8^6 EEPORTS ON TflE STAtE OF SClENCfi. 



E. Wiedemann obtained the value 0-2389 by a method similaf to 

 Regnault's. This value is probably affected by a similar error. 



J. Joly measured the mean specific heat of air at constant volume, 

 and at densities 7 to 22 times normal, by the method of the steam 

 calorimeter, between 10° and 100° C. This method has the advantage 

 of avoiding the majority of the sources of error above mentioned. 

 Joly's value for air at constant volume when reduced to the calorie at 

 20° C. and to normal density, would be 0'1732, which corresponds to a 

 value 0'2419 for the specific heat at constant pressure at a temperature of 

 55° C This is a far more probable value than Regnault's, but it must 

 be observed that the extrapolation of the experiments to atmospheric 

 pressure involves some uncertainty. 



The specific heats of air and COo at atmospheric pressure and at tem- 

 peratures of 20° and 100° C. have recently been determined by Swann at 

 the Royal College of Science by the continuous electric method previously 

 employed by Callendar - in the case of steam. In this method the actual 

 specific heat at any point is determined by observing the rise of tempera- 

 ture produced in a steady current of gas at the required temperature 

 by supplying a measured quantity of electric energy. This method 

 is better adapted than Regnault's for determining the variation of the 

 specific heat, because it gives the actual specific heat over a small range 

 (about 5°) at the required point in place of the mean specific heat over 

 a large range. It has also the advantage that systematic errors may be 

 more completely eliminated. 



The values obtained by Swann for the specific heat of air at atmo- 

 spheric pressure in terms of the calorie at 20^ C. equivalent to 4'180 

 joules were :— 



8=0-2415 at 20° C, and 8=02425 at 100° C. 



His value at 55° C. is in very good agreement with that deduced above 

 from Joly'i) experiments by the constant-volume method. Adopting a 

 linear formula, we have for the specific heat at any temperature, t, be- 

 tween 0° and 100° C. 



S,=0-2413(l+-0-00005«) (Swann). 



Holborn and Austin ^ and Holborn and Henning'' extended Regnault's 

 method for the determination of the mean specific heat to temperatures 

 up to 840° C. In working at these high temperatures the difiiculties of 

 the method are greatly increased. They found it necessary to employ 

 electric heating and to connect the heater to the calorimeter by a porce>- 

 lain tube in order to diminish conduction. The temperature of the hot 

 gas was observed with a thermo-couple near the entrance to the calori- 

 meter. The time of flow was about three minutes in each experiment, 

 and the corrections were estimated by observing the rate of change of 

 temperature of the calorimeter before and after each observation. There- 

 appeared to be some doubt whether the couple would give the true mean 

 temperature of the giis-flow, especially as the time of flow was so short, 

 x^'or this and other reasons the authors do not lay great stress on the 



' Callendar, Phil. Mag., January 1903, p, 7C. 

 ^ Proc. R.S., 1900. 



» Sitz. Akad. Wiss., Berlin. 1905, p. 175. 

 " Wied. Ann., 18, 1905, p. 739. 



