FAMILIAR LETTERS ON CHEMISTRY. 39 



upon our fields, it is highly important to keep in mind the source whence they are 

 derived. 



It is generally known, that if we deprive an animal of food, the weight of its 

 body diminishes during every moment of its existence. If this abstinence is con- 

 tinued for some time, the diminution becomes apparent to the eye ; all the fat of 

 the body disappears, the muscles decrease in firmness and bulk, and, if the animal 

 is allowed to die starved, scarcely any thing but skin, tendon, and bones, remain. 

 This emaciation which occurs in a body otherwise healthy, demonstrates to us, that 

 during the life of an animal every part of its living substance is undergoing a per- 

 petual change ; all its component parts, assuming the form of lifeless compounds, 

 are thrown off by the skin, lungs, and urinary system, altered more or less by the 

 secretory organs. This change in the living body is intimately connected with the 

 process of respiration ; it is, in truth, occasioned by the oxygen of the atmosphere 

 in breathing, which combines with all the various matters within the body. At 

 every inspiration a quantity of oxygen passes into the blood in the lungs, and 

 unites with its elements ; but although the weight of the oxygen thus daily entering 

 into the body amounts to thirty-two or more ounces, yet the weight of the body is 

 not thereby increased. Exactly as much oxygen as is imbibed in inspiration passes 

 off in expiration, in the form of carbonic acid and water ; so that with every breath 

 the amount of carbon and hydrogen in the bodjr is diminished. But the emacia- 

 tion the loss of weight by starvation does not simply depend upon the separation 

 of the carbon and hydrogen ; but all the other substances which are in combination 

 with these elements in the living tissues pass off in the secretions. The nitrogen 

 undergoes a change, and is thrown out of the system by the kidneys. Their 

 secretion, the urine, contains not only a compound rich in nitrogen, namely urea, 

 but the sulphur of the tissues in the form of a sulphate, all the soluble salts of the 

 blood and animal fluids, common salt, the phosphates, soda, and potash. The car- 

 bon and hydrogen of the blood of the muscular fibre, and of all the animal tissues 

 which can undergo change, return into the atmosphere. The nitrogen, and all the 

 soluble inorganic elements, are carried to the earth in the urine. 



These changes take place in the healthy animal body during every moment of 

 life ; a waste and loss of substance proceeds continually ; and if this loss is to be 

 restored, and the original weight and substance repaired, an adequate supply of 

 materials must be furnished whence the blood and wasted tissues may be regene- 

 rated. This supply is obtained from the food. 



In an adult person in a normal or healthy condition, no sensible increase or 

 decrease of weight occurs from day^to day. In youth the weight of the body 

 increases, while in old age it decreases. There can be no doubt that in the adult 

 the food has exactly replaced the loss of substance: it has supplied. just so much 

 carbon, hydrogen, nitrogen, and other elements, as have passed through the skin, 

 lungs, and urinary organs. In youth the supply is greater than the waste. Part 

 of the elements of the food remain to augment the bulk of the body. In old age 

 the waste is greater than the supply, and the body diminishes. It is unquestionable, 

 that, with the exception of a certain quantity of carbon and hydrogen, which are 

 secreted through the skin and lungs, we obtain, in the solid and fluid excrements 

 of man and animals, all the elements of their food. 



We obtain daily, in the form of urea, all the nitrogen taken in the food both of 

 the young and the adult ; and further, in the urine, the whole amount of the alkalies, 

 soluble phosphates and sulphates contained in all the various aliments. In the solid 

 excrements are found all those substances taken in the food which have undergone 

 no alteration in the digestive organs, all indigestible matters, such as woody fibre, 

 the green coloring matter of leaves (chlorophyle,) wax, &c. 



Physiology teaches us that the process of nutrition in animals, that is, their 

 increase of bulk, or the restoration of wasted parts, proceeds from the blood. In 

 the stomach and intestines, therefore, all those substances in the food capable of 

 conversion into blood are separated from its other constituents ; in other words, 

 during the passage of the food through the intestinal canal there is a constant 

 absorption of its nitrogen, since only azotized substances are capable of conversion 

 into blood ; and, therefore, the solid excrements are destitute of that element, except 

 only a small portion, in the constitution of that secretion which is formed to facili- 

 tate their passage. With the solid excrements, the phosphates of lime and magnesia, 

 which were contained in the food and not assimilated, are carried off, these salts being 

 insoluble in water, and therefore not entering the urine. 



We may obtain a clear insight into the chemical constitution of the solid excre- 

 ments without further investigation, by comparing the faeces of a dog with his food. 

 We give that animal flesh and bones substances rich in azotized matter and we 

 obtain, as the last product of its digestion, a perfectly white excrement, solid while 



