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XVII. Studies on Gases. 

 By Dr. H. W. Schroeder van der Kolk*. 



f§ I. Introduction. 



IN so far as the principle of the mechanical theory of heat — 

 that the condition of a body only depends on two given 

 magnitudes — can be considered to hold good, the formula jsu — kr, 

 where p is the pressure, v the volume, and r the absolute tem- 

 perature, may be regarded as valid for all bodies. In general, 

 k is then a variable magnitude dependent on temperature and 

 pressure ; if it is made constant, the formula is obtained of the 

 body which is called an ideal gas. Actual gases approximate to 

 this condition; the changes which ^temperature and pressure 

 here produce in the value of k arc very small. For some gases, 

 especially for hydrogen, air, carbonic acid, and nitrogen, Reg- 

 nault's determinations render it possible to determine more ac- 

 curately the changes in k. From his experiments in reference to 

 Mariotte's law, I have calculated for these gases the formula 

 which indicates the connexion between k andjo at the mean tem- 

 perature of 4°, as I have already communicated in a former 

 memoirf. From the coefficients of expansion between 0° and 

 100° under different pressures, the same formula may be calcu- 

 lated for 100°. It was found that in the case of hydrogen, at 

 any rate within the limits of the accuracy of Regnault's experi- 

 ments, k does not alter with the temperature, while in the case 

 of the other gases it is a function of both variables. Other de- 

 terminations of Regnault rendered it possible to test the accu- 

 racy of the formula thus obtained. 



If the principle that the condition of a body is determined by 

 means of two magnitudes, and therefore by means of the changes 

 in k, be compared with Regnault's result, that the specific heat 

 of water and of air scarcely changes at all with the temperature, 

 while that of carbonic acid, in which k undergoes a far greater 

 alteration, changes considerably, it is natural to investigate 

 whether these changes of the specific heat are not connected 

 with changes in k. 



This investigation is connected with an accurate determination 

 of the mechanical equivalent of heat. Hence I repeated the 

 calculation of Moll and van Beck's experiments on the velocity 

 of sound, in order to have a more accurate determination of the 



coefficient —J. The ordinary formula for calculating the con- 



* Translated from Poggendorff's AnnaJen, vol. exxvi. p. 333. 



t Poggendorff's Annalen, vol. cxvi. p. 429. 



% In tbis paper I have adhered to Clausius's proposal to designate the 



