380 PHILOSOPHY OF ZOOLOGY. 
whether the air is moist or dry. And in the case of frogs 
and dogs, Dr Crawrorp found that the cold was generat- 
ed even when the body was immersed in water. 
Even when the heating cause is applied only to a par- 
ticular part of the body, as (in the experiments of Dr 
Hunter) tothe urethra, its temperature is not increas- 
ed beyond that degree to which the whole body may be 
raised. 
In all these experiments in which the body is exposed 
to a high temperature, the generation of cold is effected by 
a great expenditure of the vital energy. Hence a great 
degree of weakness is produced by continuing the effort. 
As the power of the whole body for the production of 
cold appears to be limited, so is likewise the power of par- 
ticular parts, ‘“‘ which (says BuacpEn *) may be one rea- 
son why we can bear for a certain time, and much longer 
than can be necessary to fully heat the cuticle, a degree of 
heat which will at length prove intolerable.” It is proba- 
ble that this power of the human body is greatly influen- 
ced by habit, in the case of washer-women, whose hands 
are frequently immersed in warm water, and glass-blowers, 
whose bodies are exposed to a degree of heat, which to 
others would prove painful. 
3. When the body is exposed to a temperature greatly 
lower than the ordinary standard, a counteracting influ- 
ence is exerted, and heat is generated.—In the experiments 
already quoted it appears evident, that there is an inferior 
as well as a superior limit to the change of temperature in 
living bodies, from the influence of surrounding objects. 
In the experiments of Dr Currie, the heat of the human 
® Phil. Trans. 1775, p. 121. 
