54 VAPORIZATION. 



The latent heat of the vapours of several other bodies besides 

 water has been determined, and found to have a relation to the 

 volumes of the vapours. Thus, when equal weights of water 

 and oil of turpentine are converted into vapour under the same 

 pressure, the quantity of heat rendered latent by the turpentine 

 vapour is not more than one-fifth of the latent heat assumed by 

 the water vapour ; but the bulk of the latter vapour is about five 

 times greater than that of the former. The table below exhibits 

 the latent heat of equal weights of several vapours, as ascer- 

 tained by Dr. Ure. He distilled, in all cases, 200 grains of the 

 liquid, from a small retort, and condensed the vapour in a thin 

 glass globe, surrounded with a certain quantity of water at a 

 known temperature, contained in a glass basin. To prevent the 

 air from exercising an influence on the temperature of the water 

 in the basin, care was taken that the water should be three or 

 four degrees below the temperature of the air at the beginning 

 of the experiment, while it was not afterwards heated more than 

 the same number of degrees above the atmospheric temperature 

 by the condensation of the vapour. A thermometer of great 

 delicacy was continually moved through the water, and its indi- 

 cations were read off to small fractions of a degree. The latent 

 heat of each vapour was of course proportional to the rise of 

 temperature which occurred in the condensing water. 



Equal weights. Latent heat. 



Vapour of water 1000 degrees. 



Alcohol (specific gravity 0.825). . . . 457 



Ether (boiling point 112) . . . 318 



Petroleum 184 



Oil of turpentine 1 84 



Nitric acid (specific gravity 1.494) . 550 



Liquid ammonia (specific gravity 0.978) . 8C6' 



Vinegar (specific gravity 1.007) . . . 903 



Tf the latent heat of different vapours be proportional to their 

 volume, as these numbers seem to indicate, the same bulk of 

 vapour will be produced from all liquids with the same expen- 

 diture of heat, and hence there can be no advantage in substi- 

 tuting any other liquid for water, as a source of vapour, in the 

 steam engine. 



The latent heat of the vapour of water itself increases with 

 its rarity at low temperatures, and diminishes with its increasing 

 density at high temperatures. Water may easily be made to 



