ox GASEOUS EXPLOSIONS. 321 



large amount at work which has been done on explosion pressures gives 

 but little definite information as to the specific heats of gases. Never- 

 theless, it is to the study of explosion pressures that we owe such 

 knowledge as we possess of the energy function at the temperatures 

 which prevail in the gas-engine, and it is to work on these lines that 

 we must look in large measure for extension of our knowledge. A full 

 discussion of what has been done already must therefore form an important 

 part of this report. 



Let EL be the calorific value of the mixture before combustion, let A 

 be the heat lost at some point A on the record (taken on a revolving 

 drum) connecting the pressure and the time (fig. 3). The energy in the 

 gas is then H — h. The gas at this point is, however, certainly not in 

 thermal equilibrium, and is probably neither in chemical equilibrium nor 

 at rest. If, therefore, the loss of heat were suddenly arrested at A the 

 pressure would change owing to the more or Ies,s gradual attainment of 

 equilibrium in all three respects. The equilibrium value of the pressure 

 would be reached asymptotically, as shown by the dotted line. When 



"J 



Tvme 



equilibrium has been attained the energy of the gas is all thermal and 

 equal to H — h, and the temperature can be calculated in the ordinary 

 way from the pressure. The pi"oblem, therefore, is first to find or estimate 

 the heat loss h which has occurred at some point on the explosion record, 

 and then to find or estimate by how much the equilibrium value of the 

 pressure, if there were no further heat-loss, would differ from that shown 

 on the record. 



This change of pressure, marked p on the diagram, is due partly to 

 the combustion of the gas remaining unburnt at A and partly to the 

 equalisation of temperature by convection. It may also be due to some 

 extent to the damping-down of the motion of the gas set up by the 

 explosion. 



The sooner the point A is taken, the less will be the loss of heat ; 

 but the greater, on the other hand, will be the departure from equilibrium 

 conditions. The principal workers in this field. Mallard and Le Chatelier 

 and Langen, assumed that the latter might be neglected if the point A 

 were taken at the point of inflexion on the falling curve, and they 

 1908. r 



