OH GASEOUS EXPLOSIONS. 819 



Professor Calleudar has favoured the Committee with a note dealing 

 with the constant-pressure experiments. He is of opinion that the 

 results obtained by Regnault's method are too low, and that at the 

 higher temperatures reached by Holborn and Henning the error may 

 possibly amount to as much as 10 per cent. In all these experiments 

 there is a considerable flow of heat from the heater to the calorimeter. 

 This, of course, has to be deducted from the heat registered in the 

 calorimeter in order to find that which has been given up by the hot 

 gas. All the experimenters by this method have determined the amount 

 of this correction by observations when the gas was not flowing, or 

 have compensated it under the same conditions by radiation from the 

 calorimeter, making, in either case, the assumption that the amount of 

 heat conducted is the same whether the gas be flowing or not. Professor 

 Calleudar is of opinion that this heat-flow is, in fact, much less when the 

 gas is flowing. He considers that even the values obtiiined by Regnault 

 may be as much as 2 or 3 per cent, too low, and he supports this conten- 

 tion by reference to the work of other experimenters (some of which 

 has been alluded to above) and by theoretical considerations. As this 

 type of error is likely to increase greatly with rise of temperature a 

 systematic error of even 2 per cent, in Regnault's results, if established, 

 would give reason to suspect that the experiments at high temperatures 

 may be subject to erroi's of real importance for the present purpose. 



If there be systematic error in Mr. Clerk's work it seems most likely 

 that it lies in the estimate of heat-loss. The total heat-loss in the first 

 partial compression and expansion line in the diagram (b c d, fig. 2) is 

 estimated from the fall of temperature and from the net work done 

 (area bcd) in the double operation, and amounts to, roughly, half the 

 work done in expansion. This loss has to be divided between com- 

 pression and expansion, and Mr. Clerk divides it on the assumption that 

 if the mean temperature in compression and expansion were the same 

 the heat-loss would also be the same. The mean temperature in expansion 

 is, in fact, rather less than in compression, and the heat-loss calculated in 

 this way is correspondingly smaller, but the difi'erence on this account is 

 not very great, and the result is, roughly speaking, that the loss is equally 

 divided between the two operations. Thus the correction to be added 

 to the work done in expansion in order to get the total loss of energy 

 of the gas is about 25 per cent, of the work, or 20 per cent, of the 

 energy change. 



Professor Hopkinson has dealt with this point in a note which he 

 communicated to the Committee, and he is of opinion that, relative to 

 the mean temperature, tiie heat- loss is really much greater in compression 

 than it is in expansion. He supports this view by reference to some 

 experiments which he has made on the compression and expansion of a 

 charge of cold air in a gas-engine which was motored round with the 

 gas cut off". The specific heat of air being known, the loss of heat in 

 any part of the compression or expansion stroke can in this case be 

 independently estimated from the diagram. He found that while in the 

 latter half of th« compression stroke the heat lost to the walls amounted 

 to a considerable fraction of the work done, some part of this loss was 

 actually restored to the gas during the first half of the succeeding 

 expansion, and this notwithstanding the high temperature of the air, 

 which in expansion, as in compression, was much above that of the walls. 

 An estimate of the thermal capacity could, of course, be obtained from 



