Propellants. Although combustion is propagated through a quantity of gunpowder with very great rapidity, it is not done instantaneously. The time required is about one-hundredth of a second under ordinary conditions, and this interval, short though it is, is very important. When the object is to throw a projectile, the inertia of the latter has to be overcome, that is, a certain amount of force has to be applied before the heavy body begins to move. In order that the strain on the breech of the gun may be as small as possible, the pressure must be gradually developed and must reach its maximum just as the projectile begins to move. The time factor in the explosion constitutes the difference between what we now call “propellants” and “high explosive.” Propellants are explosives which develop pressure gradually, and are therefore used to launch the projectile; high explosive develops pressure instantaneously, and is therefore used as the bursting charge inside the shell, bomb, or torpedo, and also in blasting operations. Cordite, or smokeless powder, is the propellant now most used. It is made by macerating guncotton and nitroglycerine with their common solvent acetone. A pulp is thus made to which 5 per cent. of vaseline is added. The mixture is then forced through a die, and in this way it is formed into threads or rods, which harden as the acetone evaporates. Cordite produces no smoke, because all the products of its combustion are invisible gases. High Explosive. _Nitroglycerine_ and _Guncotton_ are both explosives of the instantaneous kind. The former is made by forcing glycerine, under pressure in a very fine stream, into a mixture of fuming nitric and concentrated sulphuric acids; the latter by soaking cotton-wool in a similar mixture. Both products are washed with water until quite free from acid, and subsequently dried. Nitroglycerine is a colourless oil with a burning taste. The oil itself is very dangerous to handle, for it is liable to explode spontaneously even when the utmost care has been taken in its preparation. A mere spot on a filter paper explodes with a deafening report when gently hammered on an anvil; and one drop, when heated on a stout iron plate, blows a hole through the plate. No use could be made of this substance for many years after its discovery because it was so liable to explode during transportation; now, however, it is made safer by mixing with absorbent infusorial earth or _kieselguhr_. This mixture is known as dynamite. Blasting gelatine, like cordite, is a mixture of nitroglycerine and guncotton.