Experiments on the Latent Heat of Steam. 41 



temperature at which boiling begins. It would seem that 

 this is not perfectly easy to fix. With regard to this point 

 see Regnault, loc. cit. p. 659. 



But the gravest difficulty consists in calculating t f In 

 order to do this we need to know two things : — (1) The 

 temperature t r at the moment when, all condensation having 

 previously ceased, the contents of the worm have acquired 

 the same temperature as the water in the calorimeter, and 

 (2) the amount of heat gained or lost by the calorimeter by 

 radiation, surface evaporation, etc., from the beginning of 

 the condensation to the moment when the temperature t r is 

 attained. This amount of heat would produce a change of 

 temperature called the " total correction." 



We can ascertain by special experiments how long a 

 time must elapse before the difference of temperature 

 between the contents of the worm and the calorimeter is 

 negligible. This does not amount to more than two or 

 three minutes, and we can, in a well-conducted experiment, 

 ascertain the precise moment when this occurs with sufficient 

 certainty by noticing when the march of the thermometer 

 becomes regular. 



For the calculation of the total correction, M. Berthelot 

 has himself given elsewhere a method which is irreproach- 

 able,since it is independent of any assumption (See Mecanique 

 Chiinique y Vol. I., p. 208). The correction for each minute is 

 a function of the mean temperature of the calorimeter during 

 that minute, and M. Berthelot determines this by a series of 

 blank experiments performed throughout the whole range of 

 temperature traversed during an actual determination. It is, 

 however, evidently necessary for the accurate use of this 

 method that external conditions, on which radiation and 

 evaporation depend, shall remain the same during the blank 

 experiments and the actual determination. Hence it is 

 impossible, to make a blank experiment of this kind with the 



