ON GASEOUS EXPLOSIONS'. 317 



The values at 100'^ are derived from the experiments of Wiedemaiux 

 and Regnault. Those at 600° and 1100° are based on the specific heat 

 values given by Holborn and Henniug ; in other words, they are obtained 

 by drawing tangents to the curves, iig. 1. The error at these higher 

 temperatures may be double that of the internal energy, or, say, 4 per 

 cent. The figures show tliat the volumetric heat of air increases by about 

 •0009, that of steam by -0033, and that of CO., by -0036 per degree 

 centigrade over the range 100°-1100° C. There is no evidence that the 

 rate of increase is other than constant in the case of air ; but there can hs 

 no doubt that the average rate of increase between 100° and 1100" in CO., 

 is less than half the rate of increase between 0° and 200°, as determined 

 by Wiedemann and Regnault. There is also distinct evidence in these 

 and other experiments that the rate of increase of the specific heat of 

 steam becomes greater as the temperature rises, 



(2) Clerk's Experiments,^ 



These cover about the same range of temperature as Holborn and 

 Henning. The gas used was the products of an explosion in a gas-engine, 

 and therefore consisted of a mixture of CO2, steam, and air. It was first 

 expanded in the ordinary course after the explosion, and was then heated 

 by compression on the next in-stroke of the engine, the valves being kept 

 closed for this purpose. On the next out-stroke the gas was again 

 expanded, then compressed again, and so on, the valves remaining closed 

 and the engine running on its own momentum. An indicator diagram was 

 taken of the whole operation. The change of internal energy in any 

 portion of a compression stroke {e.g., b c in fig. 2) is equal to the work 

 done less the heat lost to the cylinder walls ; in an expansion stroke (c d) 

 it is the work done plus the heat lost. The work can be obtained from 

 the indicator diagram with an accuracy which is only limited by the 

 indicating appliances. The change of temperature can also be calculated 

 from the indicator diagram subject to a knowledge of the temperature at 

 one point. Errors in the latter, however, do not greatly aflect the 

 results found for internal energy or volumetric heat, because the figure 

 for the quantity of gas present is affected by these errors in such a way 

 as to cancel out the error in temperature interval. 



The loss of heat comes in as a correction on the work done and was 

 estimated by a comparison of the compression line and the immediately 

 following expansion line (b c and c d, fig. 2). The calculation is based on 

 the assumption that the total heat loss from the hot gases during any 

 given portion of a stroke is the same in expansion and compression if 

 the mean temperature be the same. 



In the first compression the temperature of the gas rose to about 

 1100° C. (at the point c, fig. 2). During the first three tenths of the 

 following expansion stroke (c d), the temperature fell to about 700° C. 

 The work done in this part of the expansion was measured and the 

 heat loss determined as above M'as added. Thus the change of internal 

 energy corresponding to the temperature change 1100°-700° is obtained. 

 The average volumetric heat over this range is within the errors of 

 experiment equal to the volumetric heat at the mean temperature of 

 900° C, which accordingly is by this method determined direct instead 



' I'l-of. Ii'oi/. ,V(»c., A, vol. Ixxvii. 



