FAMILIAR LETTERS ON CHEMISTRY. 19 



gum, sugar, beer, wine, spirits, &c., furnish no element capable of entering into 

 the composition of blood, muscular fibre, or any part which is the seat of the vital 

 principle. It must surely be universally interesting to trace the great change our 

 views have undergone upon these subjects, as well as to become acquainted with 

 the researches from which our present knowledge is derived. 



The primary conditions of the maintenance of animal life, are a constant supply 

 of certain matters, animal food, and of oxygen, in the shape of atmospheric air. 

 During every moment of life, oxygen is absorbed from the atmosphere in the organs 

 of respiration, and the act of breathing cannot cease while life continues. 



The observations of physiologists have demonstrated that the body of an adult 

 man supplied abundantly with food, neither increases nor diminishes in weight 

 during twenty-four hours, and yet the quantity of oxygen absorbed into his system^ t/ 

 in that period, is very considerable. According to the experiments of Lavoisier t ' 

 an adult man takes into his system from the atmosphere, in one year, no less than 

 seven hundred and forty-six pounds weight of oxygen ; the calculations of Men- 

 zies make the quantity amount even to eight hundred and thirty-seven pounds ; 

 but we find his weight at the end of the year either exactly the same, or differing 

 one way or other by at most a few pounds. What, it may be asked, has become 

 of the enormous amount of oxygen thus introduced into the human system in the 

 course of one year ? We can answer this question satisfactorily. No part of the 

 oxygen remains in the body, but is given out again, combined with carbon and hy- 

 drogen. The carbon and hydrogen of certain parts of the animal body combine 

 with the oxygen introduced through the lungs and skin, and pass off in the forms 

 of carbonic acid and vapor of water. At every expiration and every moment of 

 life, a certain amount of its elements are separated from the animal organism, 

 having entered into combination with the oxygen of the atmosphere. 



In order to obtain a basis for an approximate calculation, we may assume, with 

 Lavoisier and Seguin, that an adult man absorbs into his system 32 ounces of 

 oxygen daily that is, 46,037 cubic inches = 15,661 grains, French weight ; and 

 further, that the weight of the whole mass of his blood is twenty-four pounds, of 

 which eighty per cent, is water. Now, from the known composition of the blood, 

 we know that in order to convert its whole amount of carbon and hydrogen into 

 carbonic acid and water 64.102 grains of oxygen are required. This quantity will 

 be taken into the system in four days and five hours. Whether the oxygen enters 

 into combination directly with the elements of the blood, or with the carbon and^ 

 hydrogen of other parts of the body, it follows inevitably the weight of the body 1 

 remajjiing^uncha^ed, and in a^ normal^ condition that as much of these elements \ 

 as will suffice to supply t we nty-iou r ~"p~oun ds of blood, must be taken into the ! 

 system in four days and five hours ; and this necessary amount is furnished by ! 

 the food. 



We have not, however, remained satisfied with mere approximation : we have 

 determined accurately, in certain cases, the quantity of carbon taken daily in the 

 food, and of that which passes out of the body in the faeces and urine combined 

 that is, uncombined with oxygen; and from these investigations it appears that an 

 adult man, taking moderate exercise, consumes 13.910 ounces of carbon, which pass 

 off through the skin and lungs as carbonic acid gas.* 



It requires thirty-seven ounces of oxygen to convert 13.910 of carbon into carbonic 

 acid. Again : according to the analysis of Boussingault (Annales de Chim. et de 

 Phys. Ixx. i., p. 136,) a horse consumes 79.110 ounces of carbon in twenty-four 

 hours, a milch cow 70| ounces ; so that the horse requires 13 pounds 3i ounces, and 

 the cow 11 pounds 10J ounces of oxygen. f 



As no part of the oxygen taken into the system of an animal is given off in any 

 other form than combined with carbon or hydrogen, and as in a normal condition, 

 or state of health, the carbon and hydrogen so given off, are replaced by those 

 elements in the food, it is evident that the amount of nourishment required by an 

 animal for its support, must be in a direct ratio with the quantity of oxygen taken into 

 its system. Two animals, which in equal times take up by means of the lungs and skin 

 unequal quantities of oxygen, consume an amount of food unequal in the same ratio. 

 The consumption of oxygen in a given time may be expressed by the number of 



* This account is deduced from observations made upon the average daily consumption of 

 about thirty soldiers in barracks. The food of these men, consisting of meat, bread, potatoes, 

 lentiles, peas, beans, butter, salt, pepper, &c., was accurately weighed during a month, and 

 each article subjected to ultimate analysis. Of the quantity of food, beer, and spirits, taken 

 by the men when out of barracks, we have a close approximation from the report of the 

 sergeant ; and from the weight and analysis of the faeces and urine, it appears that the carbon 

 which passe* off through these channels may be considered equivalent to the amount taken 

 in that portion of the food, and of sour-crout, which was not included in the estimate. 

 f!6 ounces=i kilogramme. 



