575 ) 



XXXVII. — TJieoretical Considerations on the Effect of Pressure in Lmcering the 

 Freezing Point of Water. By James Thomson, Esq., of Glasgow. Commu- 

 nicated by Professor William Thomson. 



(Read 2d January 1849.) 



Some time ago my brother, Professor William Thomson, pointed out to me a 

 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 mater at the freezing point may he converted into ice by a process solely mecha- 

 nical, 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 stibjected is 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 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 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 fi-om 

 the hand to the au', 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 



vol. XVI. PART V. 7 k 



