ON GASEOUS EXPLOSIONS. oSl 



oi which is necessary for getting absolute values) is obtained either by 

 estimating the quant ty of gas present in the cy'inder or by direct 

 measurem^nt with the platiaum thermometer, as was recently done by 

 Callendar and Dalby. 



The temperature scale so obtained is probably sufficiently definite, at 

 any rate for the purpose of gas-engine theory, since the mixture to whicli 

 ifc is applied dops not vary very greatly in composition, and always consists 

 mainly of nitrogen. It is not so certain, however, that this scale agrees 

 with the absolute thermodynamic scale ; and the question of the possible 

 amount of the deviations at temperatures of 1500° and over is of great 

 importance in connection with the present inquiry. 



So far as the Committee are aware, the experiments of Joule and 

 Thomson still remain the only comparison between the various gas-scales 

 and the thermodynamic scale : this comparison only extended to about 

 200° C, and is, of course, of no application to the problem now under dis- 

 cussion except in so far as it gives an idea of the differences to be expected 

 at higher temperatures. What it really shows is that thermometers 

 constructed of the more permanent gases are all so closely accordant with 

 the thermodynamic thermometer as to lead to the belief (as a matter of 

 induction, and quite independently of the kinetic or any other theory) 

 that there is really some definite cause tending to make a gas, as such and 



apart from its composition, obey the law ^=constant. It would appear 



that the small deviations from this law, sometimes one way and sometimes 

 the other, which are observed must be due to disturbing causes depending 

 on the nature of the gas, whose influence may be in either direction and 

 is of very various amount, but is at low temperatures small compared with 

 the tendency to obey the perfect gas law. This view being accepted, 

 there is a strong presumption that, if a number of thermometers con- 

 structed of different gases be compared at high temperatures and be 

 found to agree faii-ly well, then they all agree with the thermodynamic 

 scale at leafct as well as they agree with one another. It is upon the 

 agreement between different gas-thermometers that our belief in the 

 measurement of temperature is really founded, and, so far as it goes, 

 the foundation seems to be sound. 



The nitrogen thermometer has been used with an iridium bulb up to 

 1600° C.,1 but no other gas has been taken above 1100° C. At the 

 latter temperature the differences between thermometers constructed of 

 hydrogen, nitrogen, and air are quite negligible for the present purpose. 

 Rather more deviation has been observed in CO.2 ; but having regard to 

 the small percentage of this gas which is ordinarily present in the gas- 

 engine mixture, it is not likely that temperatures up to 1100° C, 

 calculated in the usual way from the indicator diagram, will differ much 

 from the true temperatures on the thermodynamic scale. 1100° C. is, 

 however, not much above the lower limit of the gas-engine range ; as to 

 what goes on in the upper part of that range we have little or no 

 evidence. 



When considerable deviation from the gas-laws at high temperatures 

 is observed in the case of any gas, it is usually ascribed to dissociation. 

 For example, if comparison be made of two constant-pressure thermo- 

 meters filled respectively with hydrogen and with iodine vapour 



' Holboin and Valentiner, Ann. d. Phys. xxii, (1007), p. 1. 



