In some larger works the apparatus is arranged upon an extensive scale, in buildings built at different heights and separated from one another by embankments, so high that the top of each covers the top part of the apparatus in the particular building it is designed to protect. The correct atomic quantities are weighed out into a large vessel called the mixing vessel, where the nitric and sulphuric acids are mixed ; they remain there for a considerable period until they cool down, so that when they are to be operated upon there ie no room for any increment of heat by chemical development. When in this condition they aro weighed off in the presence of the chief chemist, taken up an incline to a higher level termed the nitrating house, where they are mixed with the glycerine in a very large apparatus, where as much as 1500 lb. of nitro-glycerine is treated at each operation. After the nitrating the explosive is allowed to flow down by gravity into a lower building, which contains a large tank holding many tone of water, so that should the chemist be unable to control tho action he may run the charge into the water, and by thus diluting it, atop the chemical action forthwith. The stirring or Mixing arrangements are all effected by machine power, and so arranged as to be mechanically under the control of the operator in charge, who can adjust everything, so that neither quantity nor speed can be exceeded in any way.
The process of washing is effected by means of compressed air, working at a pressure of from 45 to 50 lb. on the square inch, and the whole of the contents of the large washing vessels are surged over and over again, an indefinite number of times, through the vessel, so that every particle is repeatedly submitted to the purifying process. A portion of the charge having been decanted the chemist takes samples and tests them, with regard to their neutrality, by means of liquid litmus ; if a small quantity of it discolours a sample of the nitro-glycerine, the operation of washing is continued until little or no trace, if possible, of the acid remains. As soon as the process is satisfactory, the charge is allowed to run down by gravity into the final washing and filtering house, where it is treated with alkaline solution to neutralize any remaining acidity that may cling to the particles of nitro-glycerine; it is usually then tested again, and the washing afterwards continued from 15 to 20 minutes longer.
At the end of that time a final sample is taken and tested in the laboratory both for purity and neutrality ; when if satisfactory it is allowed to flow into a filter, which is a large oval vessel sufficient to contain nearly 3 tons weight. In the centre there is a cylinder with a thick blanket fastened over the end of it by copper hooka, for the purpose of allowing the mixture to drain down into it, so that no foreign matter of any kind may pass, and any aqueous substance that may cling to the nitro-glycerine, when decanted, does not usually percolate through tho blanket, unless there is great carelessness on the part of the men who are attending to the process. The communication
between the buildings is carried on by means of troughs, and it is not touched in any way by workpeople, until it has been absorbed in the inert base with which it it is mixed, and rendered commercially available.
It has been practically very unsafe and inconvenient to use nitro-glycerine as a blasting agent unless it is mixed with some absorbent. It therefore forms the essential ingredient in a number of semi-solid mixtures, such as dynamite, lithofracteur, duolin, giant powder, rendrock, sehastin, and the like, which are of great commercial value ; of these the most important is dynamite, inasmuch as the beet quality or No. 1 consists of 7.5 per cent. nitro-glycerine and 25 per cent. kieselguhr, the latter being a siliceous earth forming a fine white powder of infusoria, which has a high absorp tive power, being capable of taking up from two to three times its weight of nitro-glyeerino without becoming pasty. Artificial silica, prepared by precipitating it from a solution of sodium silicate by sulphuric acid, has been proposed and used with some success by W. N. Hill. The process of making dynamite is simple, since the nitro-glycerine is merely mixed with fine dry powder in a leaden vessel with wooden beaters, the kieselguhr having been burnt in order to destroy any organic matter which may be present. The explosive properties of dynamite are similar to those of the nitro-glycerine contained in it, as the absorbent is quite inert. It freezes at the same temperature as its nitro-glycerine, and when in this state it is exceedingly 'difficult to fire it ; it should only by thawed by placing it in a warm pocket about the person, or in a water bath, which may be readily extemporized if necessary ; dynamite is far safer than pure nitro-glycerine, as from its softness it will bear blows better, and so is therefore not so sensitive* to percussion or to friction ; its firing point is the same as nitro-glycerine, and if it takes fire it burns with a strong flash, leaving a residuum of silica. It must not contain an over-charge of nitro-glycerine as otherwise the latter will exude, especially if exposed to high temperatures, which have a tendency to render the nitro glyceriue fluid and less easily retained. In some works when weighed out into the quantities in which it is used, the cartridges are submitted to a slight pressure in a small machine, and those which show any signs of exudation are rejected.