LATENT HEAT OF VAPORISATION. 287 



marks 100, after which the steam simply bubbles through the 

 hot water as a current of air would do, without becoming condensed 

 as it did at first. When this occurs, remove the cylinder and 

 weigh or measure the quantity of hot water now contained in it 

 (more conveniently, cork it up and wait till cool enough to measure 

 without danger of being scalded). It will be found that the 

 original 10 ounces of water has become increased to something 

 like 11-f- ounces, the extra 1-f- ounces having been produced by the 

 condensation of the steam. It thus results that whilst 1-f- ounces 

 of steam have become condensed to water at 100, a large quantity 

 of heat previously latent in the steam has become sensible, pro- 

 ducing the effect of raising the temperature of 10 ounces of water 

 from to 100 ; i.e., taking now 1 ounce as unit of weight, 

 10 x 100 = 1000 units of heat have been rendered sensible, or have 

 passed out of the latent state during the conversion of 1-f- ounces 

 of steam at 100 to water at 100 ; so that during the condensation 



1 000 

 of 1 ounce of steam, 1 6 = nearly 540 units of heat have become 



sensible. 



Hence the latent heat of steam is said to be 540 units ; that is, 

 in order to convert water at 100 into steam at 100, 540 units of 

 heat must become latent. In precisely the same way heat is 

 rendered latent during the conversion of all liquids into vapours 

 or gases by volatilisation ; the more quickly this operation is per- 

 formed the greater will be the fall in temperature of an evaporating 

 liquid, owing to the conversion of sensible into latent heat, just as 

 with a freezing mixture. Thus in Expt. 25 the rapid evaporation 

 of ether produces a sufficient degree of cold to freeze water and 

 other substances 5 by the use of more volatile substances still (such 

 as ammonia or sulphur dioxide gases reduced to the liquid state 

 by cold and pressure) an extremely quick evaporation can be pro- 

 duced, more especially when the gas or vapour formed by the 

 evaporating liquid is rapidly removed by a powerful pump ; the 

 modern forms of powerful freezing machines (Expt. 320) are pro- 

 duced by employing these or other analogous agents, waste being 

 prevented by recondensing the evolved vapour by compression and 

 cooling in another part of the apparatus, and using the recovered 

 liquid over and over again. 



