EXPERIMENTS ON SOLID AND GASEOUS EXPLOSIVES. 379 



When fully ignited, each particle is freely suspended in space, being kept from direct 

 contact with other bodies by the rush of flame issuing from its surface. It is to 

 these conditions that the law of combustion by parallel layers accurately applies. 



While the combustion is taking place, heat is being continually transmitted to the 

 walls of the enclosure, and the maximum pressure attained will therefore be less for a 

 slow explosion than for a fast one ; the actual effect may be seen by reference to 

 figs. 11, 12 and 15. 



The heat loss accounts also, as stated above, for the manner in which the curves of 

 rise and fall of pressure merge together. By the time the maximum pressure is nearly 

 reached the diameter of each particle of explosive is greatly reduced. The weight of 

 substance consumed per unit time begins therefore to decrease, although the flame is 

 actually advancing towards the axis of each cord at an ever increasing speed. Finally, 

 the combustion just counterbalances the total thermal loss, and the curve of pressure 

 remains for an instant practically constant at its maximum value. This will be seen 

 clearly in figs. 1 and 2 on Plate 21. 



Effect of the Enclosure. 



We have just referred to the thermal loss due to the cold walls of the explosion 

 chamber. The total loss, cceteris paribus, is proportional to the time. 



When the diameter of the cordite, and consequently the time occupied by the 

 combustion, is very small, the theoretical value of the maximum pressure is closely 

 approached, and the shape and size of the enclosure have but little effect (compare 

 A and B, fig. 14). These factors become, however, of considerable importance in 

 determining the maximum pressure developed by the slower burning cordite (see 

 fig. 15). 



The shape of the cooling curve depends, on the other hand, essentially on the 

 dimensions of the enclosure. In fig. 16 the facts are clearly illustrated by the results 

 of comparative experiments carried out respectively in a sphere and in the cylinder. 



It is proposed to reserve the general discussion of the questions of dissociation and 

 rate of cooling for the third part of the present research ; we shall then be dealing 

 with gaseous mixtures of simple composition which will serve as a natural introduction 

 to the consideration of more complicated questions. A few words are, however, 

 necessary with regard to the somewhat unusual conditions under which the cooling 

 of the products of combustion of a solid explosive takes place. 



Under ordinary circumstances the convection and conductivity of the gas itself are 

 the ruling factors which determine the rate of cooling. 



The thermal capacity of the gaseous mixture and the rate at which heat can be 

 transmitted through it are low compared with the corresponding properties of the 

 enclosure. These facts hold true whether the latter is water-cooled or not. 



In such cases neither the inner surface of the enclosure nor the layer of gas in 



3 c 2 



