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Liebig's Chemical Letters



My dear Sir,

    One of the most remarkable effects of the recentprogress of science is the alliance of chemistry with physiology, by which a newand unexpected light has been thrown upon the vital processes of plants and animals.We have now no longer any difficulty in understanding the different actions of aliments,poisons, and remedial agents - we have a clear conception of the causes of hunger,of the exact nature of death; and we are not, as formerly, obliged to content ourselveswith a mere description of their symptoms. It is now ascertained with positive certainty,that all the substances which constitute the food of man must be divided into twogreat classes, one of which serves for the nutrition and reproduction of the animalbody, whilst the other ministers to quite different purposes. Thus starch, gum, sugar,beer, wine, spirits, &c., furnish no element capable of entering into the compositionof blood, muscular fibre, or any part which is the seat of the vital principle. Itmust surely be universally interesting to trace the great change our views have undergoneupon these subjects, as well as to become acquainted with the researches from whichour present knowledge is derived.

    The primary conditions of the maintenance ofanimal 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 absorbedfrom the atmosphere in the organs of respiration, and the act of breathing cannotcease while life continues.

    The observations of physiologists have demonstratedthat the body of an adult man supplied abundantly with food, neither increases nordiminishes in weight during twenty-four hours, and yet the quantity of oxygen absorbedinto his system, in that period, is very considerable. According to the experimentsof Lavoisier, an adult man takes into his system from the atmosphere, in one year,no less than 746 pounds weight of oxygen; the calculations of Menzies make the quantityamount even to 837 pounds; but we find his weight at the end of the year either exactlythe same or different one way or the other by at most a few pounds. What, it maybe asked, has become of the enormous amount of oxygen thus introduced into the humansystem in the course of one year? We can answer this question satisfactorily. Nopart of the oxygen remains in the body, but is given out again, combined with carbonand hydrogen. The carbon and hydrogen of certain parts of the animal body combinewith the oxygen introduced through the lungs and skin, and pass off in the formsof carbonic acid and vapour of water. At every expiration and every moment of life,a certain amount of its elements are separated from the animal organism, having enteredinto combination with the oxygen of the atmosphere.

    In order to obtain a basis for the approximatecalculation, we may assume, with Lavoisier and Seguin, that an adult man absorbsinto his system 32ù5 ounces of oxygen daily, - that is, 46,037 cubic inches= 15,661 grains, French weight; and further, that the weight of the whole mass ofhis blood is 24 pounds, of which 80 per cent. is water. Now, from the known compositionof the blood, we know that in order to convert its whole amount of carbon and hydrogeninto carbonic acid and water, 64ù102 grains of oxygen are required. This quantitywill be taken into the system in four days and five hours. Whether the oxygen entersinto combination directly with the elements of the blood, or with the carbon andhydrogen of other parts of the body, it follows inevitably - the weight of the bodyremaining unchanged and in a normal condition - that as much of these elements aswill suffice to supply 24 pounds of blood, must be taken into the system in fourdays and five hours; and this necessary amount is furnished by the food.

    We have not, however, remained satisfied withmere approximation: we have determined accurately, in certain cases, the quantityof carbon taken daily in the food, and of that which passes out of the body in thefaeces and urine combined - that is, uncombined with oxygen; and from these investigationsit appears that an adult man taking moderate exercise consumes 13.9 ounces of carbon,which pass off through the skin and lungs as carbonic acid gas.1

    It requires 37 ounces of oxygen to convert 13ù9of carbon into carbonic acid. Again; according to the analysis of Boussingault, (Annalesde Chim. et de Phys., lxx. i. p.136), a horse consumes 79ù1 ounces of carbonin twenty-four hours, a milch cow 70ù75 ounces; so that the horse requires 13pounds 3ù5 ounces, and the cow 11 pounds 10ù75 ounces of oxygen.2

    As no part of the oxygen taken into the systemof 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 givenoff are replaced by those elements in the food, it is evident that the amount ofnourishment required by an animal for its support must be in a direct ratio withthe quantity of oxygen taken in to its system. Two animals which in equal times takeup by means of the lungs and skin unequal quantities of oxygen, consume an amountof food unequal in the same ratio. The consumption of oxygen in a given time maybe expressed by the number of respirations; it is, therefore, obvious that in thesame animal the quantity of nourishment required must vary with the force and numberof respirations. A child breathes quicker than an adult, and, consequently, requiresfood more frequently and proportionably in larger quantity, and bears hunger lesseasily. A bird deprived of food dies on the third day, while a serpent, confinedunder a bell, respires so slowly that the quantity of carbonic acid generated inan hour can scarcely be observed, and it will live three months, or longer, withoutfood. The number of respirations is fewer in a state of rest than during labour orexercise: the quantity of food necessary in both cases must be in the same ratio.An excess of food, a want of a due amount of respired oxygen, or of exercise, asalso great exercise (which obliges us to take an increased supply of food), togetherwith weak organs of digestion, are incompatible with health

    But the quantity of oxygen received by an animalthrough the lungs not only depends upon the number of respirations, but also uponthe temperature of the respired air. The size of the thorax of an animal is unchangeable;we may therefore regard the volume of air which enters at every inspiration as uniform.But its weight, and consequently the amount of oxygen it contains, is not constant.Air is expanded by heat, and contracted by cold - an equal volume of hot and coldair contains, therefore, an unequal amount of oxygen. In summer atmospheric air containswater in the form of vapour, it is nearly deprived of it in winter; the volume ofoxygen in the same volume of air is smaller in summer than in winter. In summer andwinter, at the pole and at the equator, we inspire an equal volume of air; the coldair is warmed during respiration and acquires the temperature of the body. In order,therefore, to introduce into the lungs a given amount of oxygen, less expenditureof force is necessary in winter than in summer, and for the same expenditure of forcemore oxygen is inspired in winter. It is also obvious that in an equal number ofrespirations we consume more oxygen at the level of the sea than on a mountain.

    The oxygen taken into the system is given outagain in the same form, both in summer and winter: we expire more carbon at a lowthan at a high temperature, and require more or less carbon in our food in the sameproportion; and, consequently, more is respired in Sweden than in Sicily, and inour own country and eighth more in winter than in summer. Even if an equal weightof food is consumed in hot and cold climates, Infinite Wisdom has ordained that veryunequal proportions of carbon shall be taken in it. The food prepared for the inhabitantsof southern climes does not contain in a fresh state more than 12 per cent. of carbon,while the blubber and train oil which feed the inhabitants of Polar regions contain66 to 80 per cent. of that element.

    From the same cause it is comparatively easyto be temperate in warm climates, or to bear hunger for a long time under the equator;but cold and hunger united very soon produce exhaustion.

    The oxygen of the atmosphere received into theblood in the lungs, and circulated throughout every part of the animal body, actingupon the elements of the food, is the source of animal heat.



    1.This account is deduced from observationsmade upon the average daily consumption of about 30 soldiers in barracks. The foodof these men, consisting of meat, bread, potatoes, lentils, peas, beans, butter,salt, pepper, &c., was accurately weighed during a month, and each article subjectedto ultimate analysis. Of the quantity of food, beer, and spirits, taken by the menwhen 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 carbonwhich passes off through these channels may be considered equivalent to the amounttaken in that portion of the food, and of sour-crout, which was not included in theestimate.


    2.17.5 ounces = 0.5 kilogramme.