Knowledge of the Specific Yolumes of Steam. 25 



heat at 15° according to Rowland. For since we know the 

 value of the mean specific heat from 0° to 100° in terms of 

 Regnault's calorie by Regnault's own experiments, it follows 

 that 598*9 expresses Dieterici's determination at 0° in the same 

 unit as that in which 636*7 expresses the total heat at 100°, be 

 that unit c 16 or not. Therefore the writer considers that the 

 formula expresses the total and latent heats in Regnault's 

 calorie with a maximum possible error of 0*5 per cent at the 

 lower end. 



Griffiths believes that the mean specific heat from 0° to 

 100° is almost exactly equal to c lb . He bases this conclusion 

 on the following considerations : He has expressed in terms of 

 c lb his latent heats, already referred to, very closely by the 

 linear formula 



L =596*73 — 0*601(H. 



Extrapolating to 0° and 100° there are obtained 596*73 and 

 536*63. The first is almost exactly the value of L Dieterici 

 has found at 0° in terms of c _ 100 , the second is what one would 

 get from Regnault's experiments on total heat if one supposed 

 that c _ 100 is equal to <? 15 and that Regnault's total heat at 100° 

 is expressed also in terms of c u . Thus to support a linear 

 formula, which type of formula is improbable in itself, he has 

 to make two suppositions, one that c _ 100 is the same as <? 15 , 

 which we have seen is not the best value, and the other that 

 Regnault's calorie is equal to c 1B , which we have seen to be 

 probably true. But these two assumptions are contradictory, 

 for by Regnault's own experiments on the specific heat of 

 water c _ 100 is 1*00358 times his calorie. 



From the formulae for h and H the writer has obtained the 

 latent heats and thence the specific volumes of dry saturated 

 steam. In obtaining the latter there are also required the spe- 

 cific volumes of liquid water at various temperatures, the rela- 

 tion between the pressures and temperatures of dry saturated 

 steam, and the value of the mechanical equivalent of heat. 

 The specific volume of water below 100° is for as many decimal 

 places as have been used constantly 0*001 cubic meters per 

 kilogram. Above 100° Hirn's* formula has been adopted. 

 For the relation between pressure and temperature Regnault's 

 formula H has been used above 100°, while below 100° his 

 formula C (with the constants as recalculated by Zeuner) is 

 taken. These should be sensibly correct except below 40°, 

 where an error exists due to the fact that Regnault did not 



know of the sudden change in — at 0°. The value of the 



do 



mechanical equivalent is taken as 427*03 kilogrammeters (at 



* Ann. de Chimie et de Phys. (4), x, p. 32, 1866. 



