ANIMAL HEAT. 259 



production of internal heat varies in intensity in different classes of 

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

 is in general 45 C. In mammalians it is 37 to 40; and in man. 

 about 37.5. As in these two classes the internal organs and the blood 

 are nearly always above the temperature of the air or that of the skin, 

 and, accordingly, feel warm to the touch, they are called " 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 "cold-blooded animals." This difference,, 

 however, is only in degree and not in kind. Reptiles and fish also 

 generate a certain amount of heat, which may be measured by the ther- 

 mometer. The temperature of frogs, serpents, tortoises, water-lizards, 

 and fish has been found to be from 1.7 to 4.5 above that of the sur- 

 rounding air or water. 



In invertebrate animals the heat produced is usually still less percep- 

 tible because, from the greater surface of their bodies in proportion to 

 their mass, the warmth is more rapidly dissipated. But when many 

 of them are collected in a small space, and especially when in a state 

 of activity, their heat is distinguishable by thermometric measurement. 

 The temperature of the butterfly after active motion has been found 

 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 Xewport, the 

 interior of a hive of bees may have a temperature of 9 with the 

 external atmosphere at 1.4, even while the insects are quiet ; but if 

 they be excited by tapping on the hive, it may rise to 38.8. Thus 

 so long as the insects are at rest, the thermometer indicates a very 

 moderate warmth ; but if kept for a few moments in rapid motion in a 

 confined space, they may generate sufficient heat to produce a sensible 

 elevation of temperature. 



The production of heat is not confined to animals, but takes place 

 also in vegetables. In vegetables, however, it is very rapidly lost, 

 owing to the extensive surface presented by their ramifications and 

 foliage, and the abundant evaporation of moisture. If this loss be 

 diminished by keeping the air charged with watery vapor and thus 

 preventing evaporation, the elevation of temperature becomes sensible 

 and may be measured. Dutrochet* demonstrated, by the use of the 

 thermo-electric needle, that nearly all parts of a living plant, such as 

 the green stems, the leaves, the buds, and even the roots and fruit, 

 generate heat to some degree ; the maximum temperature thus reached 

 being about 0.28 above that of the surrounding atmosphere. Sub- 

 sequent observations have shown that in certain periods of vegeta- 

 tive activit- as in those of germination and flowering, the development 



* Annale? des Sciences naturelles. Paris, 2me Serie, tome xii., p. 277. 



