498 KEPORT— 1888. 



explosion, we have Q,:=(c + inc') T', from which now that c is known c' can 

 be found for various temperatures ; and hence the specific heat of hydro- 

 gen at different temperatures can be deduced. Similarly for the specific 

 heats of oxygen and nitrogen. 



Above we have assumed the specific heats of CO2 known below the 

 temperatures of its dissociation : these are found from the results of 

 exploding CO and O with varying quantities of COo sufficient for the 

 mixture to explode at temperatures below 2000° ; from the results of such 

 experiments, the product being CO 2 alone, a formula can be found to give 

 the specific heats of COo at various temperatures below 2000°. 



It should be mentioned in reference to the specific heats of vapour of 

 water that Wiillner ' deduces, ft'om experiments of his own combined with 

 Regnault's, the number 1-173 as the ratio between the specific heat at 

 100° and 0°, and that Winckelmann,- from experiments with reference to 

 the coefficients of conduction of heat by gases and vapours, deduced for 

 the same ratio the number 1'172, which agrees closely with Wiillner's. 



For the very high temperatures produced in these explosion experi- 

 ments the observed pressures, which excluded the momentary highest 

 pressure of the explosive wave, often exceed 600 cm. of mercury, but do not 

 exceed 10 atmospheres, and are under 8 metres ; and it may therefore be 

 safely said that, apart from dissociation, the gases raised to these high 

 temperatures at constant volume behave approximately as perfect gases in 

 respect to pressure and temperature, so that for each gas the pressure is 

 nearly proportional to the absolute temperature. The equation Cp=:C„ + l '98 

 therefore holds approximately (p. 28) and gives us an easy method of 

 deducing the specific heats, and also the molecular heat-capacities of the 

 gases at constant pressure. 



MM. Mallard and Le Chatelier found that if for any volume of one of 

 the four gases hydrogen, oxygen, nitrogen, carbon monoxide we replace 

 in an explosive mixture of gases an equal volume of another of the four, 

 throughout the whole range of temperatures embraced in their experi- 

 ments the temperature of combustion remains unaltered ; these observa- 

 tions led them to propound in 1882 ^ the following law : The molecular 

 heat-capacities of perfect qases luhich are equal zo each other at ordinary 

 temperatures are also equal to each other at about 3000° and perhaps above. 

 As the temperature rises the specific heats, not only of gases and 

 vapours in general, but of those called perfect gases also, increase ; the 

 latter, it is true, very much more slowly, so that the difference between 

 the specific heats of, e.g., water-vapour or CO2 and of the so-called per- 

 fect gases is a gi-eat deal more marked at 2000° than at 0° : this is seen 

 in the numbers given below for the molecular heat-capacities at constant 

 volume of 



at 0° at 2000° 



The ' perfect ' gases 4-8 ... 6-0 



H„0 vapour 5-6 ,. . 12-2 



CO, 6-3 .. . 13-6 



HCl was shown to have the same specific heat, or nearly, as the other 

 perfect gases np to 1800°, and to dissociate not much before 2400°, the 

 temperature of explosion of HCl. A similar rapid increase in specific 

 heat with rise of temperature was observed for chlorine as for CO2 and 

 HjO. These results were confirmed subsequently by Berthelot and 

 Vieille. 

 » Wied. Ann. i, 1878, p. 7. ^ p^^^_ ^,,,j_ 159_ i^jq^ p. 177. 3 (j. R. 96, 1883. 



