SECT. II. PHYSIOLOGY. 19 
which coloured surfaces part with their caloric, 
this circumstance is most remarkable in negroes, 
whose skin usually feels cold to the touch of a 
white person. 
xxi. In tropical countries, the temperature 
of the atmosphere being often equal to that of 
the human body, no caloric can, in such circum- 
stances, be taken off by the air respired ; but the 
absence of this cooling process is counterpoised 
by the vast increase of cuticular evaporation, which 
is supported by a comparative diminution of the 
aqueous part of the urine. 
xxiv. From these chemical sources of cold, 
and from the human body being an indifferent 
conductor of caloric, it can for a short time resist, 
without injury, the influence of an atmosphere 
heated. to 212°, and even upwards according to 
some. statements. ; 
xxv. As the blood necessarily acquires the 
temperature of the organs through which it circu- 
lates, it loses part of its caloric in the capillaries 
of the extremities. Dr. Davy states the differ- 
ence of heat, in arterial and venous blood, to be 
one degree, thermometers being placed in both 
ventricles of the heart. There is no valid proof, 
however, that this difference is owing to re- 
spiration. 
xxvi. After the relation of all the above facts, 
it appears superfluous to insist on the necessary 
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