The Complete Works in Philosophy, Politics and Morals of the late Dr. Benjamin Franklin, Vol. 2 [of 3]

By Benjamin Franklin

Page 65

cold is nothing more than
the absence of heat or fire. Now if the quantity of fire before
contained or diffused in the snow and salt was expelled in the
uniting of the two matters, it must be driven away either through
the air or the vessel containing them. If it is driven off thro' the
air, it must warm the air, and a thermometer held over the mixture,
without touching it, would discover the heat, by the rising of the
mercury, as it must, and always does in warm air.

This, indeed, I have not tried, but I should guess it would rather
be driven off through the vessel, especially if the vessel be metal,
as being a better conductor than air; and so one should find the
bason warmer after such mixture. But, on the contrary, the vessel
grows cold, and even water, in which the vessel is sometimes placed
for the experiment, freezes into hard ice on the bason. Now I know
not how to account for this, otherwise than by supposing, that the
composition is a better conductor of fire than the ingredients
separately, and, like the lock compared with the wood, has a stronger
power of attracting fire, and does accordingly attract it suddenly
from the fingers, or a thermometer put into it, from the bason that
contains it, and from the water in contact with the outside of the
bason; so that the fingers have the sensation of extreme cold, by
being deprived of much of their natural fire; the thermometer sinks,
by having part of its fire drawn out of the mercury; the bason grows
colder to the touch, as, by having its fire drawn into the mixture,
it is become more capable of drawing and receiving it from the hand;
and through the bason, the water loses its fire that kept it fluid;
so it becomes ice. One would expect, that from all this attracted
acquisition of fire to the composition, it should become warmer; and,
in fact, the snow and salt dissolve at the same time into water,
without freezing.

I am, Sir, &c.

B. FRANKLIN.

FOOTNOTE:

[14] Dr. Lining. _Editor_.




TO THE SAME ON THE SAME SUBJECT.


_London, June 17, 1758._

DEAR SIR,

In a former letter I mentioned the experiment for cooling bodies by
evaporation, and that I had, by repeatedly wetting the thermometer
with common spirits, brought the mercury down five or six degrees.
Being lately at Cambridge, and mentioning this in conversation with
Dr. Hadley, professor of chemistry there, he proposed repeating the
experiments with ether, instead of common spirits, as

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and rejoice that it has been published.
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