326 Mr. F. J. Brcmwell [Feb. 27, 



would be the most destructive of all, producing throughout the mass 

 a rise of temperature to the inflaming point, and thus causing a 

 detonation under the following circumstances: — Powder compressed 

 to its fullest specific gravity, powder heated up all through to the 

 point of explosion, and powder ignited at every portion of its bulk at 

 the same moment. 



With respect to the heating, I find I have not called your attention 

 to the importance — the twofold importance — of this. The first way 

 in which it is important is, that heating renders the chemical changes 

 which occur when the powder is inflamed more rapid, and thereby 

 causes the explosion to be more violent than it would be if the 

 temperature were not raised. The second way in which the 

 heating up is important is owing to the fact, that, when powder 

 explodes, the effect of the explosion is due, not alone to the ex- 

 pansion which arises from the conversion of the solid into a gas, 

 but very largely to the further expansion which arises from the 

 heating up of the gas produced. Anything therefore which abstracts 

 heat from the products of the explosion, diminishes the expansion of 

 the gas, and thereby diminishes the effect produced. Now, assuming 

 the inflaming point of gunpowder to be about 660 degrees, and 

 assuming it to be of the temperature of the atmosphere, or say 

 60 degrees, the whole weight of gunpowder has to be heated 600 

 degrees before it attains the point of combustion, and this heat is 

 abstracted from that which would otherwise go to augment the bulk 

 of the gases. I am not speaking now of the heat requisite to convert 

 the solid into a gas, for that must be expended in any event, but I 

 am speaking of the heat needful to raise the solid from the ordinary 

 temperature of the air to the temperature of ignition. 



We have again to thank Professor Abel for an experiment which 

 will show you the effect on the burning of powder of the rapid 

 abstraction of heat. 



There are here two similar pieces of pebble powder. One has 

 been cooled down to freezing, the other has been heated up to about 

 350°. On firing them you will see that while the first burns slowly, 

 the second burns more rapidly. 



Another mode of showing the effect arising from the abstraction 

 of heat was devised by Professor Abel many years ago. There is 

 here an exhausted receiver, containing powder at the ordinary tem- 

 perature. On igniting it by a wire heated electrically up to white- 

 ness, it will be found that the powder, instead of exploding, will burn 

 in the immediate neighbourhood only of the wire ; indeed, if we can 

 succeed in throwing the image on the screen, the powder will appear 

 to be boiling, and producing a vapour ; and it will not be until after 

 such an interval that the production of gas has partially restored 

 pressure in the receiver, or until air is admitted, that the residue 

 of the powder will explode, the reason being that in the vacuous 

 condition existing here, the gas generated expands with great readi- 

 ness, and takes up the heat rapidly, and this goes on until, as has 



