TRANSFORMATION AND EQUIVALENCE OF FORCES. 217 



of force; and this force is supplied by the Sun. In what 

 manner the decomposition is effected we do not know. But 

 we know that when, under fit conditions, plants are exposed 

 to the Sun's rays, they give off oxygen and accumulate car- 

 bon and hydrogen. In darkness this process ceases. It 

 ceases too when the quantities of light and heat received are 

 greatly reduced, as in winter. Conversely, it is active when 

 the light and heat are great, as in summer. And the like re- 

 lation is seen in the fact that while plant-life is luxuriant in 

 the tropics, it diminishes in temperate regions, and disap- 

 pears as we approach the poles. Thus the irresistible infer- 

 ence is, that the forces by which plants abstract the materi- 

 als of their tissues from surrounding inorganic compounds 

 the forces by which they grow and carry on their functions, 

 are forces that previously existed as solar radiations. 



That animal life is immediately or mediately dependent 

 on vegetal life is a familiar truth ; and that, in the main, the 

 processes of animal life are opposite to those of vegetal life is 

 a truth long current among men of science. Chemically 

 considered, vegetal life is chiefly a process of de-oxidation, 

 and animal life chiefly a process of oxidation: chiefly, we 

 must say, because in so far as plants are expenders of force 

 for the purposes of organization, they are oxidizers (as is 

 shown by the exhalation of carbonic acid during the night) ; 

 and animals, in some of their minor processes, are probably 

 de-oxidizers. But with this qualification, the general truth 

 is that while the plant, decomposing carbonic acid and water 

 and liberating oxygen, builds up the detained carbon and 

 hydrogen (along with a little nitrogen and small quantities 

 of other elements elsewhere obtained) into branches, leaves, 

 and seeds; the animal, consuming these branches, leaves, 

 and seeds, and absorbing oxygen, recomposes car- 

 bonic acid and water, together with certain nitrogenous 

 compounds in minor amounts. And w T hile the decom- 

 position effected by the plant, is at the expense of certain 

 forces emanating from the sun, which are employed in 

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