Upon leaving the filter house, where it has been washed and filtered, and has satisfactorily passed the heat test, it is drawn off from the lowest tank in indiarubber buckets, and poured down the conduit leading to the precipitating house, where it is allowed to stand for a day, or sometimes longer, in order to allow the little water it still contains to rise to the surface.  In order to accomplish this, it is sufficient to allow it to stand in covered-in tanks of a conical form, and about 3 or 4 feet high.  In many works it is previously filtered through common salt, which of course absorbs the last traces of water.  It is then of a pale yellow colour, and should be quite clear, and can be drawn off by means of a tap (of vulcanite), fixed at the bottom of the tanks, into rubber buckets, and is ready for use in the preparation of dynamite, or any of the various forms of gelatine compounds, smokeless powders, &c., such as cordite, ballistite, and many others.

Mikolajezak (Chem.  Zeit., 1904, Rep. 174) states that he has prepared mono- and di-nitro-glycerine, and believes that the latter compound will form a valuable basis for explosives, as it is unfreezable.  It is stated to be an odourless, unfreezable oil, less sensitive to percussion, friction, and increase of temperature, and to possess a greater solvent power for collodion-cotton than ordinary nitro-glycerine.  It can thus be used for the preparation of explosives of high stability, which will maintain their plastic nature even in winter.  The di-nitro-glycerine is a solvent for tri-nitro-glycerine, it can therefore be mixed with this substance, in the various gelatine explosives in order to lower the freezing point.

The Waste Acids.—­The waste acids from the separating house, from which the nitro-glycerine has been as completely separated as possible, are run down the conduit to the secondary separator, in order to recover the last traces of nitro-glycerine that they contain.  The composition of the waste acids is generally somewhat as follows:—­Specific gravity, 1.7075 at 15 deg.  C.; sulphuric acid, 67.2 per cent.; nitric acid, 11.05 per cent.; and water, 21.7 per cent., with perhaps as much as 2 per cent. of nitric oxide, and of course varying quantities of nitro-glycerine, which must be separated, as it is impossible to run this liquid away (unless it can be run into the sea) or to recover the acids by distillation as long as it contains this substance.  The mixture, therefore, is generally run into large circular lead-lined tanks, covered in, and very much like the nitrating apparatus in construction, that is, they contain worms coiled round inside, to allow of water being run through to keep the mixture cool, and a compressed air pipe, in order to agitate the mixture if necessary.  The top also should contain a window, in order to allow of the interior being seen, and should have a leaden chimney to carry off the fumes which may arise from decomposition.  It is also useful to have a glass tube of 3 or 4 inches in diameter substituted for about a foot of the lead chimney, in order that the man on duty can at any time see the colour of the fumes arising from the liquid.  There should also be two thermometers, one long one reaching to the bottom of the tank, and one to just a few inches below the surface of the liquid.