FAMILIAR LETTERS ON CHEMISTRY. 29 



is, for the production of blood ; substances which may be entirely dispensed with 

 in their nourishment in the adult state. In the young of carnivorous birds, the 

 want of all motion is an obvious cause of diminished waste in the organized parts ; 

 hence, milk is not provided for them. 



The nutritive process in the carnivora thus presents itself under two distinct 

 forms ; one of which we again meet with in the graminivora. 



In graminivorous animals, we observe, that during their whole life, their exist- 

 ence depends on a supply of substances having a composition identical with that 

 of sugar of milk, or closely resembling it. Every thing that they consume as food 

 contains a Certain quantity of starch, gum, or sugar, mixed with other matters. 



The function performed in the vital process of the graminivora by these sub- 

 stances is indicated in a very clear and convincing manner, when we take into con- 

 sideration the very small relative amount of the carbon which these animals con- 

 sume in the nitrogenized constituents of their food, which bears no proportion 

 whatever to the oxygen absorbed through the skin and lungs. 



A horse, for example, can be kept in perfectly good condition, if he obtain as 

 food fifteen pounds of hay and four and a half pounds of oats, daily. If we now 

 calculate the whole amount of nitrogen in these matters, as ascertained by analysis 

 (1.5 per cent, in the hay, 2.2 per cent, in the oats), in the form of blood, that is, as 

 h'brine and albumen, with the due proportion of water in blood (eighty per cent.), 

 the horse receives daily no more than four and a half ounces of nitrogen, corres- 

 ponding to about eight pounds of blood. But along with this nitrogen, that is, 

 combined with it in the form of fibrine or albumen, the animal receives only about 

 fourteen and a half ounces of carbon. 



Without going further into the calculation, it will readily be admitted, that the 

 volume of air inspired and expired by a horse, the quantity of oxygen consumed, 

 and, as a necessary consequence, the amount of carbonic acid given out by the 

 animal, is much greater than in the respiratory process in man. But an adult man 

 consumes daily about fourteen ounces of carbon, and the determination of Bous- 

 singault, according to which a horse expires seventy-nine ounces daily, cannot be 

 very far from the truth. 



In the nitrogenized constituents of his food, therefore, 'the horse receives rather 

 less than the fifth part of the carbon which his organism requires for the support 

 of the respiratory process ; and we see that the wisdom of the Creator has added to 

 his food the four-fifths which are wanting, in various forms, as starch, sugar, &c., 

 with which the animal must be supplied, or his organism will be destroyed by the 

 action of the oxygen. 



It is obvious, that, in the system of the graminivora, whose food contains so small 

 a portion, relatively, of the constituents of blood, the process of metamorphosis in 

 existing tissues, and consequently their restoration or reproduction, must go on far 

 less rapidly than in the carnivora. Were this not the case, a vegetation a thousand 

 times more luxuriant than the actual one would not suffice for their nourishment. 

 Sugar, gum, and starch, would no longer be necessary to support life in these 

 animals, because, in that case, the products of the waste, or metamorphosis of the 

 organized tissues, would contain enough of carbon to support the respiratory 

 process. 



LETTER X. 



MY DEAR SIR : 



Let me now apply the principles announced in the preceding letters to the 

 circumstances of our own species. Man, when confined to animal food, requires 

 for his support and nourishment extensive sources of food, even more widely 

 extended than the lion and tiger, because, when he has the opportunity, he kills 

 without eating. 



A nation of hunters, on a limited space, is utterly incapable of increasing its 

 numbers beyond a certain point, which is soon attained. The carbon necessary 

 for respiration must be obtained from the animals, of which only a limited number 

 can live on the space supposed. These animals collect from plants the constituents 

 of their organs and of their blood, and yield them, in turn, to the savages who live 

 by the chase alone. They, again, receive this food unaccompanied by those com- 



