310 



EBULLITION. 



the same circumstances ; that it will not retain the vaporous form on being 

 compressed, nor increase its elasticity ; but that, on the contrary, as the piston 

 is depressed, it will be partially restored to the liquid state, and that the por- 

 tion which remains in the vaporous form will retain the same density and elas- 

 ticity as it had before the piston was moved. In fact, if the piston be de- 

 pressed so as to reduce the space occupied by the steam to one half its origi- 

 nal dimensions, it has been assumed that in that case one half the steam under 

 the piston would be restored to the liquid form, and would become water of 

 the temperature of 212, while the remaining half would still retain the vapor- 

 ous form, and have the same temperature and density as before.* 



From this statement, however universally admitted, I must most distinctly 

 dissent, unless it be assumed, at the same time, that a large quantity of heat 

 has been abstracted from that portion of the steam which is reduced to the 

 liquid form. If this do not happen, and the same quantity of heat remain in 

 the vapor under the piston, no change to the liquid form can, in my opinion, 

 take place. The steam originally contained in the cylinder below the piston 

 has that quantity of latent and sensible heat which is necessary and sufficient 

 to maintain it in the vaporous form in all degrees of density. If the steam be 

 compressed by the piston, we cannot suppose a portion of it to be condensed 

 into a liquid, without at the same time supposing that portion to part with about 

 1,000 of latent heat ; but this supposition cannot be admitted, unless we sup- 

 pose the heat so dismissed to pass off to some external object, the contrary of 

 which is the supposition upon which I have here argued. 



I consider that the effects of the compression of steam thus enclosed would 

 be the same as already described with respect to air. The temperature and 

 pressure will be increased, but no portion of it will be condensed into a liquid. 

 In every state of density to which it will be reduced by compression it will 

 take that temperature and pressure which steam of the same density raised im- 

 mediately from water would have. If the piston be depressed so as to reduce 

 the steam lo one half its original bulk, then, its density being doubled, it will 

 acquire that temperature at which steam of double the degree of density would 

 be raised from water. The steam will be in all respects, both with regard to 

 its latent and sensible heat, its density and its elasticity, the same as steam 

 raised from water boiled at the increased temperature. Similar observations 

 may be applied to any degree of compression whatever ; and it will follow, not 

 only that no part of the steam will be restored to the liquid form by reducing 

 its bulk, but that no degree of compression whatever will be capable of redu- 

 cing any part of it to the liquid stale. If the piston could be moved toward the 

 bottom, so as to reduce the dimensions of the steam to those which it had 

 when it existed in the liquid state, which would be accomplished by advancing 

 it within a distance of the bottom of the cylinder equal to about the seventeen 

 hundredth part of its original distance, it would continue to be steam, but would 

 have a prodigiously increased elastic force, and a temperature of 1,212. The 

 steam would in such case be reduced to the state explained in page 308, and 

 would be identical with water raised in a close vessel to the temperature of 

 1,212. It is obvious that the practical exhibition of such effects as here de- 

 scribed would be obstructed by the difficulty of preventing the escape of the 

 sensible heat developed in the compression of the steam. 



The true cause of the conversion of any part of a vapor to the liquid form, I 

 consider to be the diminution of that sum of sensible and latent heat which is es- 

 sential to the existence of vapor. Such a loss of heat would equally cause the 

 vapor to return to the liquid state, whether compressed into a less bulk or ex- 



* See Biot, Traite de Physique, torn, i., p. 266, and physical and chemical writers generally. 



