ON GASEOUS_£XPLOSlOKS. 3-35 



Effective Temperature and Effective Specific Heat, 



2. Since the temperature of a mass of gas, when exploded in a closed 

 vessel oi' in the cylinder of a gas-engine is far from uniform, and since 

 the actual distribution of temperature is necessarily somewhat uncertain, 

 it is evident that the variation of the specific heats of the constituents 

 with temperature cannot be certainly deduced from a knowledge of the 

 heats of combustion and the efiective temperature, even apart from 

 difficulties inseparably connected with the determination of the cooling 

 corrections. It is possible, however, by explosion experiments to deduce 

 values of the apparent or effective specific heats which, in so far as they 

 approximate to the conditions actually existing in the gas-engine, may be 

 of greater practical utility than the true specific heats would be if they 

 could be independently determined. The method of Dugald Clerk, in 

 which the specific heat is directly determined from the work done on 

 the charge after ignition, appears to be particularly appropriate for this 

 purpose. 



It is well known that the values of the specific heats deduced from 

 explosion experiments are generally higher than those deduced by more 

 direct methods, and it has been customary to explain the discrepancy 

 largely by possible errors inherent in the explosion method. Such errors 

 undoubtedly exist, and require careful investigation, but in arriving at 

 a decision it is most important to subject other experimental methods to 

 an equally close scrutiny. 



Experimental Errors in the Determination of the Specific Heats of Gases 

 by the Constant'piressure Method. 



3. Apart from errors in the measurement of the temperature of the 

 gas and of the calorimeter, which are not likely to be serious at low 

 temperatures, there is an important source of error in this method, as 

 applied by Regnault and subsequent observers, which has generally been 

 overlooked. In Regnault's experiments, the rate of gain of heat from 

 the heating vessel by the calorimeter was observed before and after the 

 experiment proper, while the gas was not passing through the connecting 

 tube, and was assumed to be the same whether the gas was passing 

 or not. The correction amounted, when the heater was at 200° C, to 

 between 4 per cent, and 5 per cent, of the heat supplied by the gas. 



The effect of the gas curi'ent would certainly be to change the 

 temperature gradient in the connecting tube in such a manner as to 

 diminish the heat conducted from the heater during the passage of the 

 gas. The error from this cause cannot be exactly determined, but would 

 probably amount to between 2 per cent, and 3 per cent, in Regnault's 

 experiments at 200° C, and would have the effect of making the values 

 as determined by Regnault too low. The true variation of the specific 

 heat of water was unknoM'-n in Regnault's time, and he was also unable 

 to correct his thermometers accurately to the absolute scale. These 

 considerations introduce minor uncertainties which might amount to as 

 much as 1 per cent, on the result. 



The specific heat of air considered as a mixture of perfect diatomic 

 gases, taking the calorie at 20° C. as equivalent to 4-180 joules, should 

 be 0'2405. Since air is not a perfect gas the actual value must be some- 

 what greater than this. Regnault's value, 0'2375, is evidently too low. 



