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Liebig's Chemical Letters



My dear Sir,

    The source of animal heat, its laws, and theinfluence it exerts upon the functions of the animal body, constitute a curious andhighly interesting subject, to which I would now direct your attention.

    All living creatures, whose existence dependsupon the absorption of oxygen, possess within themselves a source of heat, independentof surrounding objects.

    This general truth applies to all animals, andextends to the seed of plants in the act of germination, to flower-buds when developing,and fruits during their maturation.

    In the animal body, heat is produced only inthose parts to which arterial blood, and with it the oxygen absorbed in respiration,is conveyed. Hair, wool, and feathers, receive no arterial blood, and, therefore,in them no heat is developed. The combination of a combustible substance with oxygenis, under all circumstances, the only source of animal heat. In whatever way carbonmay combine with oxygen, the act of combination is accompanied by the disengagementof heat. It is indifferent whether this combination takes place rapidly or slowly,at a high or at a low temperature: the amount of heat liberated is a constant quantity.

    The carbon of the food, being converted intocarbonic acid within the body, must give out exactly as much heat as if it had beendirectly burnt in oxygen gas or in common air; the only difference is, the productionof the heat is diffused over unequal times. In oxygen gas the combustion of carbonis rapid and the heat intense; in atmospheric air it burns slower and for a longertime, the temperature being lower; in the animal body the combination is still moregradual, and the heat is lower in proportion.

    It is obvious that the amount of heat liberatedmust increase or diminish with the quantity of oxygen introduced in equal times byrespiration. Those animals, therefore, which respire frequently, and consequentlyconsume much oxygen, possess a higher temperature than others, which, with a bodyof equal size to be heated, take into the system less oxygen. The temperature ofa child (102ø) is higher than that of an adult (99ù5ø). That of birds(104ø to 105ù4ø) is higher than that of quadrupeds (98ù5øto 100ù4ø) or than that of fishes or amphibia, whose proper temperatureis from 2ù7 to 3ù6ø higher than that of the medium in which they live.All animals, strictly speaking, are warm-blooded; but in those only which possesslungs is the temperature of the body quite independent of the surrounding medium.

    The most trustworthy observations prove thatin all climates, in the temperate zones as well as at the equator or the poles, thetemperature of the body in man, and in what are commonly called warm-blooded animals,is invariably the same; yet how different are the circumstances under which theylive!

    The animal body is a heated mass, which bearsthe same relation to surrounding objects as any other heated mass. It receives heatwhen the surrounding objects are hotter, it loses heat when they are colder, thanitself.

    We know that the rapidity of cooling increaseswith the difference between the temperature of the heated body and that of the surroundingmedium; that is, the colder the surrounding medium the shorter the time requiredfor the cooling of the heated body.

    How unequal, then, must be the loss of heatin a man at Palermo, where the external temperature is nearly equal to that of thebody, and in the polar regions, where the external temperature is from 70ø to90ø lower!

    Yet, notwithstanding this extremely unequalloss of heat, experience has shown that the blood of the inhabitant of the arcticcircle has a temperature as high as that of the native of the south, who lives inso different a medium.

    This fact, when its true significance is perceived,proves that the heat given off to the surrounding medium is restored within the bodywith great rapidity. This compensation must consequently take place more rapidlyin winter than in summer, at the pole than at the equator.

    Now, in different climates the quantity of oxygenintroduced into the system by respiration, as has been already shown, varies accordingto the temperature of the external air; the quantity of inspired oxygen increaseswith the loss of heat by external cooling, and the quantity of carbon or hydrogennecessary to combine with this oxygen must be increased in the same ratio.

    It is evident that the supply of the heat lostby cooling is effected by the mutual action of the elements of the food and the inspiredoxygen, which combine together. To make use of a familiar, but not on that accounta less just illustration, the animal body acts, in this respect, as a furnace, whichwe supply with fuel. It signifies nothing what intermediate forms food may assume,what changes it may undergo in the body; the last change is uniformly the conversionof its carbon into carbonic acid, and of its hydrogen into water. The unassimilatednitrogen of the food, along with the unburned or unoxidised carbon, is expelled inthe urine or in the solid excrements. In order to keep up in the furnace a constanttemperature, we must vary the supply of fuel according to the external temperature,that is, according to the supply of oxygen.

    In the animal body the food is the fuel; witha proper supply of oxygen we obtain the heat given out during its oxidation or combustion.In winter, when we take exercise in a cold atmosphere, and when consequently theamount of inspired oxygen increases, the necessity for food containing carbon andhydrogen increases in the same ratio; and by gratifying the appetite thus excited,we obtain the most efficient protection against the most piercing cold. A starvingman is soon frozen to death. The animals of prey in the arctic regions, as everyone knows, far exceed in voracity those of the torrid zone.

    In cold and temperate climates, the air, whichincessantly strives to consume the body, urges man to laborious efforts in orderto furnish the means of resistance to its action, while, in hot climates, the necessityof labour to provide food is far less urgent.

    Our clothing is merely an equivalent for a certainamount of food. The more warmly we are clothed the less urgent becomes the appetitefor food, because the loss of heat by cooling, and consequently the amount of heatto be supplied by the food, is diminished.

    If we were to go naked, like certain savagetribes, or if in hunting or fishing we were exposed to the same degree of cold asthe Samoyedes, we should be able with ease to consume 10 lbs. of flesh, and perhapsa dozen of tallow candles into the bargain, daily, as warmly clad travellers haverelated with astonishment of these people. We should then also be able to take thesame quantity of brandy or train oil without bad effects, because the carbon andhydrogen of these substances would only suffice to keep up the equilibrium betweenthe external temperature and that of our bodies.

    According to the preceding expositions, thequantity of food is regulated by the number of respirations, by the temperature ofthe air, and by the amount of heat given off to the surrounding medium.

    No isolated fact, apparently opposed to thisstatement, can affect the truth of this natural law. Without temporary or permanentinjury to health, the Neapolitan cannot take more carbon and hydrogen in the shapeof food than he expires as carbonic acid and water; and the Esquimaux cannot expiremore carbon and hydrogen than he takes in the system as food, unless in a state ofdisease or of starvation. Let us examine these states a little more closely.

    The Englishman in Jamaica perceives with regretthe disappearance of his appetite, previously a source of frequently recurring enjoyment;and he succeeds, by the use of cayenne pepper, and the most powerful stimulants,in enabling himself to take as much food as he was accustomed to eat at home. Butthe whole of the carbon thus introduced into the system is not consumed; the temperatureof the air is too high, and the oppressive heat does not allow him to increase thenumber of respirations by active exercise, and thus to proportion the waste to theamount of food taken; disease of some kind, therefore, ensues.

    On the other hand, England sends her sick tosouthern regions, where the amount of the oxygen inspired is diminished in a verylarge proportion. Those whose diseased digestive organs have in a greater or lessdegree lost the power of bringing the food into the state best adapted for oxidation,and therefore are less able to resist the oxidising influence of the atmosphere oftheir native climate, obtain a great improvement in health. The diseased organs ofdigestion have power to place the diminished amount of food in equilibrium with theinspired oxygen, in the mild climate; whilst in a colder region the organs of respirationthemselves would have been consumed in furnishing the necessary resistance to theaction of the atmospheric oxygen.

    In our climate, hepatic diseases, or those arisingfrom excess of carbon, prevail in summer; in winter, pulmonary diseases, or thosearising from excess of oxygen, are more frequent.

    The cooling of the body, by whatever cause itmay be produced, increases the amount of food necessary. The mere exposure to theopen air, in a carriage or on the deck of a ship, by increasing radiation and vaporisation,increases the loss of heat, and compels us to eat more than usual. The same is trueof those who are accustomed to drink large quantities of cold water, which is givenoff at the temperature of the body, 98ù5ø. It increases the appetite, andpersons of weak constitution find it necessary, by continued exercise, to supplyto the system the oxygen required to restore the heat abstracted by the cold water.Loud and long continued speaking, the crying of infants, moist air, all exert a decidedand appreciable influence on the amount of food which is taken.

    We have assumed that carbon and hydrogen especially,by combining with oxygen, serve to produce animal heat. In fact, observation provesthat the hydrogen of the food plays a no less important part than the carbon.

    The whole process of respiration appears mostclearly developed, when we consider the state of a man, or other animal, totallydeprived of food.

    The first effect of starvation is the disappearanceof fat, and this fat cannot be traced either in the urine or in the scanty faeces.Its carbon and hydrogen have been given off through the skin and lungs in the formof oxidised products; it is obvious that they have served to support respiration.

    In the case of a starving man, 32ù5 oz.of oxygen enter the system daily, and are given out again in combination with a partof his body. Currie mentions the case of an individual who was unable to swallow,and whose body lost 100 lbs. in weight during a month; and, according to Martell(Trans. Linn. Soc., vol. xi. p.411), a fat pig, overwhelmed in a slip of earth, lived160 days without food, and was found to have diminished in weight, in that time,more than 120 lbs. The whole history of hybernating animals, and the well-establishedfacts of the periodical accumulation, in various animals, of fat, which, at otherperiods, entirely disappears, prove that the oxygen, in the respiratory process,consumes, without exception, all such substances as are capable of entering intocombination with it. It combines with whatever is presented to it; and the deficiencyof hydrogen is the only reason why carbonic acid is the chief product; for, at thetemperature of the body, the affinity of hydrogen for oxygen far surpasses that ofcarbon for the same element.

    We know, in fact, that the graminivora expirea volume of carbonic acid equal to that of the oxygen inspired, while the carnivora,the only class of animals whose food contains fat, inspire more oxygen than is equalin volume to the carbonic acid expired. Exact experiments have shown, that in manycases only half the volume of oxygen is expired in the form of carbonic acid. Theseobservations cannot be gainsaid, and are far more convincing than those arbitraryand artificially produced phenomena, sometimes called experiments; experiments which,made as too often they are, without regard to the necessary and natural conditions,possess no value, and may be entirely dispensed with; especially when, as in thepresent case, Nature affords the opportunity for observation, and when we make arational use of that opportunity.

    In the progress of starvation, however, it isnot only the fat which disappears, but also, by degrees all such of the solids asare capable of being dissolved. In the wasted bodies of those who have suffered starvation,the muscles are shrunk and unnaturally soft, and have lost their contractibility;all those parts of the body which were capable of entering into the state of motionhave served to protect the remainder of the frame from the destructive influenceof the atmosphere. Towards the end, the particles of the brain begin to undergo theprocess of oxidation, and delirium, mania, and death close the scene; that is tosay, all resistance to the oxidising power of the atmospheric oxygen ceases, andthe chemical process of eremacausis, or decay, commences, in which every part ofthe body, the bones excepted, enters into combination with oxygen.

    The time which is required to cause death bystarvation depends on the amount of fat in the body, on the degree of exercise, asin labour or exertion of any kind, on the temperature of the air, and finally, onthe presence or absence of water. Through the skin and lungs there escapes a certainquantity of water, and as the presence of water is essential to the continuance ofthe vital motions, its dissipation hastens death. Cases have occurred, in which afull supply of water being accessible to the sufferer, death has not occurred tillafter the lapse of twenty days. In one case, life was sustained in this way for theperiod of sixty days.

    In all chronic diseases death is produced bythe same cause, namely, the chemical action of the atmosphere. When those substancesare wanting, whose function in the organism is to support the process of respiration,when the diseased organs are incapable of performing their proper function of producingthese substances, when they have lost the power of transforming the food into thatshape in which it may, by entering into combination with the oxygen of the air, protectthe system from its influence, then, the substance of the organs themselves, thefat of the body, the substance of the muscles, the nerves, and the brain, are unavoidablyconsumed.

    The true cause of death in these cases is therespiratory process, that is, the action of the atmosphere.

    A deficiency of food, and a want of power toconvert the food into a part of the organism, are both, equally, a want of resistance;and this is the negative cause of the cessation of the vital process. The flame isextinguished, because the oil is consumed; and it is the oxygen of the air whichhas consumed it.

    In many diseases substances are produced whichare incapable of assimilation. By the mere deprivation of food, these substancesare removed from the body without leaving a trace behind; their elements have enteredinto combination with the oxygen of the air.

    From the first moment that the function of thelungs or of the skin is interrupted or disturbed, compounds, rich in carbon, appearin the urine, which acquires a brown colour. Over the whole surface of the body oxygenis absorbed, and combines with all the substances which offer no resistance to it.In those parts of the body where the access of oxygen is impeded; for example, inthe arm-pits, or in the soles of the feet, peculiar compounds are given out, recognisableby their appearance, or by their odour. These compounds contain much carbon.

    Respiration is the falling weight - the bentspring, which keeps the clock in motion; the inspirations and expirations are thestrokes of the pendulum which regulate it. In our ordinary time-pieces, we know withmathematical accuracy the effect produced on their rate of going, by changes in thelength of the pendulum, or in the external temperature. Few, however, have a clearconception of the influence of air and temperature on the health of the human body;and yet the research into the conditions necessary to keep it in the normal stateis not more difficult than in the case of a clock.