CHANGE OF STATE LIQUID VAPOUR. 181 



tube drop by drop, and, falling on to the silver, evaporated at the pressure 

 corresponding to the vapour pressure for the temperature of the calori- 

 meter. The vapour was pumped out as fast as it was formed, its latent 

 heat being supplied by the oil in the calorimeter. The oil, of course, 

 tended to fall in temperature, but its temperature was maintained 

 constant by an electric heating coil and by very rapid stirring. The 

 heat equivalent of the energy given to the oil by the current and by 

 the stirring was determined, and this gave the latent heat of the steam 

 formed. Griffiths' results at the two temperatures agree with the 

 formula 



L = 596-73 --601*, 



where the unit of heat adopted is the 15 0. calory, which agrees very 

 nearly with the mean calory from 0. to 100 0. This gives L 596'73, 

 closely agreeing with Dieterici's value, and L 100 = 536'6, closely agreeing 

 with Regnault's value L 100 = 537, and at this higher temperature Reg- 

 riault's work is probably very accurate. Griffiths' value for the total 

 heat of steam is 



Q = 596-73 + -399*. 



Henning (Ann. d. Physik, xxi., 5, 1906, p. 849) measured the steam 

 generated from water kept boiling continuously at constant temperature 

 by an electric heater, in which the heat supply was measured. Over 

 the range used, 30 to 100 0., L = 598'8 - 0'5994 fairly represents the 

 results, but L = 94*21(365 - )' 31249 gives a closer agreement on the 

 whole. The 15 calory was taken as equal to 4' 188 joules. 



By superheating the vapour, that is, raising its temperature above 

 the condensing-point, while maintaining the pressure the same, Regnault 

 used the higher temperature apparatus devised for the determination of 

 latent heat of a vapour to find also its specific heat at constant pressure. 

 For, suppose that the latent heat of the vapour at t" is L, the specific 

 heat a; and let the vapour be superheated in two different cases to t + l 

 and t + # 2 . Then the heat given up in condensing is L + d^ and L + # 2 cr. 

 We may determine both of these quantities, and knowing 6 l and $ 2 , we 

 may obtain both L and <r. 



Regnault found in this way that the specific heat of steam at constant 

 pressure is constant within the limits of errors of observation, and the 

 value he found is '4805. He also determined the specific heat of other 

 vapours. But it is to be remarked that in the expression L + 6^0-, L is 

 much the larger quantity, and the value of L + d-p will be affected 

 by the errors of L. Hence, o- cannot be determined in this way with 

 very great accuracy. 



It is important to distinguish this specific heat of vapour at constant 

 pressure from the specific heat of saturated vapour. In the former, the 

 pressure, of course, remains constant during the rise of 1. In the latter, 

 the pressure varies, being always that at which the vapour is just on the 

 point of condensation. For instance, the specific heat of saturated steam 

 at 100 is the heat required to raise the temperature from 100 to 101, 

 the pressure being increased meanwhile from 760 mm. to 787-63 mm., 

 the latter being the vapour-pressure at 101. 



It is easily shown from the second latent heat equation (chap, xix.), 

 that the specific heat of saturated steam at 100 is negative, but we may see 



