6 RINCIPLES OF ANIMAL NUTRITION. 
means of determining directly, but it is commonly estimated from 
the amount of nitrogen upon two assumptions: first, that all the 
substances of the protein group contain 16 per cent. of nitrogen, and 
second, that all the nitrogen of feeding-stuffs exists in the proteid 
form. On the basis of these assumptions, protein is, of course, 
equal to total nitrogen X 6.25. 
Although it was never claimed that this method of estimating 
protein was strictly accurate, it was for a long time assumed that 
the two sources of error involved were not serious. Later investi- 
gations, however, have dispelled this pleasing illusion. Further 
investigations of the true proteids, notably those of Ritthausen and 
of Osborne, have shown a very considerable variation in the per- 
centage of nitrogen contained in them, while, on the other hand, the 
researches of Scheibler, E. Schulze, Kellner, and others have shown 
the presence in many feeding-stuffs of relatively large amounts of 
nitrogenous matters of non-proteid nature. The results of these 
latter investigations have made it necessary to subdivide the total 
nitrogenous matter of feeding-stuffs into two groups, called respec- 
tively “proteids” and “‘non-proteids,” while the name “protein” 
has been retained in the sense of total nitrogen X 6.25 or other con- 
ventional factor. For various classes of human foods, Atwater and 
Bryant * propose the following factors, based on the results in- 
dicated in the next two paragraphs, for the computation of protein 
from nitrogen: 
Animal 0008) 05 6c. deg oe tnogeee i Geregeewe yee es 6.25 
Wheat, rye, barley, and their manufactured products 5.70 
Maize, oats, buckwheat and rice, and their manufactured 
PROdU Cte ss aes es ieee ae Se REE 6.00 
Dried seeds of legumes .........-.....-0.0 0002000, 6.25 
VeSStableS: accede AES ee A eased 5.65 
PRUTGS eexey dees ar ecsuanat tinier ie evens mean Guy ata Wiese gee 5.80 
Proteids.—In the absence of any adequate knowledge regarding 
the very complex molecular structure of the proteids, both the 
classification and the terminology of these bodies are in a very con- 
fused state. For convenience, however, we may adopt here those 
* Storrs (Conn.) Ag. Ex. St., Rep. 12, 79. 
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