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 when I say that though it was a true explosive, burning with extreme rapidity, it was a poor conductor of heat. When the charge was fired all the pieces commenced to burn on the surface, and the combustion was so rapid that it spread itself through each piece of the material more quickly than the high temperature could arrive at the centre of that piece by conduction of heat. Hence it followed that the piece always burnt from the outside inwards and the explosion never commenced inside the piece, so that by changing the shape and size of the pieces into which the material was cut and thus making the extent of their surface large or small compared with their bulk, you could make a powder which would burn more or less quickly. For instance, a favourite method of increasing the rapidity of burning is by perforating the pieces so as to increase the surface without increasing the bulk. In this way there can be made out of the same substance powders suitable for small arms which require a very rapidly burning powder and others suitable for 21