28 FAMILIAR LETTERS ON CHEMISTRY. 



constituents of food, a certain amount of carbon ; or, as in the case of butter, of 

 carbon and hydrogen ; that is, an excess of elements, which cannot possibly be 

 employed in the production of blood, because the nitrogenised substances contained 

 in the food already contain exactly the amount of carbon which is required for the 

 production of fibrine and albumen. 



The following considerations will show that hardly a doubt can be entertained, 

 that this excess of carbon alone, or of carbon and hydrogen, is expended in the 

 production of animal heat, and serves to protect the organism from the action of 

 the atmospheric oxygen, which is required for the production of fibrine and 

 albumen. 



In an adult carnivorous animal, which neither gains nor loses weight, perceptibly, 

 from day to day, its nourishment, the waste of organized tissue, and its consump- 

 tion of oxygen, stand to each other in a well-defined and fixed relation. 



The carbon of the carbonic acid given off, with that of the urine; the nitrogen 

 of the urine, and the hydrogen given off as ammonia and water ; these elements, 

 taken together, must be exactly equal in weight to the carbon, nitrogen, and hydro- 

 gen of the metamorphosed tissues, and since these last are exactly replaced by the 

 food, to the carbon, nitrogen, and hydrogen of the food. Were this not the case, 

 the weight of the animal could not possibly remain unchanged. 



But, in the young of the carnivora, the weight does not remain unchanged ; on 

 the contrary, it increases from day to day by an appreciable quantity. 



This fact presupposes, that the assimilative process in the young animal is more 

 energetic, more intense, than the process of transformation in the existing tissues. 

 If both processes were equally active, the weight of the body could not increase ; 

 and were the waste by transformation greater, the weight of the body would 

 decrease. 



Now, the circulation in the young animal is not weaker, but, on the contrary, 

 more rapid ; the respirations are more frequent ; and, for equal bulks, the consump- 

 tion of oxygen must be greater rather than smaller in the young than in the adult 

 animal. But, since the metamorphosis of organized parts goes on more slowly, 

 there would ensue a deficiency of those substances, the carbon and hydrogen of 

 which are adapted for combination with oxygen ; because, in the carnivora, it is the 

 new compounds, produced by the metamorphosis of organized parts, which nature 

 has destined to furnish the necessary resistance to the action of the oxygen, and to 

 produce animal heat. What is wanting for these purposes Infinite Wisdom has 

 supplied to the young animal in its natural food. 



The carbon and hydrogen of butter, and the carbon of the sugar of milk, no part 

 of either of which can yield blood, fibrine, or albumen, are destined for the support 

 of the respiratory process, at an age when a greater resistance is opposed to the 

 metamorphosis of existing organisms ; or, in other words, to the production of com- 

 pounds, which, in the adult state, are produced in quantity amply sufficient for the 

 purpose of respiration. 



The young animal receives the constituents of its blood in the caseine of the 

 milk. A metamorphosis of existing organs goes on, for bile and urine are 

 secreted ; the matter of the metamorphosed parts is given off in the form of urine, 

 of carbonic acid, and of water ; but the butter and sugar of milk also disappear ; 

 they cannot be detected in the faeces. 



The butter and sugar of milk are given out in the form of carbonic acid and 

 water, and their conversion into oxidized products furnishes the clearest proof that 

 far more oxygen is absorbed than is required to convert the carbon and hydrogen 

 of the metamorphosed tissues into carbonic acid and water. 



The change and metamorphosis of organized tissues going on in the vital process 

 in the young animal, consequently yield, in a given time, much less carbon and 

 hydrogen in the form adapted for the respiratory process than corresponds to the 

 oxygen taken up in the lungs. The substance of its organized parts would undergo 

 a more rapid consumption, and would necessarily yield to the action of the oxygen, 

 were not the deficiency of carbon and hydrogen supplied from another source. 



The continued increase of mass, or growth, and the free and unimpeded develop- 

 ment of the organs in the young animal, are dependent on the presence of foreign 

 substances, which, in the nutritive process, have no other function than to protect 

 the newly-formed organs from the action of the oxygen. It is the elements of these 

 substances which unite with the oxygen ; the organs themselves could not do so 

 without being consumed ; that is, growth, or increase of mass in the bodythe 

 consumption of oxygen remaining the same would be utterly impossible. 



The preceding considerations leave no doubt as to the purpose for which Nature 

 has added to the food of the young of carnivorous mammalia substances devoid of 

 nitrogen, which their organism cannot employ for nutrition, strictly so called, that 



