THE COMPOSITION OF FEEDING STUFFS 9 
of organic plant substances. It was formerly believed that all pro- 
teins contain about 16 per cent nitrogen, and, since this element 
can be readily determined by the.chemist, the content of protein in 
a substance was obtained by multiplying the nitrogen content by422, 
or 6.25. Later investigations showed that the different protein sub- 
stances vary considerably in chemical composition, and that the per- 
centage of nitrogen they contain may range from 15 to over 19 per 
cent. Hence, the factor 6.25 is often not correct, but it is generally 
applied, since our knowledge of the composition of pure proteins 
from different sources is still incomplete. , We shall, therefore, con- 
tinue the use of the factor 6.25-in this book until chemists have 
agreed on specific factors to be used in the case of plant materials 
and feeding stuffs of different origin. 
Besides nitrogen, proteins contain between 50 and 55 per cent 
of carbon, 6 to 7 per cent hydrogen, 20 to 24 per cent oxygen, 
0.3 to 2.3 per cent sulfur; the phosphorus content of the proteins 
in which this element is present ranges between 0.4 and 0.9 per 
cent. The average composition of protein substances may be given 
as follows: 
Carbon, 53 per cent. . Nitrogen, 16 per cent. 
Hydrogen, 7 per cent. Sulfur, 2 per cent. 
Oxygen, 22 per cent. nab 
The proteins form a most important group of nutrients, since 
they furnish the materials for building up body tissues and fluids ; 
other nutrients cannot take their place for this purpose. We shall 
see, however, that protein may also serve other purposes than to 
furnish material for tissue building when necessary, viz.: To supply 
energy that may be used for maintaining body heat, for performing 
work, or for storage as body fat. 
Classification of Protcins—Protein substances are generally classified 
as (1) simple, (2) conjugated, and (3) derived proteins. 
1. Simple Proteins —The most important compounds in this group 
are given below. 
a, Albumins.—These are soluble in pure water and are coagulated and 
rendered insoluble by heat. They are present in small amounts in the sap. 
and seeds of plants. The main proteins found in the animal body belong 
to this class, viz.: Those of the muscle, blood, milk, and eggs. Leucosin 
found in the cereals, legumelin found in leguminous seeds, ricin in castor 
bean, and tuberin in potatoes, belong to this class. 
6. Globulins are insoluble in water, but soluble in a 10 per cent sodium 
chloride solution. The globulins are abundant in plant materials and have 
been identified in many seeds of plants. The following are present in 
the cereals and other common seeds: Maysin in corn kernels, edestin in 
corn, wheat, cotton seed, and flaxseed, avenalin in oats, legumin and 
vicilin in leguminous seeds (peas, lentils, horse beans), glycin in soy- 
beans, and conglutin in lupines. 
