HEALED POWDER.--4t is scarcely safe to grind gun powder either in a mill or a mortar, unless both of these be made of wood, which would of course require separate machines. If it can be procured, it is much preferable to purchase gunpowder dust from the mills, as it is a mere prejudice among firework makers to suppose that it, is not so strong as powder that has been mealed. If, however, this cannot be obtained, the powder must be reduced to this state by the artificer himself. A large mortar of elm, with a pestle of lig. num vita, will answer this purpose ; but there is an inconvenience in this machine, arising from the adhe sion of the powder in hard lumps to the bottom of the pestle. It is a better and speedier plan to grind gun powder by means of an iron shot of 18 or 24 pounds weight, rolled in a large wooden vessel, turned out of some hard wood ; either beech or elm. What is called the mealing table is also used for the same purpose. This is, as the name expresses, a table made of smooth ed elm, surrounded with a margin, on which the pow der is placed. By means of a flat piece of lignum vita, or other hard wood, of about six inches square, and furnished with a proper handle, the powder is easily rubbed down. This method is, however, less safe than the preceding ; because, if any sand were present, an explosion might easily be produced. In all such cases, it is most important that the tables, or vessels of any kind, should be provided with close covers when not in use ; nor should more than a pound be pulverized at a time. All these operations should also be carried on, where there is room, in tents. and not in houses of any kind ; as there is, in such situations, much less mischief pro duced by accidental explosions. Further, no more powder should at any time be at hand than is imme diately required ; and whatever is finished should be removed without delay. Mealed powder must be care fully sifted from the grains that may remain, by means of the lawn sieve.
On the Mixing of Compositions.
This is a circumstance which requires considerable care, as the accurate performance of sky-rockets, anti of many other fireworks, depends very much upon it. Tt is impossible to be too particular with regard to these, it any regard is to be had to their accuracy, or if they are intended for signals, or for long flights. In all sorts of illumination and fixed lights, it is even more necessary to be accurate in this respect. In these, the perfection consists in their burning with a steady light, in their all giving out the same size of flame, and in all lasting the same time. Otherwise, as so many are used together, and always disposed in various figures, the effect of a firework is apt to be materially injured or altogether spoiled. There are many other alterations required in this case, but it is the accuracy of mixture which is our present concern.
Supposing that all the ingredients of any composition are determined on, as much should be made at once as is sufficient for all the fires, whatever these may be, that are to be introduced together, or to form any one piece. This is one of the methods of ensuring accuracy in time, and correctness in performance. All the com positions that consist of fine materials, such as nitre, sulphur, charcoal. and mealed powder, may be mixed for an indefinite time, because they cannot be over done. There are different methods of doing this, and the one or the other may be chosen according to the quantity of the materials or the scale on which the ope rator is to work.
On a large scale, the most speedy and effectual me thod is by a hopper contrived for this purpose. The composition having been first mixed with the hands as far as it can, is placed in this machine, which is pro vided with a long and square wooden tube, like a com mon house gutter, for conducting rain from the roofs. Within this, and in a part of it which is inlayed for that purpose, there is placed an axis, carrying four or more light vanes of tin or of wood, so adapted to the inside of this receptacle or tube, as to fit it easily, and capable of revolving in a vertical direction. The axis Communicates with a crank handlo outside, which the operator manages. As the mixture begins to descend down the tube, the upper part of which has its opening regulated by a valve, which the same person can con tract or enlarge at pleasure, it falls on the vanes of this little mill, which, being turned quickly round, pro duces an intimate and accurate union of all the parts, and thus it is delivered complete at the lower end of the tube. If it be judged necessary, it may pass through two mills instead of one; and, by means of a band or string, both of these may beset in motion by one band.
On a smaller scale, the mixture may be made in a cylinder of wood, placed on its end, like an apothecary's mortar, and provided with a well-fitted cover. Through a hole in this, there passes an axle and handle, car rying vanes, placed vertically. The handle may be turned between the hands, or, what is much better, by means of a drill bow, somewhat in the manner prac tised with chocolate ; and, in this manner, a small quantity of mixture can be completed in two or three minutes.
The third method is to introduce the ingredients in to a sieve of moderate fineness, provided with a top and bottom cover, so as to be completely enclosed. By agitating this, the ingredients are carried through the sieve into the receptacle below, and thus become accu rately mixed.
If now it becomes necessary to mix fine and coarse, or heavy and light ingredients together, a different prac tice must be adopted. Supposing that antimony, or orpiment, or any weighty substance in fine powder is required, the mixture cannot be effectually performed in either of these ways, to any extent ; as the unequal weights of the materials will prevent the composition from being uniform. The proper combustible ingre dients, therefore, or the fundamental substances above mentioned, must first be mixed in the requisite propor tion, and these additional matters may afterwards be in troduced and mixed up with them either by the hand or by a comb with broad teeth. Coarser powdered char coal, intended to yield sparks, must be managed in a similar manner. Greater care still is required, where metallic filings, and particularly where large particles of iron are to be used ; as these are apt to fall through the lighter and liner dust, and thus to become unequally disposed. They should, therefore, first be sifted as equally as possible over the surface of the general or fundamental mixture; after which, with a little care, they may easily he diffused throughout in a regular manner. Owing to their brilliancy and distinctness, the eye is a good judge of the correctness of such mixtures as this.