ON GASEOUS EXPLOSIONS. 313 



for purposes of thermodynamic calculation, and it has the further 

 advantage that it expresses the actual quantity measured. In nearly 

 all the experiments on the specific heats of gases the increase of energy 

 in unit volume associated with a large rise of temperature is measured ; 

 and in most the lower limit of temperature is near that of the room. The 

 rate of change with temperature, of the energy so determined, is some- 

 times called the ' true ' or ' instantaneous ' speciflc heat, and sometimes 

 ' thermal capacity.' The Committee are of opinion that a definite name 

 should be given to this important quantity, and they suggest the name 

 ' volumetric heat,' which if adopted should include in its significance 

 that the measurement to which it relates is made at constant volume, 

 and is referred to unit volume of the gas. The term ' specific heat ' could 

 then be restricted to its usual meaning, which refers to unit mass of the 

 substance. Convenience of calculation is promoted if the unit of volume 

 taken is that corresponding to the gramme molecule under standard con- 

 ditions which is sufficiently nearly the same for each of the gases under 

 consideration and equal to 22-25 litres.' In this report internal energy 

 and volumetric heat are expressed as calories ' per 22-25 standard litres ; 

 and the zero of temperature from which the energy is reckoned (except 

 where otherwise stated) is taken to be 100° C, in order that steam may 

 be included on the same basis as the other gases. The results are con- 

 veniently exhibited as curves in which the energy is the ordinate, and 

 the excess of the temperature over 100° C. is the abscissa. The slope 

 of such a curve represents the volumetric heat C, and the ordinate 

 divided by the abscissa for any tempeiature represents the mean volu- 

 metric heat from 100° C. to that temperature, here denoted by C. 



The experimental work done on this subject may be divided into three 

 classes : — 



(1) Constant-pressure experiments : Regnault, Wiedemann, Witkow- 

 ski, Lussana, Holborn and Austin, Holborn and Henning. The gas is 

 heated from an external source in these experiments, and is at atmo- 

 spheric pressure. 



(2) Experiments in which both volume and pressure are varied, the 

 gas being heated by compression. The recent experiments of Clerk and 

 the determinations of the velocity of sound in hot gas by Dixon and 

 others belong to this class. 



(3) Constant-volume experiment. To this category belong the ex- 

 plosion experiments of Mallard and Le Chatelier, Clerk, Langen, Petavel, 

 Hopkinson, and others, and Joly's determinations with the steam 

 calorimeter. In the explosion experiments the gas is heated by internal 

 combustion. 



' The volumes of the gramme molecule for the several gases are : — 



H., . . . . 22-24 1 CO ... , 22-21 



N.J . . . . 22-28 I CO., . . .' . 22-O.S 



0„ . . , . 22-22 



in litres at 0° C. and under a pressure of 760 mm. of mercury. 



It may be noted here that 1 calorie per gramme molecule is equivalent to 3-06 

 foot-pounds per cubic foot. 



^ There is some difference in the energy value of the calorie according to the 

 temperature at which it is measured. The difference between the maximum and 

 mmimum value over the range 0° to 100° C. amounts to about 1 per cent. This is 

 of no importance for the purposes of this report, except in one or two places ; but 

 where it is necessary to be so precise the calorie at 15° C— namely, the quantity of 

 ^eat required to w,arm 1 gramme of water from 14^-° C, tp IR^° C. -is meant 



