45 



the conflict between the he at -producing power and the cooling 

 influences to wheh the animal is subjected. 



Keeping these considerations in view, we may now enter 

 into the question of the circumstances by which the heat- 

 producing power of animals are modified. And, in doing 

 so, I will here anticipate a general conclusion, which will 

 be drawn in a future part of this paper, when treating on 

 the sources of animal heat, and upon which all physiologists 

 are agreed, that the sources of animal heat are the chemical 

 changes produced under vital influences in the animal body. 

 The circumstances modifying the production of heat by 

 the animal, which will be considered, are the size and age 

 of the animal, the period of the day, the season of the 

 year, and the climate. 



First, with regard to size, there ought to be (cceteris 

 paribus) a greater power of generating heat in the small 

 animal than in the large. In general it is found as a rule 

 that the mean temperature, in moderate heat of atmosphere, 

 in small animals, is rather greater than that of large animals. 

 Their motions are more rapid, and their vital functions are 

 performed with greater rapidity. These circumstances, 

 inasmuch as they increase the insensible transpiration, are 

 unfavourable to the preservation of heat, and, therefore, 

 taken along with the greater loss by conduction and radia- 

 tion, from their proportionately greater surface, indicate a 

 power of generating heat in the small animal much greater 

 than in the large, to enable it to preserve the same tempera- 

 ture. If we remove them both from a moderately warm 

 atmosphere to an extremely cold or an extremely hot one, 

 it will be found that the large animal bears the change much 

 better in the former case, because it loses proportionately 

 less heat by conduction and radiation from its surface ; in 

 the latter case it can better bear the loss by transpiration, 

 and more copiously supply the fluid requisite for this pur- 

 pose. We iind, moreover, as a general rule, that where there 



