the 
force which produces all photographic phenomena ; and elec- 
tricity regulating the magnetic conditions of this globe. Com- 
bined in action, these solar radiations carry out the conditions 
necessary to animal and vegetable organization, in all their 
varieties, and create out of a chaotic mass forms of beauty 
rejoicing in life. 
To confine our attention to the one subject before us. Every 
person knows that to grow a tree or a shrub healthfully, it must 
have plenty of sunshine. In the dark we may force a plant to 
