EXPANSION AND CONDENSATION. 



independent of the process of evaporation, and, indeed, of every 

 property by which vapours are distinguished from air or gases, 

 inasmuch as these latter, being similarly compressed, would 

 similarly expand, and would develope in their expansion precisely 

 the same force. 



15. III. FOECE DEVELOPED BY CONDENSATION. 



It has been already explained* that as heat converts water 

 into steam, so, on the other hand, will cold convert steam into 

 water ; and as water, in passing from the liquid to the vaporous 

 state, is swelled into a vastly increased volume, so, on the other 

 hand, in passing from the vaporous to the liquid state, it suffers 

 a proportionate diminution of volume. Thus if the evaporation 

 take place under a pressure of 15 lb., a cubic inch of water is 

 dilated into a cubic foot of steam, Now, if by the application of 

 cold this steam is converted into water, it will resume its original 

 dimensions, and will become a cubic inch of water. This change 

 of vapour into water has therefore been called CONDENSATION, 

 inasmuch as- the matter of which it consists, contracting into a 

 much smaller volume, is rendered proportionally more dense. 



This property has supplied another means of rendering steam a 

 mechanical agent, Let us suppose that after the piston p, fig. 1, 

 has been raised 140 feet high by the evaporation of a cubic inch 

 of water, the counterpoise, w, having descended through the same 

 height, an additional weight of 15 lb. is placed upon TV, and, at 

 the same time, the lamp withdrawn from the tube and cold 

 applied to its external surface. The steam by which the piston 

 was raised will then be converted into water, or condensed, and 

 will, as at first, fill the bottom of the tube to the height of an 

 inch. The space within the tube above the surface of the water 

 extending to the height of 140 feet, will then be a vacuum, and 

 the atmospheric pressure acting above the piston, not being 

 resisted by any corresponding pressure below it, will force the 

 piston down with a force of 15 lb., and will raiso the weight w, 

 loaded with the additional 15 lb. through the same height. 



Thus, it appears that when steam is condensed, or reconverted 

 into water, by producing a vacuum, it developes a mechanical 

 force equal to that which was developed in the conversion of 

 water into vapour. 



The mechanical power developed by the evaporation of water 

 has been sometimes called the direct power, and that produced 

 by the conversion of vapour into water the indirect power of 



* See Tract on Water, 



203 



