398 Royal Society : — 



of a boiler partly filled with water, and partly with steam, an inverted 

 cup, or bell-shaped vessel, be suspended, and if it then, being full of 

 steam, be forced down under the water, mechanical work will be im- 

 parted as potential energy to the system of which the steam and 

 water in the boiler form one part, and the earth is the other part ; 

 though, for brevity of expression, the work may be spoken of as 

 applied to the steam and water. In this case there is perfect freedom 

 for the steam forced under the water to condense and cause by com- 

 munication of its latent heat the generation of an equal quantity of 

 steam at the [surface of the water under which the bell was sunk. 

 The occurrence of this change of state will enable the system to lose 

 the potential energy which had been imparted to it by the submer- 

 sion of the steam, or will release that energy which had been stored, 

 and the system will pass into the changed state ; that is to say, a 

 certain part of the steam will change to water, and, instead of that, a 

 different part of the water will be changed to steam ; and this change 

 will be accompanied by a transmission of heat from the part con- 

 densing to the part evaporating. This is all in accordance with the 

 axiom ; and we know otherwise that it must take place, as the steam 

 being pressed when submerged must condense and give its latent heat 

 to the water, and that heat must generate an equal quantity of steam 

 at the surface of the water, where the pressure is less. Thus the truth 

 of the axiom is confirmed. 



If a quantity of ice and water be enclosed in a cylinder with a 

 water-tight piston, and if this be put into a completely closed vessel 

 filled with other ice and water, and if the piston then be pulled with 

 any given force and fixed in its new position (which might be done 

 in many ways, as for instance, by the use of an axle passing air-tight 

 through the side of the outer vessel), mechanical work will be intro- 

 duced as potential energy into the system consisting of all the things 

 enclosed in the outer vessel. But there is perfect freedom for the 

 water enclosed in the cylinder to proceed to freeze, obtaining the 

 requisite cold from the ice in the water confined around the cylinder 

 and within the outer vessel. The occurrence of this change would be 

 accompanied by the system's losing or giving up the potential energy 

 which had been stored in it. According to the axiom, then, the 

 change ought to occur. But we know otherwise that it must occur ; 

 because the diminution of hydrostatic pressure in the cylinder raises 

 the freezing-point of the enclosed water, and makes it freeze by the 

 cold of the surrounding mixture of ice and water, which, besides, by 

 being itself subjected to increased pressure, tends to give out cold by 

 the lowering of its freezing-point. Thus the truth of the axiom is 

 again confirmed. 



Lastly, if a bar of ice in ice-cold water be subjected to any stress 

 (a pull for instance) tending to change its form, it will receive me- 

 chanical work from the force, or forces, applied, and that work will 

 be stored as potential energy in the elasticity of the ice. Now, if 

 there be another piece of ice in juxtaposition with this piece, seeing 

 that, at the beginning, both these pieces were free from externally 

 applied forces, and were both in the state in which either was perfectly 



