332 EEPOKTS ON THE STATE OF SCltKCEi 



tiiey will be found to agree up to about 1000° Abs. ; the iodine ttiermo- 

 meter will then begin to I'ead higher than the hydrogen thel^mometer 

 until, when the latter reads about 1800° Abs., the former will read 

 about double that amount. When the gases are hotter still the 

 temperature shown on the iodine thermometer Avill continue to be 

 double that shown on the other. In this case the departure between 

 the two thermometers is accompanied by a change in the absorption 

 spectrum of the iodine vapour ; and the whole phenomenon is expressed 

 by saying that the iodine molecule has been split up or dissociated. In 

 the case of a compound gas this dissociation sometimes takes the form 

 of an actual separation of the constituents, which can be detected by 

 diffusion. A great deal of experimental work has been done with the 

 object of ascertaining to what extent the gases CO2 and steam split up 

 at high temperatures. These gases are constituents in most gas-engine 

 mixtures, and if they dissociate to any considerable extent there will be 

 a corresponding effect upon the pv9 relations of the mixture of which 

 they form a part. So far, however, there is, in the opinion of the 

 Committee, no conclusive evidence that either steam or CO2 is dis- 

 sociated to an extent which is material for the present purpose. Slight 

 traces of dissociation have undoubtedly been found in both cases, but 

 the method of experiment is such as to leave it doubtful how far these 

 have been conditioned by the nature of the walls through which the 

 dissociated gas is diffused. It must be observed, moreover, that CO2 

 and steam usually form only a small part of the mixture in the gas- 

 engine, and that therefore a considerable amount of dissociation of these 

 gases would be necessary to produce much effect upon the pressure of the 

 whole. Again, such dissociation, if it occurs, must have an effect upon 

 the energy of the gas out of all proportion to the effect which it has 

 upon its temperature. Take, for example, the case of a mixture formed 

 by the explosion of CO and air and containing 10 per cent, of CO2, the 

 remainder being nitrogen. If, by heating, one tenth part of the CO2 

 be split up into CO and oxygen, the resulting change of pressure of the 

 whole mixture will be only one two-hundredth part ; but this amount of 

 dissociation could only be effected by the absorption of an amount of 

 heat of the order of 10 per cent, of the total heat of combustion of the 

 gas. In other words, the mean specific heat of the mixture, as deter- 

 mined by the explosion, would be roughly 10 per cent, lower than if 

 there had been no dissociation. Any considerable departure from the 

 gas-laws in such a mixture, if it be ascribed to dissociation at all, must 

 therefore be put down to dissociation of the nitrogen, which might con- 

 ceivably occur at 2000° C, just as iodine vapour is dissociated at a much 

 lower temperature. It does not seem likely, h -wever, that if nitrogen 

 dissoiia'es its splitting-up would be accompanied by any visible change 

 in its physical properties, such as is observed in the case of iodine. The 

 phenomenon in this case would be rendered evident only by the departure 

 from the gas-law, and possibly by absorption of heat. 



It would appear, therefore, that our knowledge of thermometry at 

 these temperatures is more likely to be advanced by direct experiments 

 on the rt-lafiou between the pressure or volume and the temperature 

 than by looking for other evidences of dissociation. The difficulty in 

 carrying the comparison of d ffe'ent gas-thermometers to very high 

 temperatures has hitherto lain in the absence of any material sufficiently 

 refractory to withs'.and such tenipe;atures, and at the same time suf- 



