ORGANIC CHEMISTRY 457 



So far as animal tissues are concerned the problem of their 

 constitution and how they are wasted and renewed is, from one 

 point of view, simpler than the corresponding problem presented 

 by plants, inasmuch as animals find the materials they require 

 in their food, ready formed in the vegetable matter from which 

 directly or indirectly they derive nourishment. No animal is 

 known to assimilate any element except oxygen from the air, or 

 to build up fats, carbohydrates, or proteins from such simple 

 materials as carbon dioxide, water, and ammonia. The case of 

 the plant is very different. The forest of timber trees equally 

 with the humble moss or lichen with which their trunks are 

 clothed derives the whole of the carbon which is the chief com- 

 ponent of wood, leaves, and fruit from the carbon dioxide of the 

 air. And this gas is found in the atmosphere to the extent of 

 only 3 to 4 parts in 10,000 under ordinary conditions. How is 

 this accomplished ? That is the great problem on which chemists 

 and physiologists have been more or less engaged since chemistry 

 and physiology began. It was proved by experiment two hundred 

 years ago that a willow tree planted in a tub of pure sand and 

 watered with rain water grew and flourished without limit (it 

 actually weighed 60 pounds at the end of the experiment), and 

 though the result seemed to prove, at that time, that vegetable 

 matter consisted only of water it was shown by later experiments 

 made in the first instance by Priestley, and subsequently con- 

 firmed by many other chemists, including Sir Humphry Davy, 

 that the growth is entirely due to the use of carbon derived from 

 the carbon dioxide of the atmosphere. In Davy's Lectures on 

 Agricultural Chemistry given in 1813 he describes experiments 

 in which, first a square of turf and in another case a branch of 

 vine, was exposed to an atmosphere containing a large quantity 

 of carbonic acid. Under the influence of sunshine the carbonic 

 acid disappeared and was replaced by oxygen. 



It is unnecessary to cite the further experiments of the older 

 observers, but among the many researches in connection with 

 this question which have been recorded during more recent 

 times, there is none more important than those of Dr. Horace T. 

 Brown published during the years 1893 to 1905 on the nature of 

 the substances formed in the leaf during exposure to sunlight 

 and the rate of decomposition of the atmospheric carbon dioxide. 

 In order that this constituent of the air may come into contact 

 with the active surface it is necessary for it to penetrate into the 



