In a further patent (Eng.  Pat. 3,020, 1903) the authors propose with the object of preventing the formation and separation of nitro-glycerine in the waste acids, after the nitro-glycerine initially formed in the nitrating vessel has been separated and removed, to add a small quantity of water to the waste acids; this is carried out as follows.  A relatively small quantity of water is added, and this prevents all further separation of nitro-glycerine, and at the same time the strength of the waste acids is so slightly reduced that their separation and re-concentration are not affected.  “After-separation” is thus done away with, and the nitro-glycerine plant simplified and its output increased.  After nitration separation is commenced at a temperature such that when all the displacing acid has been added, and the separation of the nitro-glycerine is complete, the temperature of the contents of the nitrating vessel shall not be lower than 15 deg.  C. A sufficient quantity of the displacing acid is then run off through the waste-acid cock to allow of the remaining acids being air-stirred without splashing over the top.  A small quantity of water, from 2 to 3 per cent. according to strength of acid; if waste consists of sulphuric acid (monohydrate), 62 per cent.; nitric acid (anhydrous), 33 per cent. and water 5 per cent.; temperature 15 deg.  C., then 2 per cent. of water is added; if waste acids contain less than 4 per cent. of water of temperature lower than 15 deg.  C., from 3 to 5 per cent. of water may have to be added.  The water is added slowly through the separator cylinder, and the contents of the nitrator air-stirred, but not cooled, the temperature being allowed to rise slowly and regularly as the water is added—­usually about 3 deg.  C. for each per cent. of water added.  When air-agitation has been stopped, the acids are kept at rest for a short time, in order to allow of any small quantity of initially formed nitro-glycerine adhering to the coils and sides of the vessel rising to the top.  When this has been separated by displacement, the acids are ready for denitration, or can be safely stored without further precaution.

Separation.—­The nitro-glycerine, together with the mixed acids, flows from the nitrating house to the separating house, which must be on a lower level than the former.  The separating house contains a large lead-lined tank, closed in at the top with a wooden lid, into which a lead pipe of large bore is fixed, and which is carried up through the roof of the building, and acts as a chimney to carry off any fumes.  A little glass window should be fixed in this pipe in order that the colour of the escaping fumes may be seen.  The conduit conveying the nitro-glycerine enters the building close under the roof, and discharges its contents into the tank through the pipe G (Fig. 8).  The tank is only about two-thirds filled by the charge.  There is in the side of the tank a small window of thick plate glass, which enables the workman to see the level of the charge, and also to observe the progress of the separation, which will take from thirty minutes to one hour.