328 ' Review of Renwick 



into vapor at the temperature of 212° expands nearly 1700 times; 

 but at the temperature of 419'^, it expands but 37 times. Dr. 

 Thomson, in his recent work on Heat and Electricity, adds, " that it 

 is probable that at a temperature not much higher than^ 500°, the 

 steam of water would not much exceed double the bulk of the water 

 from which it was generated. The expansive force of such steam 

 would be truly formidable. It would, when it issued into the atmos- 

 phere, suddenly expand almost 650 times. We do not know at what 

 temperature water would become vapor without any increase of vol- 

 ume. But it would then support a column of mercury 3243 feet in 

 height, and exert a force of 19.459 lbs. upon every square inch of 

 the vessel containing it."* 



4. The absolute quantity of heat is always the same in the same 

 weight of steam, whatever may be the temperature of that steam. 

 When the vapor is formed at a low temperature, nearly all the heat 

 that enters it is in the latent state ; but as we heat it to a higher de- 

 gree, its proportion of sensible heat is constantly augmented, and of 

 latent heat diminished in the same ratio, so that the sum of the two 

 is the same constant quantity. f 



"If (says Dr. Thomson) we could apply such a pressure to water, 

 that we could heat it till its sensible heat arose to 1212°, it is obvious 

 that it would be converted into steam having the specific gravity, and 

 consequently the volume of the original water. The latent heat of 



' Outline of the Sciences of Heat and Electricity, p. 222. 



t As some of our readers may not be familiar with the precise signification of the 

 terms latent heat, specific heaf, and capacity for heat, which occur in connexion 

 with this subject, we will briefly explain them. Latent heat is that which enters 

 into a body while changing its state from solid to liquid or from liquid to agriform, 

 which portion of heat is not sensible to the thermometer, but disappears or becomes 

 latent as water does when added to quick lime. Thus, when water is converted into 

 steam in the common process of ebullition, a quantity of heat enters into the water 

 to convert it into steam, which if applied to v^^ater would be sufficient to raise it near- 

 ly 1000 degrees, but which does not raise the temperature of either the water or the 

 steam in the least degree. Hence the latent heat of steam is said to be 1000. ^Spe- 

 cific heat is the absolute quantity of heat which a body contains compared with an- 

 other body of the same weight and temperature, taken as the standard unit. Thus, 

 steam is said to have a specific heat of 1.7778, because if we take equal weights of 

 air and steam at the same temperature, it can be proved that the actual quantities of 

 heat which these two bodies contain are in that ratio to each other. The term ca- 

 pacity for heat is sometimes employed as synonymous with specific heat : where 

 any distinction is intended, it is this, that the one denotes the ratio of the actual 

 quantities of heat contained, while the other denotes the ratio of the powers of con- 

 taining these respective quantities, which ratios arc evidently the same in both cases. 



