liebig's organic chemistry. 
191 
trogen are superadded; or, in other words, proteine only re- 
quires the addition of these elements to constitute any ele- 
mentary tissue. 
" How beautiful and admirably simple, (exclaims the au- 
thor,) with the aid of these discoveries, appears the process 
of nutrition in animals, the formation of their organs, in 
which vitality resides!" 
These substancesall contain nitrogen; but other substances, 
which do not contain this element, appear essential to the 
well-being of a certain class of animals ; these are, sugar, 
starch, gum, &c. These three substances are nearly similar; 
all consisting of the same amount of carbon, united with dif- 
ferent amounts of hydrogen and oxygen, with proportions to 
form water. 
The author considers that it is the carbon alone which 
here acts an efficient part in the animal economy, and that 
it is destined to supply, in gramnivorous animals, that ele- 
ment essential to the production of heat, and which would 
otherwise be insufficient, as the amount of carbon taken in 
with the food would be incapable of producing, with the 
oxygen, the quantity of carbonic acid which is thrown off, 
in addition to that which is present in the secretions. These 
views are supported by a well-chosen series of arguments, 
in which the process of conversion is traced in all its de- 
tails. 
It is to the substances destitute of nitrogen that our author 
looks for the formation of fat,— a substance in the animal 
structure destitute of nitrogen, and without traces of organi- 
zation. These substances being required to supply carbon, 
which, by its combination with oxygen, forms the expired 
carbonic acid, let us look to the result of continuing their 
supply, and diminishing its necessity. An illustration is 
drawn from the stall-fed animal: it eats, and reposes merely 
for digestion. Without exercise, and protected from cold 
it absorbs less oxygen, and, consequently, less carbonic acid 
is produced. The excess of carbon must be appropriated to 
