32 i REroRTs o>: the state of science. 



question of a general character awaiting solution in connection with 

 gaseous explosions when regarded as a means of investigating the 

 properties of gases at higher temperatures. It will be convenient to 

 discuss each of the corrections enumerated above with special regard 

 to this question. 



Loss of Heat. — That much of the heat-loss goes on by direct con- 

 duction to the walls, and is, therefore, a surface phenomenon, is obvious. 

 But there is reason to believe that the loss by radiation, which certainly 

 exists in any flame, is practically important. 



(a) Measurements of the temperature reached in an explosion by 

 means of a platinum thermometer, under circumstances which render 

 very improbable any loss of heat by conduction from the gas whose 

 temperature is measured, show that that temperature is considerably 

 lower than is to be expected from the heat of combustion of the gases 

 and the specific heat of the products. Professor Callendar pointed out, 

 in the discussion of these experiments, that there was probably a good 

 deal of radiation, and stated that he had found that an ordinary Bunsen 

 flame might radiate up to 15 per cent, of its heat.' 



(6) Recent experiments, in which the loss of heat during an ex- 

 plosion was directly measured by finding the rise of temperature of the 

 walls, showed that in a certain coal-gas explosion it amounted to about 

 12 per cent, of the whole heat at the moment of maximum pressure. 

 Estimated by Mallard and Le Chatelier's extrapolation method, the loss 

 was at most 5 per cent.- 



The prevailing opinion seems to be that most simple gases cannot 

 be made to radiate by direct heating. If this be so the radiation must 

 take place in the act of combustion. It seems very probable that when, 

 say, liydrogen and oxygen combine a certain part of the energy of 

 combination passes into the form of internal vibrations of the steam 

 molecule, and that a large proportion, if not all, of this part is ultimately 

 radiated away. If this b(3 the case a definite proportion of the heat 

 produced in combustion is always lost, and a comparison of explosions 

 in vessels of diflferent sizes would not reveal this loss. 



Thermal Equilibrium. — When an explosive mixture of gases is 

 ignited in a closed vessel the effect of the change of pressure during 

 the progress of the flame from the point or points of ignition is to raise 

 the temperature round about those points much above the mean tempera- 

 ture, and, on the other hand, the temperature attained at those places 

 which are last reached by the flame, and where the gas is compressed 

 before instead of after ignition, is much below the mean. Even in a 

 vessel whose walls are impervious to heat the diflerence of temperature 

 between the points first and last inflamed might amount to 700'' C. at the 

 moment of maximum pressure.^ In a real explosion the cooling effect of 

 the walls causes the temperature to range from perhaps 300° or more 

 above the mean (as shown by the pressure) right down to the wall tem- 

 perature at points close to the metal. The existence of large temperature 

 differences in the gas close to the walls of an engine cylinder was first 

 experimentally demonstrated by Professor Burstall with the aid of 

 platinum thermometers 



If the volumetric heat of th« gas were constant the equalisation of 



' Proc. E.S., A, \o\ Ixxvii. p. 400. ^ Ibid., A, vol. Ixxix. p. 147. 



' Ibid., A, vol. Ixsvii. p. 389. 



