The tank should be in connection with a drowning tank, as the charge sometimes gets very dangerous in this building.  It must also be connected by a conduit with the filter house, and also to the secondary separator by another conduit.  The tank should also be fitted with a compressed air pipe, bent in the form of a loop.  It should lie upon the bottom of the vat.  The object of this is to mix up the charge in case it should get too hot through decomposition.  A thermometer should of course be fixed in the lid of the tank, and its bulb should reach down to the middle of the nitro-glycerine (which rests upon the surface of the mixed acids, the specific gravity of the nitro-glycerine being 1.6, and that of the waste acids 1.7; the composition of the acids is now 11 per cent.  HNO_{3}, 67 per cent.  H_{2}SO_{4}, and 22 per cent. water), and the temperature carefully watched.

[Illustration:  FIG. 8.—­SEPARATOR. A, Compressed Air Pipes; G, Nitro-glycerine enters from Nitrator; N, Nitro-glycerine to P; L, Lantern Window; W, Window in Side; S, Waste Acids to Secondary Separator; T, Tap to remove last traces of Nitro-glycerine; P, Lead Washing Tank; A, Compressed Air; W, Water Pipe; N, Nitro-glycerine from Separator.]

If nothing unusual occurs, and it has not been necessary to bring the compressed air into use, and so disturb the process of separation, the waste acids may be run away from beneath the nitro-glycerine, and allowed to flow away to the secondary separator, where any further quantity of nitro-glycerine that they contain separates out after resting for some days.  The nitro-glycerine itself is run into a smaller tank in the same house, where it is washed three or four times with its own bulk of water, containing about 3 lbs. of carbonate of soda to neutralise the remaining acid.  This smaller tank should contain a lead pipe, pierced and coiled upon the bottom, through which compressed air may be passed, in order to stir up the charge with the water and soda.  After this preliminary washing, the nitro-glycerine is drawn off into indiarubber buckets, and poured down the conduit to the filter house.  The wash waters may be sent down a conduit to another building, in order to allow the small quantity of nitro-glycerine that has been retained in the water as minute globules to settle, if thought worth the trouble of saving.  This, of course, will depend upon the usual out-turn of nitro-glycerine in a day, and the general scale of operations.

[Illustration:  FIG. 9.—­FILTERING AND WASHING PLANT. W, Lead Washing Tank; WP, Water Pipe; L, Lid; S, Nitro-glycerine from Separator; A, B, C, Filtering Tanks; B2, Indiarubber Bucket.]