LATENT HEAT OF STEAM. 825 



stand, so that the end of the glass tube is directed 

 downwards. The water is heated, and as soon as a 

 strong jet of steam issues from the tube a beaker con- 

 taining 360 grammes of water, heated (or in the 

 summer cooled) to about 20, is placed under the end of 

 the glass tube as shown in fig. 399, so that the end dips 

 in the water. The water is now constantly stirred with 

 a thermometer, and the steam allowed to enter the 

 water until the temperature has risen to 40. The 





FIG. 399 ( real size). 



beaker is then quickly withdrawn and weighed. The 

 weight of the water will be found to have increased by 

 about 12 grammes ; that is, 12 grammes of vapour have 

 been condensed into water. In this experiment 360^ 

 of water, that is, O kgr *36, which were originally in the 

 beaker, have been raised from 20 to 40, and have there- 

 fore taken up O36 x 20 = 7'2 thermal units. The 

 whole of these 7*2 thermal units have, however, not been 

 supplied by the latent heat of the 12 gr of vapour ; for 

 these 12 gr or O kgr< 012 of vapour have, after being con- 

 densed to water of 100, been further cooled from 100 

 to 40, and have given up 0*012 + 60 = 0'72 thermal 

 units which have been expended in heating the water 

 originally in the beaker. It follows that of the 7 '2 



