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XXXVII.—Theoretical Considerations on the Liffect of Pressure in Lowering the 
Freezing Point of Water. By James Tuomson, Esq., of Glasgow. Commu- 
nicated by Professor W1LL1AM THOMSON. 
(Read 2d January 1849.) 
Some time ago my brother, Professor WiLL1am THomson, pointed out to mea 
curious conclusion to which he had been led, by reasoning on principles similar to 
those developed by Carnot, with reference to the motive power of heat. It was, 
that water at the freezing point may be converted into ice by a process solely mecha- 
mcal, and yet without the final expenditure of any mechanical work. This at first 
appeared to me to involve an impossibility, because water expands while freezing ; 
and, therefore, it seemed to follow, that if a quantity of it were merely enclosed 
in a vessel with a moveable piston, and frozen, the motion of the piston, conse- 
quent on the expansion, being resisted by pressure, mechanical work would be 
given out without any corresponding expenditure; or, in other words, a per- 
petual source of mechanical work, commonly called a perpetual motion, would 
be possible. After farther consideration, however, the former conclusion ap- 
peared to be incontrovertible; but then, to avoid the absurdity of supposing that 
mechanical work could be got out of nothing, it occurred to me that it is neces- 
sary farther to conclude, that the freezing point becomes lower as the pressure. to 
which the water is subjected 1s increased. 
The following is the reasoning by which these conclusions are proved. Let 
there be supposed to be a cylinder, and a piston fitting water-tight to it, and 
capable of moving without friction. Let these be supposed to be formed of a 
substance which is a perfect non-conductor of heat; also, let the bottom of the 
cylinder be closed by a plate, supposed to be a perfect conductor, and to possess 
no capacity for heat. Now, to convert a given mass of ice into water without the 
expenditure of mechanical work, let this imaginary vessel be partly filled with air 
at 0° C., and let the end of it be placed in contact with an indefinite mass of 
water, a lake for instance, at the same temperature. Now, let the piston be 
pushed towards the bottom of the cylinder by pressure from some external reser- 
voir of mechanical work, which, for the sake of fixing our ideas, may be supposed 
to be the hand of an operator. During this process the air in the cylinder would 
tend to become heated on account of the compression, but it is constrained to re- 
main at 0° by being in communication with the lake at that temperature. The 
change, then, which takes place is, that a certain amount of work is given from 
the hand to the air, and a certain amount of heat is given from the air to the 
water of the lake. In the next place, let the bottom of the cylinder be placed in 
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