[Illustration: FIG. 32.--PLAN OF THE BOX CONTAINING THE EXPLOSIVE, IN M'ROBERTS' MACHINE.] The application of heat in the manufacture of the jelly from collodion- cotton and nitro-glycerine is absolutely necessary, unless some other solvent is used besides the nitro-glycerine, such as acetone, acetic ether, methyl, or ethyl-alcohol. (They are all too expensive, with the exception of acetone and methyl-alcohol, for use upon the large scale.) These liquids not only dissolve the nitro-cellulose in the cold, but render the resulting gelatine compound less sensitive to concussion, and reduce its quickness of explosion (as in cordite). They also lower the temperature at which the nitro-glycerine becomes congealed, i.e., they lower the freezing point[A] of the resulting gelatine. [Footnote A: It has been proposed to mix dynamite with amyl alcohol for this purpose. Di-nitro-mono-chlorhydrine has also been proposed.] The finished gelatine paste, upon entering the cartridge huts, is at once transferred to the cartridge-making machine, which is very like an ordinary sausage-making machine[A] (Fig. 33). The whole thing must be made of gun-metal or brass, and it consists of a conical case containing a shaft and screw. The revolutions of the shaft cause the thread of the screw to push forward the gelatine introduced by the hopper on the top to the nozzle, the apex of the cone-shaped case, from whence the gelatine issues as a continuous rope. The nozzle is of course of a diameter according to the size of cartridge required. [Footnote A: G. M'Roberts, _Jour. Soc. Chem. Ind._, 31st March 1890, p. 266.] [Illustration: FIG. 33.--CARTRIDGE-MAKING MACHINE FOR GELATINE EXPLOSIVES.] The issuing gelatine can of course be cut off at any length. This is best done with a piece of hard wood planed down to a cutting edge, i.e., wedge-shaped. Mr Trench has devised a kind of brass frame, into which the gelatine issuing from the nozzle of the cartridge machine is forced, finding its way along a series of grooves. When the frame is full, a wooden frame, which is hinged to one end of the bottom frame, and fitted with a series of brass knives, is shut down, thereby cutting the gelatine up into lengths of about 4 inches. It is essential that the cartridge machines should have no metallic contacts inside. The bearing for the screw shaft must be fixed outside the cone containing the gelatine. One of these machines can convert from 5 to 10 cwt. of gelatine into cartridges per diem, depending upon the diameter of the cartridges made.