576 JAMES THOMSON, ESQ., ON THE EFFECT OF PRESSURE 



contact with the mass of water at 0", which is proposed to be converted into ice, 

 and let the piston be allowed to move back to the position it had at the com- 

 mencement of the first process. Dm-ing this second process, the temperature of 

 the air would tend to sink on account of the expansion, but it is constrained to . 

 remain constant at by the air being in communication with the freezing water, 

 which cannot change its temperature so long as any of it remains unfrozen. 

 Hence, so far as the air and the hand are concerned, this process has been exactly 

 the converse of the former one. Thus the air has expanded through the same 

 distance through which it was formerly compressed ; and, since it has been con- 

 stantly at the same temperature during both processes, the law of the variation 

 of its pressure with its volume must have been the same in both. From this it 

 follows, that the hand has received back exactly the same amount of mechanical 

 work in the second process as it gave out in the first. By an analogous reason it 

 is easily shewn, that the air also has received again exactly the same amount of 

 heat as it gave out during its compression ; and, hence, it is now left in a condi- 

 tion the same as that in which it was at the commencement of the first process. 

 Tlte only change which has heen produced, then, is, that a certain quantity of heat 

 has been abstracted from a small mass of water at , and dispersed through an in- 

 definite mass at the same temperature, the small mass having thus been converted into 

 ice. This conclusion, it may be remarked, might be deduced at once by the appli- 

 cation, to the freezing of water, of the general principle developed by Carnot, 

 that no work is given out when heat passes from one body to another without a 

 fall of temperature ; or rather by the application of the converse of this, which of 

 course equal!}' holds good, namely, that no worlc requires to be expended to make 

 heat pass from one body to another at the same temperature. 



Next, to prove that the freezing point of water is lowered by an increase of 

 the pressure to which the water is subjected : — Let a cylinder, of the same ima- 

 ginary construction as that used in the foregoing demonstration, contain some 

 air at 0° C. Let the bottom of the cylinder be placed in contact with the 

 water of an indefinitely large lake, of which the temperature is above 0" by an 

 infinitely small quantity ; and let the air be subjected to compression by pressure 

 applied by the hand to the piston. A certain amount of work is thus given from 

 the hand to the air, and a certain amount of heat is given out from the air to the 

 lake. Next, let the bottom of the cylinder be placed in communication with a 

 small quantity of water at 0°, enclosed in a second imaginary cylinder similar in 

 character to the first ; and let this water be, at the commencement, subject merely 

 to the atmospheric pressm-e. Let, however, resistance be offered by the hand to 

 any motion of the piston of this second cylinder which may take place. Things 

 being in this state, let the piston of the cylinder containing the air move back to 

 its original position. During this process part of the heat of the air becomes latent 

 on account of the increase of volume. Thus the temperature of the air, from being 



