.1)4 



Professor Sir James Deivar 



[Mcircli 2>5 



and several observations on them are i^iven. With regard to these 

 bodies, it may be noted that the values found are not far removed 

 from those of the specific heats at constant volume in the gaseous 

 state, and I have no doubt that if the experiments had been 

 extended to temperatures between that of liquid air and hydrogen 

 these results would all have been below the gas constant. The other 

 bodies examined all show diminution of specific heat at the lower 

 temperatures, the most marked examples being the hydrocarbons, 

 paraffin and naphthaline. 



While the experiments on the specific heats of diamond graphite 

 and ice were being carried on, the frequent determination of the 

 quantities of gas evaporated by lead in the same circumstances as the 

 diamond graphite or ice under investigation, afforded means for the 

 direct measurement of the latent heats of evaporation of hydrogen, 

 nitrogen, air, and oxygen. Lead had been selected as the metal of 

 comparison for the following reasons : — its low specific heat enabled 



Fig. 6. 





small quantities of heat to be conveyed into the calorimeter while 

 the mass of the metal was still considerable ; further, the variations 

 of the specific lieat of lead with temperature are small, and its 

 specific heat may hence be treated as a linear function of the tem- 

 perature ; and lastly, the metal is easily obtained very pure. 



In this manner the latent lieat of hydrogen, whether determined 

 by dropping small pieces of lead through a range of temperature as 

 great as 270°, or through as short a range as 64°, was found to be 

 about 121 to 122 gramme-calories. In my Bakerian Lecture,* 

 having assumed what at the time appeared to be the value of the 

 latent heat of hydrogen, namely, 2()() gramme-calories, and having ob- 

 served experimentally that 15 per cent, of the liquid had to Ix) quickly 

 evaporated under exhaustion to reduce the temperature to the melting 

 point of hydrogen, I deduced the mean specific heat of the liquid 

 between the boiling point and the freezing point to be about G. The 

 present investigation enables me to correct this statement, the value 



Proc. Roy. Soc, Juno 1001, vol. Ixvlii. p. 



361. 



