EXrERIMENTAL KNOWLEDGE OF THE PEOPERTIES OF MATTER. 501 



about 2800° for the specific heats of hydrogen, oxygen, nitrogen (the so- 

 called ' perfect gases '), as also of carbon monoxide and hydric chloride, 

 they found results agreeing with the formula for C,., the specific heat at 

 constant volume 



C,=4-8 + -0006f. 



Berthelot and Vieille found by experiments for greater pressures and 

 higher temperatures values for C,. greater than were given by this for- 

 mula ; and it is cei'tain that the above formula cannot legitimately be 

 used for temperatures much higher than those on observations at which 

 the formula is founded ; on the other hand, it is doubtful whether the 

 results obtained from observations concerned only with the very highest 

 pressures are so accurate as the others. However this may be, Berthelot 

 and Vieille find that the mean specific heats of equal volumes of hydro- 

 gen, oxygen, nitrogen, and carbon monoxide at constant volume is 

 doubled in passing from 0° to 4500° ; and they give the formula 

 C,=67-f--0016 (i-2800) for temperatures above 2800°. Except in 

 respect of these very high temperatures, the results agree for the gases 

 named remarkably well with those of Mallard and Le Chatelier. 



Gases such as chlorine, nitrous oxide, carbon dioxide, which have 

 specific heats between 0° and 200° greater than those of the gases men- 

 tioned, and which, according to Regnault's results and those of E. Wiede- 

 mann, have nearly equal specific heats at these lower temperatures, have 

 nearly equal specific heats at about 1800°. 



For vapour of water the molecular heat-capacity was found to be 

 18-12 at 3240°. Mallard and Le Chatelier had found 16-6 at 3.350° and 

 that there was no dissociation at this temperature : these results are in 

 fair accordance with those of Berthelot and Vieille. 



These authoi-s go on to discuss the absorption of heat in the form of 

 specific heat and in dissociation ; attributing the former to actions on the 

 molecules themselves either in increasing the kinetic energy of rotation 

 and vibration of the molecules or of their component parts, or to a disgre- 

 gation of the molecules without dissociation ; as in the cases of CO2 and 

 NoO, in which cases heat is absorbed up to 200°, as shown by Regnault's 

 results as to the specific heat of these gases and the increase of specific 

 heat with rise of temperature. 



This absorption of heat during the disgregation of molecules, &c., as 

 shown by the rise of specific heat with rise of temperature has an 

 important bearing on the heat of combustion at different temperatures, as, 

 e.g., in the cases of CO and H2 to form CO2 and HjO. In the case of the 

 formation of HCl from 1 gram of hydrogen at constant pressure, althongh 

 this gives 22,000 thermal units at 0°, yet at 2000° the heat evolved is 

 about 26,000, the difference being due (apart from dissociation) to the very- 

 great increase in the specific heat of chlorine between 0° and 2000° ; 

 whereas 1 gram of hydrogen, in combining with oxygen to form water, 

 evolves at 0° 29,000 units, and at 2000° (in consequence of the great rise 

 in the specific heat of HjO vapour from 0° to 2000°) 25,300 ; hence the 

 heat of combination of H,C1 is greater than that of ^ (Hg,©) at 2000°, 

 although less at 0°, at constant volume. In the explosions of hydrogen 

 and oxygen, and of hydrogen and chlorine, in closed vessels there appears 

 to be little or no dissociation of HCl or of HjO at 2000°. 



At p. 84 of this volume of the 'Ann. Chim.' Berthelot questions 

 the validity of Avogadro's hypothesis and its applicability in particular 



