In order to find out how long any sample of cotton requires to be dried, a sample should be taken from the centre of several boxes, well mixed, and about 1,000 grms. spread out on a paper tray, weighed, and the whole then placed in the water oven at 100° C., and dried for an hour or so, and again weighed, and the percentage of moisture calculated from the loss in weight. This will be a guide to the time that the cotton will probably require to be in the drying house. Samples generally contain from 20 to 30 per cent. of water. After drying for a period of forty-eight hours, a sample should be again dried in the oven at 100° C., and the moisture determined, and so on at intervals until the bulk of the cotton is found to be dry, i.e., to contain from 0.25 to 0.5 per cent. of moisture. It is then ready to be sifted. During the process of removing to the sifting house and the sifting itself, the cotton should be exposed to the air as little as possible, as dry nitro-cotton absorbs as much as 2 per cent. of moisture from the air at ordinary temperatures and average dryness.

The drying house usually consists of a wooden building, the inside of which is fitted with shelves, or rather framework to contain drawers, made of wood, with brass or copper wire netting bottoms. A current of hot air is made to pass through the shelves and over the surface of the cotton, which is spread out upon them to the depth of about 2 inches. This current of air can be obtained in any way that may be found convenient, such as by means of a fan or Root's blower, the air being passed over hot bricks, or hot-water pipes before entering the building. The cotton should also be occasionally turned over by hand in order that a fresh surface may be continually exposed to the action of the hot air. The building itself may be heated by means of hot-water pipes, but on no account should any of the pipes be exposed. They should all be most carefully covered over with wood-work, because when the dry nitro-cotton is moved, as in turning it over, very fine particles get into the air, and gradually settling on the pipes, window ledges, &c., may become very hot, when the slightest friction might cause explosion. It is on this account that this house should be very carefully swept out every day. It is also very desirable that the floor of this house should be covered with oilcloth or linoleum, as being soft, it lessens the friction.

List shoes should always be worn in this building, and a thermometer hung up somewhere about the centre of the house, and one should also be kept in one of the trays to give the temperature of the cotton, especially the bottom of the trays. The one nearest to the hot air inlet should be selected. If the temperature of the house is kept at about 40° C. it will be quite high enough. The building must of course be properly ventilated, and it will be found very useful to have the walls made double, and the intervening space filled with cinders, and the roof covered with felt, as this helps to prevent the loss of heat through radiation, and to preserve a uniform temperature, which is very desirable.

The dry cotton thus obtained, if not already fine enough, should be sifted through a brass sieve, and packed away ready for use in zinc air-tight cases, or in indiarubber bags. The various gelatine compounds, gelignite, gelatine dynamite, and blasting gelatine, are manufactured in exactly the same way. The forms known as gelatine dynamite differ from blasting gelatine in containing certain proportions of wood-pulp and potassium nitrate, &c. The following are analyses of some typical samples of the three compounds:—

Gelatine Blasting
Gelignite. Dynamite. Gelatine.

Nitro-glycerine 60.514 71.128 92.94 per cent.
Nitro-cellulose 4.888 7.632 7.06 "
Wood-pulp 7.178 4.259 … "
Potassium nitrate 27.420 16.720 … "
Water … 0.261 … "

The gelignite and gelatine dynamites consist, therefore, of blasting gelatine, thickened up with a mixture of absorbing materials. Although the blasting gelatine is weight for weight more powerful, it is more difficult to make than either of the other two compounds, it being somewhat difficult to make it stand the exudation and melting tests. The higher percentage of nitro-cotton, too, makes it expensive.

When the dry nitro-cotton, which has been carefully weighed out in the proportions necessary either for blasting gelatine or any of the other gelatine explosives, is brought to the gelatine making house, it is placed in a lead-lined trough, and the necessary quantity of pure dry nitro- glycerine poured upon it. The whole is then well stirred up, and kept at a temperature of from 40° to 45° C. It should not be allowed to go much above 40° C.; but higher temperatures may be used if the nitro-cotton is very obstinate,[A] and will not dissolve. Great caution must, however, be observed in this case. The mixture should be constantly worked about by the workman with a wooden paddle for at least half an hour. At a temperature of 40° to 45° the nitro-glycerine acts upon the nitro-cotton and forms a jelly. Without heat the gelatinisation is very imperfect indeed, and at temperatures under 40° C. takes place very slowly.

[Footnote A: Generally due to the nitro-cotton being damp.]

[Illustration: FIG. 30.—WERNER, PFLEIDERER, & PERKINS' MIXING MACHINE.]