AND PHYSICAL FORCES 15 



always take carbon from the carbonic acid of the air, hydrogen 

 from the water which bathes them, and frequently azote from the 

 air. . . . The soil he used for the growth of his plants, the 

 subjects of experiment, was a siliceous sand, which was first sifted, 

 then kept at a red heat for some time, in order to destroy every 

 trace of organic matter within it. It was then moistened with 

 distilled water, and the seeds sown ; after an interval of a few 

 days, the seeds which did not germinate were removed. . . . Peas 

 planted in a soil absolutely barren, and watered with pure water, 

 may attain to complete maturity, passing through all the phases 

 of their natural growth, and bearing flowers and ripe seeds. 

 During this process, they fix a large quantity of azote, which they 

 must derive either from the air dissolved in the water which 

 they absorb by their roots, or from the air that surrounds their 

 stalks and leaves." 



Animals, as a rule, are powerless for the creation of organic 

 matter ; they can assimilate and modify the organic substances 

 which have been built up for them in the tissues of plants ; but 

 they cannot abstract from earth, air, and water the elementary 

 constituents of organic matter, and force them to enter into such 

 and such combinations. They use the materials which have been 

 elaborated for them by plants, since they all feed either directly 

 upon members of the vegetable kingdom, or else indirectly by 

 living upon animals which have been so nourished. Plants, then, 

 are the great factors of organic matter — the vegetable kingdom 

 is nature's laboratory, within whose sacred precincts not-living 

 matter is coerced into more elevated and complex modes of being, 

 and is made to display those more subtle characteristics which 

 we find in living tissues. Using only the great forces of nature — 

 availing themselves only of the subtle motions emanating from the 

 Sun under the names of heat, light, and actinism — plants compel 

 carbonic acid to yield up its carbon, water its hydrogen, and 

 nitrate of ammonia its nitrogen. 



It might be thought that plants derive the principal part of the 

 ingredients with which they build up their own structures from 

 the soil ; but the experiments of Boussingault just referred to 

 have long since disproved this formerly favoured assumption, 

 and shown that the atmosphere is the great storehouse for the 

 pabulum of plants. Carbon is the most fundamental ingredient 

 of the vegetable kingdom ; all plants fix this substance, and all 

 obtain it from carbonic acid — either abstracting it directly from 



