SECT, ii.] PROPERTIES OF STEAM. 49 



73. The heat combined or disengaged by a change of the state of a 

 body, called latent heat, is measured in the same manner as specific heat ; that is, 

 by means of the quantity of heat necessary to raise the temperature of water 

 one degree at 60. 



It was the eminent Dr. Black who first discovered (in 1762) that a change of 

 state in natural bodies requires a certain addition or diminution of heat ; and that 

 the quantity is different for different bodies, and also different according to the 

 nature of the change. The importance of this discovery to general science is great, 

 and its finest practical application is to the principles of the steam engine. 



74. The additional heat in the vapour or steam of any liquid is not very 

 easily determined ; but since the discovery of Dr. Black, experiments have been 

 made by several philosophers, distinguished for their accuracy and skill in such 

 delicate researches. The method adopted by Dr. Black is simple and easily tried, 

 but not accurate. When a vessel containing water is placed on a fire, the water 

 gradually becomes hotter till its temperature reaches 212, but after that its tempe- 

 rature does not increase. The water is flying off" in steam, and the heat not raising 

 the temperature higher, as we know it would do if the vessel were closed, we 

 must conclude, that the heat which would be communicated to the water in a 

 close vessel combines with the steam in an open one, and yet does not increase the 

 temperature of that steam to more than that of boiling water. To ascertain the 

 quantity of heat which is combined with steam, Dr. Black put some water in a tin 

 plate vessel upon a red hot iron. The water was of the temperature 50; in four 

 minutes it began to boil, and in twenty minutes it was all boiled off". During the 

 first four minutes it had received 162 or 40|- per minute. If we suppose that it 

 received as much per minute during the whole process of boiling, the heat which 

 entered into the water and converted it into steam would amount to 40j x 20 = 

 810. This 810 degrees of heat is not indicated by the thermometer, for the tem- 

 perature of steam is only 212; therefore Dr. Black called it latent heat. * But the 

 result is obviously inaccurate, because steam is formed during the heating of the 

 water to the boiling point, and the vessel is losing heat from its surfaces in un- 

 equal quantities ; and the effect of the fire is also unequal, being less as the heat of 

 the water increases. 



75. The heat required to form steam may be more accurately determined by 

 condensing the steam by a cold fluid, and the heat communicated to the fluid by 

 a given weight of steam gives the additional quantity of heat contained in the 

 steam. Mr. Watt made various trials in this manner in 1781, and those on which 

 he placed the most reliance gave 950 for the additional heat in the steam of 



1 Dr. Thomson's System of Chemistry, vol. i. p. 101. 



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