NUTRITIVE PRINCIPLES OF PLANTS. 247 
of nitrogen; the only animal substances indeed which do not 
contain that element, are water and fat, both without any di- 
stinct form, both without vital action, and serving only as con- 
nexions in the organization. Animals must receive as much 
nitrogen in their food, as is excreted and removed from the body 
by the decomposition of the organs ; young animals must receive 
more, for the growth of these organs. When the proper quan- 
tity of nitrogen necessary for the reproduction of the decomposed 
parts is not found in the nutriment, the equilibrium is of course 
destroyed, the body can no longer grow, but must, on the con- 
trary, decrease. Experiments have been made on the nutrition 
of animals, by means of sugar, gum, and starch, and it has been 
sufficiently proved that the vital action of animals is inadequate 
to the production of nitrogen, or any other element ; for animals 
fed only with the nutriments mentioned invariably die of star- 
vation. 
The first case supposed above is the only one in which sub- 
stances destitute of nitrogen can be of use in supplying the loss 
sustained in the organs of animals ; namely, if the azotized prin- 
ciples of vegetables contain the same number of atoms of nitrogen, 
but a smaller proportion of carbon, than the blood and other 
component parts of animals. Then may sugar, and bodies of a 
similar composition, be available in the formation of the organism 
of animals, by imparting carbon. 
Several talented and skilful chemists and physicians have de- 
voted themselves, in this laboratory, during the past year, to the 
‘investigation of the composition of albumen, fibrin, the mem- 
branes, and gelatinous parts of animals, and the azotized com- 
pounds of vegetables. The preparation of the vegetable sub- 
stances for analysis was undertaken by myself, and it is my ob- 
ject at present to communicate the principal conclusions of the 
whole inquiry. 
Drs. Scherer and Jones will publish, in their own papers, a de- 
scription of their experiments, with the more detailed results of 
their analyses; and I shall content myself at present with speak- 
ing only of their general relations, and of the results, which are 
independent of the percentage of the elements. 
Fibrin, albumen, and casein, as they are found in nature, differ 
very much in their external properties and structure, and in their 
relations towards water and heat. 
Fibrin derived from the blood is perfectly insoluble in cold 
i 
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