wn 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.]

~Filtering and Washing.~--The filter house (Fig. 9), which must of course
be again on a somewhat lower level than the separating house, must be a
considerably larger building than either the nitrating or separating
houses, as it is always necessary to be washing some five or six charges
at the same time. Upon the