4 A. W. Witkowski on the 



the pressure of one atmosphere (1033*24 -^— 2 ) ; thus it follows 



that m = 0*001293 grammes. Assuming, further, J = 42,700 

 gramme-centimetres per gramme-calorie, the last equation 

 becomes 



Cp = Cl -18-714*J ±£dp (4) 



Similarly, instead of (2) we obtain 



<F 



c,=c p + l8-7Ut^- (5) 



§ 4. It has been already remarked that the investigation 

 ought to bear in the first place on the quantity c x . The 

 specific heat of atmospheric air under constant pressure has 

 been repeatedly measured. In the first rank there stands 

 the fundamental work of Regnault* ; fourteen years later 

 these experiments were repeated by E. Wiedemann f, with 

 the help of a simpler form of apparatus. Regnault's experi- 

 ments cover a range of temperature from +200° to —31°; 

 air of ordinary density was tried, as w T ell as condensed air, 

 up to 12 atmospheres. Notwithstanding these relatively 

 extended limits Regnault was not able to discover any marked 

 variation of the quantity c p ; hence there arose the very 

 general belief, that the specific heat of air is a constant, inde- 

 pendent of pressure and temperature — an opinion which, as 

 will be seen further on, is very far from being correct. 

 Wiedemann's experiments, so far as air is concerned, were 

 intended to control Regnault's value of the specific heat. 

 The results obtained by the two investigators are only in 

 approximate agreement. According to Regnault the most 

 probable value of the specific heat c x between 0° and + 100° 

 is 0*23741 ; this is the mean value of single determinations 

 varying between the limits 0*23536 and 0*23890. 



Wiedemann's result is 0*2389, the limits of single determi- 

 nations being 0*2374 and 0*2414. Although the principal 

 aim of my own experiments was to find the variations of the 

 specific heat of air through as wide a range of temperatures 

 as possible, yet I thought it worth while to redetermine once 

 more the exact value of this important constant ; this seemed 



* Relation des experiences fyc., tome ii. pp. 41-110, ani pp. 206-224. 

 (Paris, 1862.) 

 t u Spez. Warme de3 Gase," Pogg. Ann. vol. clvii. (1376), pp. 19-22. 



