NUTRITIVE PRINCIPLES OF PLANTS. 249 
of lead is used, in the place of oxide of copper, and due precau- 
tions taken, the analysis is much more accurate, and the quan- 
tity of carbon approaches nearer to that obtained by the direct 
methods. The dried flesh of animals, and the dried muscular 
fibre of the ox, freed from fat, give, when burnt with oxide of 
copper, 1 volume of nitrogen to 7 volumes of carbonic acid. 
The result is the same with pure albumen, burnt in the same 
manner. The muscular fibre of the ox and roe, boiled or roasted, 
blood dried at 212°, and the dried flesh of fish, such as pike and 
cod, which so much resembles coagulated albumen, all give the 
same proportions of gas—(Dr. Playfair). Proceeding, then, on the 
foundation which these experiments afford, to compare the com- 
position of azotized vegetable substances with the principal com- 
ponent parts of animals, a most important fact is at once discover- 
ed, namely, that all those nutritive vegetable principles, what- 
ever they may be, possess either the same composition as fibrin, 
albumen, and casein, or, if the percentage be different, still have 
the same proportion of nitrogen and carbon as the animal sub- 
stances possess. It is remarkable also that this resemblance 
goes still further, for these vegetable substances conduct them- 
selves in a similar manner with chemical reagents, so that we 
may say that their form is merely changed, when animals pro- 
duce blood and muscular fibre from them ; for they are obtained 
from plants in a perfect state, as far as the proportion of their 
elements is concerned. 
Graminivorous animals are fed on vegetable albumen, fibrin, 
and casein, which have therefore, chemically considered, the 
Same composition, and in most cases the same properties, as 
their own blood, albumen, and muscular fibre. 
The azotized principles of vegetables may be divided into 
three modifications, from their behaviour towards ammonia and 
acetic acid, at the ordinary temperature. I have given the name 
_ of vegetable fibrin to that ingredient of the cereals, of wheat, rye, 
| barley, oats, buck-wheat, maize, and rice, which is insoluble in 
water and ammonia. It is not found in leguminous plants. 
The name vegetable albumen is applied by me to that azotized 
ingredient of the juice or other parts of plants which is held in 
solution, or is soluble in water and coagulates like animal albu- 
men, when boiled, and is not precipitated from solution by acetic 
acid. 
Vegetable casein is soluble in cold water: its solution does not 
VOL. I11. PART Xx. 8 
