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



not be easily planned out or procured. Another, and a better, mode of proceeding 

 has, however, occurred to me: and by it we can deduce, from the known expansion 

 of water iu freezing, together with data founded on the experiments of Regnault 

 on steam at the freezing point, a formula which gives the freezing point in terms 

 of the pressure ; and which may be applied for any pressure, from nothing up to 

 many atmospheres. The following is the investigation of this formula : — 



Let us suppose that we have a cylinder of the same imaginary construction 

 as that of the one desci-ibed at the commencement of this paper ; and let us use it 

 as an ice-engine analogous to the imaginary steam-engine conceived by Carnot, 

 and employed in his investigations. For this purpose, let the entire space en- 

 closed within the cylinder by the piston be filled at first with as much ice as 

 would, if melted, form rather more than a cubic foot of water, and let the ice 

 be subject merely to one atmosphere of pressure, no force being applied to the 

 piston. Now, let the following four processes, forming one complete stroke of the 

 ice-engine be performed. 



Process 1 . Place the bottom of the cylinder in contact with an indefinite lake 

 of water at , and push down the piston. The effect of the motion of the piston 

 is to convert ice at 0° into water at 0°, and to abstract from the lake at 0° the 

 heat Avhich becomes latent during this change. Continue the compression till one 

 cubic foot of water is melted from ice. 



Process 2. Remove the cylinder from the lake, and place it with its bottom on 

 a stand which is a perfect non-conductor of heat. Push the piston a very little 

 farther down, till the pressure inside is increased by any desired quantity which 

 may be denoted, in pounds on the square foot, by ^;. During this motion 

 of the piston, since the cylinder contains ice and water, the temperature of the 

 mixture must vary with the pressure, being at any instant the freezing point 

 which corresponds to the pressure at that instant. Let the temperature at the 

 end of this process be denoted by — t C. 



Process 3. Place the bottom of the cylinder in contact with a second inde- 

 finitely large lake at — t % and move the piston upwards. During this motion the 

 pressure must remain constant at p above that of the atmosphere, the water in 

 the cylinder increasing its volume by freezing, since, if it did not freeze, its pres- 

 sm-e would diminish, and therefore its temperature would increase, which is im- 

 possible, since the whole mass of water and ice is constrained by the lake to remain 

 at — t . Continue the motion till all the heat has been given out to the second 

 lake at — t , which was taken in during Process 2, from the first lake at .* 



* This ste]), as well as the coi'responding one in Carnot's investigation, it must be observed, in- 

 volves (lirticiilt (juestions, which cannot as yet be satisfactorily answered, regarding the possibility of the 

 absolute formation or destruction of heat as an equivalent for the destruction or formation of other 

 agencies, such as mechanical work ; but, in taking it, I go on the almost universally adopted suppo- 

 sition of the perfect conservation of heat. 



