338 Mr. J. M. Gray on Regnault's Determination 



same calorimeter. It was found that the heat given up by the 

 steam at 225° exceeded that given up by the steam at 125° by 

 48*051 units of heat; and, dividing by the difference of tem- 

 perature (100), Regnault found 0-48051 for the specific heat 

 of steam at constant pressure. 



In these experiments the superheating to 125° was, no 

 doubt, intended to thoroughly dry the steam, so as to get, in 

 the quantity of heat abandoned in the first set of experiments, 

 the entire heat of complete gasefication, because, if still some 

 moisture remained iu the steam, the latent heat of evaporation 

 of that moisture would be included in the difference-quantity 

 supposed to be due to the specific heat of temperature-raising, 

 and, by that amount, the result would be too high. 



It appears to me that the completion of the evaporation of 

 suspended moisture cannot be accomplished between 100° and 

 125°; but it will be more likely to be carried on between 125° 

 and 225°. Particles of liquid remain at the pressure-tempe- 

 rature whatever be the superheated temperature of the gas in 

 which they are suspended. The rate of evaporation of those par- 

 ticles will therefore depend upon the temperature-difference ; 

 and in the lower range of temperatures but little of the mois- 

 ture will be evaporated in its rapid passage through a worm 

 heated to only 25° in excess of the pressure-temperature. In 

 the second set of experiments the excess of temperature 

 was 125°. 



To test this, let similar experiments be made at 100°; and 

 if the resulting apparent specific heats between 100° and 

 125° are higher than those obtained by Regnault between 

 between 125° and 225°, then my argument is demolished; but 

 if the results are smaller, then my suspicions have been jus- 

 tified, and a correction is required on the results as given by 

 Regnault. Fortunately for my object, Regnault has left us (in 

 vol. i., at page 695) the data of thirty-eight reliable experi- 

 ments on the " total heat " of steam at 100° down to 0°, giving 

 the mean = 636*70 units of heat. I have, with this " total 

 heat " for 100°, compared the results of the experiments on the 

 steam of 125° — in precisely the same way as Regnault worked 

 for the interval between 125° and 225° (these experiments are 

 recorded in vol. ii. ; pages 167-178). The results of my calcu- 

 lations are given below. The first two series were merely pre- 

 paratory trials to arrive at the best form of apparatus; and only 

 a few grammes of vapour were passed through the worm at 

 each of those experiments. In the third and the fourth series 

 ten times as much vapour was passed through on each occasion. 

 Regnault says: — "J'aipu operer ainsi sur des quantites de 

 vapeur beaucoup plus considerables, et diminuer l'importance 



