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important element in the food of plants. These four substances consti¬ 
tute the organic part of vegetable and animal bodies. In vegetables used 
as food for man and cattle, they exist in the following different propor- 
i ons in the plants in the dry state : 
Carbon forms nearly one half by weight. 
Oxygen rather more than one third. 
Hydrogen little more than five per cent. 
Nitrogen from two to four per cent.— Johnston. 
Many vegetable products, as starch, gum, and sugar, do not con¬ 
tain any nitrogen, and their use in the animal economy is to supply car¬ 
bon for respiration, and to be stored up in the animal tissue, as fat. The 
muscles and blood of animals, the albumen or white of egg, and the 
caseine of milk, contain a certain proportion of nitrogen. The gluten of 
wheat and other products of plants, known as vegetable albumen and 
caseine, are similar in composition to animal fibrine, albumen and 
caseine; hence it is concluded that vegetable substances containing these 
nitrogenous, or azotized products, as they are usually called, are most 
essential food, especially for young growing animals. The composition 
of starch is 12 equivalents of carbon (C{ 2 .) 10 equivalents of hydro¬ 
gen (Hio.) and 10 equivalents of oxygen (Oio.) Cane sugar consists 
of Ci 2 Hj O.j plus 2 of water 2 HO.) Gluten or fibrine, albu¬ 
men and caseine have been found to contain the same proportions of 
carbon, hydrogen, oxygen and nitrogen, combined with different propor¬ 
tions of sulphur and phosphorus. The name of protein compounds 
has, in consequence, been given to them. Protein consists of C^s H 36 
N G O14. Gluten or fibrine is protein, plus sulphur (txj S) plus phos¬ 
phorus (X Ph.) Albumen is protein IxJ Sj Ph. Caseine is protein 
X S. As these azotized substances contain the same amount of ni¬ 
trogen, chemists, instead of in all cases making a complete analysis of 
vegetable substances to determine their value as articles of food, simply 
burn a quantity of the perfectly dry plant and determine the per-centage 
of nitrogen which it contains, they thus obtain the per-centage of protein 
compounds. 
The inorganic constituents of plants form a very small proportion of 
their whole weight, they are nevertheless of great importance. Different 
species growing in the same soil afford ashes which differ in quantity and 
