75 
Experiments upon Heat . 
In furs, for instance, the attraction between the particles of 
air, and the fine hairs in which it is concealed, being greater 
than the increased elasticity, or repulsion of those particles with 
regard to each other, arising from the heat communicated to 
them by the animal body, the air in the fur, though heated, is 
not easily displaced ; and this coat of confined air is the real 
barrier which defends the animal body from the external cold. 
This air cannot carry off the heat of the animal, because it is 
itself confined, by its attraction to the hair or fur ; and it 
transmits it with great difficulty, if it transmits it all, as has 
been abundantly shown by the foregoing experiments. 
Hence it appears why those furs which are the finest, longest, 
and thickest, are likewise the warmest ; and how the furs of 
the beaver, of the otter, and of other like quadrupeds which 
live much in water, and the feathers of water-fowls, are able to 
confine the heat of those animals in winter, notwithstanding 
the extreme coldness and great conducting power of the water 
in which they swim. The attraction between these substances, 
and the air which occupies their interstices, is so great, that this 
air is not dislodged even by the contact of water, but remain- 
ing in its place, it defends the body of the animal at the same 
time from being wet, and from being robbed of its heat by the 
surrounding cold fluid ; and it is possible that the pressure of 
this fluid upon the covering of air confined in the interstices of 
the fur, or feathers, may at the same time increase its warmth, 
or non-conducting power, in such a manner that the animal 
may not, in fact, lose more heat when in water, than when in 
air: for we have seen by the foregoing experiments, that, under 
certain circumstances, the warmth of a covering is increased, 
by bringing its component parts nearer together, or by 
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