824 LATENT HEAT OF WATER. 



down to is expended in melting the ice, and we may 

 say that in melting 1 kilogramme of ice 80 thermal 

 units have become latent, or more briefly that the 

 latent heat of water is 80. 



The determination of the latent heat of water cannot well be 

 actually made in the simple manner just mentioned, because the last 

 portion of the ice melts so slowly, in consequence of the temperature 

 being not very much above when the greater portion is already 

 melted, that a considerable quantity of heat is taken up from the sur- 

 rounding air by the vessel containing the liquid and the ice. The ex- 

 periment may be made in the following manner. In a cylindrical 

 vessel of thin glass with a very thin bottom a so-called ' beaker ' 

 500s r of water is heated to 60 in a sand bath, or over a layer of iron 

 filings spread on a disc of sheet metal. When the water is at 60, 

 20O r of ice is thrown into it, the mixture stirred, and when all the 

 ice is just melted, the temperature is observed. It will be about 20. 

 In this experiment <500s r or O kgr '5 of water has had its tempera- 

 ture lowered from 60 to 20. and the heat given up by the water 

 is therefore 40 x 0*5 = 20 thermal units. These 20 units have 

 melted 200 gr of ice and converted it into water at 20. Now 200& r 

 or O k s r> 2 requires 20 X 0'2 = 4 thermal units to raise 'its tempera- 

 ture from to 20 ; hence of the 20 thermal units given up by the 

 water only 20 4 = 16 have been expended in melting the 200 gr 

 of ice. If O ksr< 2 of ice requires 16 thermal units, the quantity 

 required by l kgr will be found from the proportion 



0-2 : i : : 16 : x 



x =. 80 thermal units ; 

 that is, the latent heat of water is 80. 



While the latent heat of water may be actually found 

 by observing how much heat a quantity of ice with- 

 draws from a quantity of water during the passage 

 from the solid into the liquid state, the latent heat ot 

 vapour is better determined by observing how much 

 heat is given up by a certain quantity of vapour in 

 passing from the gaseous into the liquid state. 



A retort nearly filled with water is provided with a 

 glass tube bent at right angles, and clamped in a retort- 



