314 REPORTS ON THE STATE OF SCIENCE. 



(1) Constant-jyressure Exjoeriinents. 



Tlie constant-pressure experiments have been carried to a tem- 

 perature of about 1400° C. The gas under atmospheric pressure flows 

 steadily through a heater and then through a calorimeter, where it is 

 cooled. The temperature just before entering and just after leaving the 

 calorimeter and the quantity of heat evolved per gramme molecule of 

 the gas are measured. This quantity of heat less the work done in the 

 contraction, which is 1-98 times the fall of temperature, is the change of 

 internal energy corresponding to that fall. 



Regnault applied the method to air, H.,, CO, COn, and other gases over 

 the range 0-200° C. 



Wiedemann ' repeated Regnault's experiments with some modifica- 

 tions of the apparatus. On account of the small range of temperature 

 these experiments must be regarded as only giving the slope of the 

 internal energy curve at the origin ; but as they give this with an 

 accuracy at least equal to that with which the ordinate is known at 

 higher temperatures, they are of considerable importance in constructing 

 the cjirve. The following table shows the values of the mean volumetric 

 heat C. over the range 0° to 100° C, found by these two observers for 

 air, H, and CO. Witkowski's value for air, by the same method, is in 

 exact agreement with Regnault's. ^ 



These results give a good idea of the accuracy attained in these 

 experiments. In both sets the different observations ranged about 

 H per cent, above and below tlie mean in each determination. Later 

 work shows that the value of C for air is probably about 1 per cent, 

 greater over the range to 200 than over the range to 100. Regnault 

 was unable to detect this difference though he looked for it. 



The volumetric heat of air has also been determined by Joly by 

 means of the steam calorimeter. He found the specific heat of air at 

 constant volume for the range 10° to 100° C. and at a pressure of about 

 twenty atmospheres to be 0*172. There were distinct signs of an 

 increase of specific heat with density, and assuming this to follow the 

 linear law given by Joly the specific heat at normal density would be 

 0-1715, equivalent to 4-93 calories per gramme molecule. Professor 

 Callendar points out, however, that this is based upon Regnault's number 

 for the latent heat of steam which is of doubtful accuracy, and that 

 more probably Joly's determination when reduced to the 15° calorie 

 should be 0-1732, or 4-98 calories per gramme molecule. According to 

 some unpublished experiments by a constant-pressure method, which 

 have been made by Mr. Swann in Professor Callendar's laboratory, and 

 in which it is believed that some sources of systematic error inherent in 

 the earlier experiments of this type have been avoided, the volumetric 

 heat of air is 5-0. These results are distinctly higher than those obtaineii 

 by Wiedemann and Regnault, but the difference is of no importance for 

 the present purpose except as an indication of the possibility of 



' Annalen der Physili, 1876, vol. clvii. 

 « Phil Mag., vol. xlii. (1896), p. 5. 



