320 titePORTS ON THE STATE OF SCIENCE. 



this diagi-am by the application of Mr. Clerk's method, and it would lead 

 to a result considerably in excess of the truth. Mr. Clerk has himself 

 tried this same experiment of compressing and expanding air, and lie 

 also has found that the resulting value of the specilic heat of air is 

 too high and that the air takes in heat during expansion. Professor 

 Hopkinson thinks it possible that the heat lost during the partial 

 compression line in Clerk's diagram may be more than twice as great 

 as the loss during expansion. If this were so, the correction for heat-loss 

 in expansion would be less than 16 per cent, of the work done instead 

 of 25 per cent., and the true change of energy would be less than that 

 calculated on the assumption of equal heat-loss in compression and 

 expansion by 7 per cent, or more. 



It will be seen that the errors believed to affect each method of 

 experiment are in such a direction as to account for the divergence of the 

 results ; and it is quite probable that when these errors are completely 

 allowed for, the discrepancy will largely disappear. Meanwhile the 

 internal energy of the products of combustion in the gas-engine at 

 1200° C, if taken as the mean of Clerk's and of Holborn and Henning's 

 results, must be regarded as subject to a possible error of about 5 per 

 cent. 



Under these circumstances it does not seem necessary to discuss the 

 possibility that there may be a real difference between the energy values 

 obtained by the two methods due to the different conditions of experi- 

 ment. It may be pointed out, however, that Clerk's gas was at the 

 maximum temperature from fifteen to twenty times as dense as Holborn 

 and Henning's. This difference in the condition of the gas is such that 

 a comparison of the results obtained by the two methods, when freed 

 from experimental errors, will be of great interest and importance. 



(3) JExplosion Exjyeriments. 



If a combustible mixture of gases be fired in a closed vessel im- 

 pervious to heat, and if sufficient time elapse to allow of the attainment 

 of complete thermal and chemical equilibi-ium, the internal energy of the 

 products of combustion after the explosion will be equal to the chemical 

 energy before explosion. The latter is capable of accurate measurement. 

 The temperature reached after explosion can be inferred from the 

 pressure, assuming the gaseous laws to hold. The pressure can also be 

 measured without difficulty and with considerable accuracy. 



In the study of explosion pressures we ha\e therefore a very con- 

 venient and simple means of getting the internal energy function at high 

 temperatures, provided that it is possible to make the necessary correc- 

 tions for deducing from the pressures observed in a real explosion the 

 pressure reached in an explosion under the ideal circumstances postulated 

 above. Moreover, the gaseous laws on which the temperature estimations 

 are based can themselves be checked, and if necessary corrected, by com- 

 parison of the pressures reached by mixtures of the same composition 

 but of different densities. Thus explosion experiments are capable of 

 furnishing a complete account of all the thermal properties of gases at 

 the temperatures reached by combustion, subject always to the possibility 

 of making the corrections referred to above. The difficulty of finding 

 these corrections is, however, very great, and in consequence of the 

 uncertainty which prevails even as to their order of magnitude, the 



