668 



ANIMAL HEAT. 



early life, it is difficult to suppose that a young 

 animal will resist the action of intense cold in 

 the same manner as an adult. This inference 

 is fully borne out by the following experiment. 

 A young guinea-pig a month old, the tempe- 

 rature of whose body was high and steady, the 

 temperature of the external air being mild, was 

 exposed along with an adult to the same decree 

 of diminished temperature the air was at 0c. 

 (32 F.). In the course of an hour the young 

 creature had lost 9 c. in temperature, whilst 

 the adult had only lost 2,5 c. This experi- 

 ment, repeated several times with the same 

 species of animal, always gave the same result. 

 Young and adult birds of the same species, 

 treated in a similar manner, showed the same 

 diversity in their powers of resisting the effects 

 of external cold, from which we may infer that 

 the law is quite general. Several young mag- 

 pies, for instance, whose temperature was sta- 

 tionary in a mild spring atmosphere, were 

 placed with an adult in air cooled to +4 c. 

 After the lapse of twenty minutes, one of the 

 young ones was found to have lost 14 of tem- 

 perature. The others, examined at intervals, 

 none of which exceeded one hour and ten mi- 

 nutes in length, had cooled from 14 to 16 c. 

 The adult bird, on the contrary, similarly cir- 

 cumstanced, did not suffer a greater depression 

 of temperature than 3 c. The loss of heat 

 sustained by the young birds was so great as to 

 be incompatible with life, if continued ; that 

 endured by the old one was trifling in amount, 

 and not inconsistent with health. It is quite 

 true that the difference in point of size and 

 quantity of plumage has an influence upon this 

 inequality of cooling ; but at the period of de- 

 velopment, when the experiment was tried, the 

 difference was not remarkable in regard to 

 either point ; nevertheless it is only proper to 

 take notice of it. By prolonging the period 

 during which the adults were exposed to the 

 cooling process, the advantages they derive 

 from their greater size and closer plumage may 

 be counterbalanced or compensated. It is es- 

 sential to observe that iu the course of the first 

 hour the adult bird had only lost temperature 

 in the proportion of one-fifth of that lost by the 

 young birds, which obviously bears no ratio to 

 the difference in point of size, plumage, &c. 

 And then, the operation of the cold being con- 

 tinued, the adult suffered no further depres- 

 sion of temperature : it fell three degrees cen- 

 tigrade, and then became stationary. We can- 

 not, therefore, ascribe the entire difference in 

 the cooling to that of the physical conditions of 

 size and plumage ; a difference of constitution 

 must go for a great deal ; there are inherent 

 diversities of constitution, favourable or the re- 

 verse, to the production of heat. The truth of 

 this conclusion appears much more clearly if 

 we continue to subject young birds to the same 

 kind of experiment at successive epochs not so 

 close to one another. The rapid progress they 

 make in the power of evolving heat is, indeed, 

 a very remarkable fact. A few days later, and 

 they lose temperature in a much less considera- 

 ble degree when exposed to cold under the 

 same circumstances, although there was little 



or no apparent difference in the external appear- 

 ance of the birds. And this is a new and con- 

 vincing proof that the inequality in the disposi- 

 tion to lose heat obvious at different periods 

 of life under exposure to a low external tempe- 

 rature, is principally owing to inherent inequa- 

 lity in the faculty of producing caloric. 



It is of great importance that a precise idea 

 be formed of this expression. Up to a very 

 recent period in the investigation of animal 

 heat, no one thought of comparing animals 

 save with reference to the temperature of their 

 bodies only : and when it was found that this 

 was the same or different by so much, the ac- 

 count was closed, the comparison was pushed 

 no farther, under the impression that every 

 thing was included under this single ostensible 

 character. Undoubtedly, it must be granted 

 that, all else being alike, equality of tempera- 

 ture is an indication of equality in the capacity 

 to produce heat. But animals in one set of 

 circumstances may actually produce the same 

 quantity of caloric, and not continue to do this 

 the circumstances being changed. It is of 

 consequence to distinguish the actual produc- 

 tion, from the power to produce under different 

 conditions. The one is an act, the other a fa- 

 culty, a distinction of the highest importance in 

 philosophical language in general, and espe- 

 cially in that of physiology. But animals of 

 the same size, subjected to the same variations 

 of external conditions, if they continue to ex- 

 hibit corresponding degrees of temperature, 

 whether these are higher or lower, have evi- 

 dently the same faculty of producing heat. If, 

 on the contrary, they present different degrees 

 under the influence of precisely similar exter- 

 nal variations of circumstance, it is obvious 

 that they must possess the faculty of producing 

 heat in different degrees. Unless we be actu- 

 ally persuaded of the value of this expression, 

 so simple in other respects, and so constantly 

 held in view in all analogous circumstances, 

 the study of the phenomena of animal heat 

 would remain as it were barren, whilst the in- 

 vestigation of the diversities of constitution in 

 relation with this faculty is fertile in interesting 

 and useful applications. 



We have seen how constitutions differed in 

 this respect according to age in the earlier 

 period of life and in the adult state. It is 

 probable that there are other varieties depend- 

 ent on other causes ; for example, differences 

 of season, climate, &c. This point it will be 

 our next business to examine. 



INFLUENCE OF SEASONS IN THE PRODUC- 

 TION OF ANIMAL HEAT. 



The temperature of an animal is the result, 

 1st, of the heat which it produces ; 2d, of that 

 which it receives ; 3d, of that which it loses. 

 The proportion of heat which is lost depends 

 on two principal conditions, the relatively 

 colder temperature of the atmosphere, and the 

 amount of evaporation that takes place from 

 the surface of the animal. In cold and tem- 

 perate climates these two conditions of cooling- 

 are in inverse relations to one another in the 

 opposite seasons of winter and summer. In 



