ON GASEOUS EXPLOSIONS. 333 



ficiently impervious to the gas. Dr. Harker, to whom the Committee 

 are greatly indebted for much information upon this subject, bel eves, 

 however, that he is now in possession of a material which will satisfy 

 both of these conditions up to a temperature of 1800° C, and he has 

 suggested that an attempt should be made to compare thermometers 

 constructed with nitrogen, with CO.2, and with argon up to that 

 temperature. If the nitrogen and argon thermometers are found to 

 agree, then, by reason of the great difference in the constitution of 

 these gases, it is almost certain, as explained above, that each agrees 

 with the thermodynamic scale. If, on the other hand, they do not 

 agree, then the presumption is in favour of the argon thermometer, 

 because this gas is supposed to be monatomic and to be incapable of 

 dissociation. The Committee venture to express a hope that a research 

 on these lines will be commenced and carried to a conclusion. They 

 believe that the results obtained will be of very great importance in 

 the investigation of explosions and in the theory of the gas-engine, and 

 it seems to them an inquiry eminently fitted for the National Physical 

 Laboratory. 



The comparison of gas thermometers is, however, not the only way 

 in which the problem of thermometry at high temperatures may be 

 attacked. Another method, and one that is more satisfactory in some 

 ways because it is more fundamental, is to investigate the dependence 

 of the energy upon the density of the gas. As pointed out at the 

 commencement of this Report, any interdependence between f^nergy and 

 density at a given temperature must be accompanied by a corresponding 

 deviation from the perfect gas-law, and investigation of change of energy 

 with density must be the ultimate basis of gas thermometry. The Joule- 

 Thomson experiment was, of course, of this character. Since then Joly 

 has determined the change of specific heat of CO2 at pressures ranging 

 up to the critical pressure. But these determinations refer only to 

 temperatures of the order of 100° C. As was pointed out at the 

 commencement of the section of this Report dealing with explosion?, 

 the corresponding measurement at very high temperatures can be very 

 easily made when once the various corrections necessary to determine 

 internal energy by explosion experiments have been satisfactorily per- 

 formed. It is only necessary to compare the pressures reached in 

 explo.sions of mixtures identical in composition but of d liferent density. 

 Should the pressures after explosion, when corrected, be proportional to 

 the pressures before explosion, then the energy is independent of the 

 density, and we have proof that the gaslaw hrlds up to the temperature 

 reached by the combustion. On the other hand, a departure from the 

 proportionality would imply a corresponding departure from the 

 gas-laws, the amount of which could be calculated. Mallard, Le 

 Chatelier, and Langen have made very careful comparisons of this kind, 

 and they have found that the actual maximum pressures reached in the 

 explosions are in many cases very approximately proportional to the 

 pressures befoi'e explosion. Petavel has found that this proportionality 

 is not much altered even when the density of the gas is increased 

 severity times. This may be regarded to some extent as evidence that 

 there is no very great difference between the gas scale and the thermo- 

 dynamic scale at the temperatures of 1700°, or more, which were reached 

 in these experiments. But it must be observed that this inference is 

 subject to the same limitations as the determinations of internal energy 



