DEVELOPMENT OF HEAT. 683 



and Despretz.* Although, therefore, the Chemical doctrine of Calorification 

 cannot be regarded as yet perfected as to its details, there can be no reasonable 

 doubt that it is altogether sufficient to account for the phenomena in question. 

 And it may be stated as a general fact, that the production of Animal Heat is 

 due to the various changes in Chemical composition, that are continually 

 taking place within the system; of which changes, the absorption of Oxygen, 

 and the disengagement of Carbonic Acid, are the two chief external manifesta- 

 tions: and that the degree of Caloric liberated bears a close relation to the 

 activity of these changes, either in regard to the body at large, or to any 

 portion of it. 



893. The researches of Dr. Edwards upon Animal Heat have brought to 

 light some very interesting facts, regarding the diversity which exists as to 

 the power of generating heat, in the same species of animal, at different ages, 

 and at different periods of the year. 



a. It appears to be a general fact, that the younger the animal, the less is its independent 

 calorifying power. The development of the embryo of oviparous animals is entirely de- 

 pendent upon the amount of external warmth supplied to it; and there are many kinds of 

 Birds, which, at the time they issue from the egg, are so deficient in the power of generating 

 heat, that their temperature rapidly falls, when they are removed from the nest and placed 

 in a cold atmosphere. It was shown by collateral experiments, that the loss of heat was 

 not to be attributed to the absence of feathers, nor to the extent of surface exposed in com- 

 parison with the bulk of the body; and that nothing but an absolute deficiency in the power 

 of generating it, would account for the fall of temperature. This is quite conformable to 

 facts well ascertained in regard to Mammalia. The fetus, during intra-uterine life, has 

 little power of keeping up its own temperature; and in many cases it is much dependent 

 on external warmth, for some time after birth. The degree of this dependence, however, 

 differs greatly in the various species of Mammalia, as among Birds: being less, in propor- 

 tion as the general development is advanced. Thus, young Guinea pigs, which can run 

 about and pick up food for themselves almost as soon as they are born, are from the first 

 independent of parental warmth ; whilst, on the other hand, the young of Dogs, Cats, Rab- 

 bits, &c., which are born blind, and which do not, for a fortnight or more, acquire the same 

 development with the preceding, rapidly lose their heat when withdrawn from contact with 

 the body of the mother. 



b. In the Human species it is well known, that external warmth is necessary for the 

 Infant; but the fact is too often neglected (under the erroneous idea of hardening the con- 

 stitution) during the early years of childhood. It is to be carefully remembered, that the 

 development of Man is slower than that of any other animal; and that his calorifying 

 power is closely connected with his general bodily vigour. In the case of children born 

 very prematurely, the greatest attention must be given to the sustenance of the heat of the 

 body ( 932) ; and though the infant becomes more independent of it as development 

 advances, it is many years before the standard can be maintained without assistance, 

 throughout the ordinary vicissitudes of external temperature. The calorifying power, 

 which is fully possessed by adults, decreases again in advanced age. Old people complain 

 that their " blood is chill;" and they suffer greatly from exposure to cold, the temperature of 

 their whole body being lowered by it. 



c. Tbese facts have a very interesting connection with the results of statistical inquiries, 

 as to the average number of deaths at different seasons, recorded by M. Quetelet.j- 



* It has been recently shown by Liebig, that the discrepancy between the actual amount 

 of heat generated, and the amount which was calculated to have been produced by the 

 union of Carbon and Hydrogen with Oxygen, to form the Carbonic Acid and Water exhaled, 

 in these experiments, may be nearly reconciled by adopting a more correct estimate of the 

 heat generated by combustion of given quantities of Carbon and Hydrogen respectively. But 

 all tlrese calculations proceed upon the supposition, that the whole amount of Oxygen ab- 

 sorbed, which is not exhaled as Carbonic acid, is exhaled in combination with Hydrogen, as 

 Water; and thus no account is taken of other combustion-processes going on in the body, by 

 which a greater amount of heat may be generated, than by the combustion of Hydrogen. 

 We have no means whatever of ascertaining how much of the watery vapour thrown off 

 by the lungs and skin is actually formed within the body, and how much is the mere super- 

 fluity of the liquid ingested. 



| Essai de Physique Sociale, torn. i. p. 197. 



