ANIMAL HEAT. 301 



the warmest apartment, never rises to the heat of the living body, which 

 is still the only source of its own vital temperature. 



Differences of Temperature in Different Classes of Animals. The 

 intensity of the production of internal heat varies in different classes 

 of animals. As a rule, it is most active in birds, whose temperature is 

 in general 45. In the mammalians it is 37 to 40; in man about 

 ST. 5. As in these two classes the internal organs and the blood are 

 nearly always much above the temperature of the air or of the surface 

 of the skin, and accordingly feel warm to the touch, they are called the 

 "warm-blooded animals." In reptiles and fish, on the other hand, the 

 production of heat is much less rapid, and preponderates so little over 

 that of the air or water which they inhabit, that no marked difference is 

 perceptible on cursory examination ; and as their internal organs have 

 a lower temperature than our own integument, and consequently feel 

 cool to the touch, they are called the "cold-blooded animals." This 

 difference, however, is only one in degree and not in kind. Reptiles 

 and fish also generate heat within their bodies, which may be measured 

 by the thermometer. The temperature of frogs, serpents, tortoises, 

 water-lizards, and fish has thus been found to be from 1.7 to 4.5 

 above that of the surrounding air or water. 



In the invertebrate animals, as insects and the like, the heat produced 

 is still less easily perceptible because, from the great extent of the sur- 

 face presented by their bodies in proportion to their mass, the warmth 

 is more rapidly dissipated. But when many of them are collected in a 

 small air-space, or when they are in a state of activity, it is still distin- 

 guishable by thermometric measurement. The temperature of the butter- 

 fly after active motion has been found to be from 2.77 to 5 above that 

 of the air; that of the humble-bee from 1.5 to 5.5 higher than the 

 exterior. According to the experiments of Newport, the interior of a 

 hive of bees may have a temperature of 9 when the external atmos- 

 phere is at 1.4, even while the insects are quiet ; but if they be excited 

 to activity by tapping on the outside of the hive, it may rise to 38.8. 

 Thus, while the insects are at rest, the thermometer indicates a very 

 moderate temperature ; but if kept in rapid motion in a confined space, 

 they may generate a sufficient amount of heat to produce a sensible 

 elevation in the course of a few minutes. 



The production of heat is not confined to animal organisms, but takes 

 place also in vegetables. Here, however, it is still more rapidly dissi- 

 pated than in insects, owing to the great extent of surface presented 

 by the ramifications and foliage, and to the abundant evaporation of 

 moisture from the leaves, by which the heat generated is in great 

 measure consumed without becoming perceptible by the ordinary ther- 

 mometer. If this loss of heat from the plant be diminished by keeping 

 the air charged with watery vapor and thus preventing evaporation, the 

 elevation of temperature becomes sensible and may be measured. Du- 

 trochet 1 first demonstrated, by the use of the thermo-electric needle, that 



1 Annales des Sciences naturelles. Paris, 2me S6rie, tome xii. p. 277. 



