126 Dr. Schroeder van der Kolk on Gases. 



obtained on calculation showed the inadmissibility of this as- 

 sumption. 



Hence the entire calculation reverted to an accurate measure- 



dk 

 ment of -- at various teinperaures. If, now, k were known for 



three different temperatures, — - might be obtained with sufficient 



(XT 



accuracy by means of an interpolation formula ; Regnant' s de- 

 terminations give, however, only two values of k, at 4° and 100°. 

 Hence a third value of k had to be determined in an indirect 

 manner, which calculation is given in § III. By means of this 

 value J, or the mechanical equivalent of heat, could easily be 

 determined; and the same formula could inversely be used for 

 determining the difference of the two specific heats for other gases 

 under different pressure and different temperature. 



As now the external work of the gases, performed in expansion 

 during heating under constant pressure, was known, the internal 

 work in expansion could also be determined; in other words, it 

 could be determined how much more energy was accumulated in 

 a given quantity of a gas of constant temperature with a greater 

 volume than with a smaller one. 



These different subjects are more minutely discussed in the 

 following paragraphs. 



§ II. On the Determination of k. 



The results previously obtained may, in so far as they are ap- 

 plied in the sequel, be here; briefly repeated. 



The coefficient of expansion under constant pressure changes 

 in all cases with the pressure, except in the case of hydrogen ; in 

 this case llegnault found, for the barometric height h, 



A= 0-760, 0-36613, 

 £=2-545, 0-36616, 



values, therefore, which arc to be considered equal. 



Hydrogen is alone in this respect. If, therefore, different 

 volumes of hydrogen, which are under different but constant 

 pressures, arc allowed to expand, the proportion of the volumes 

 at 0° and 100° will be the same in each case. 



With an ideal gas, that is, one in which k is constant, the 

 same is the case ; for from the formula? 



pv = #T, 



pv l = kr l , 



in which p is the pressure in kilogrammes on the square metre, 

 we get by division 



