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 arrival of the nitro-glycerine at this house, it first flows into a lead-lined wooden tank (W), containing a compressed air pipe, just like the one in the small tank in the separating house. This tank is half filled with water, and the compressed air is turned on from half to a quarter of an hour after the introduction of the charge. The water is then drawn off, and fresh water added. Four or five washings are generally necessary. The nitro-glycerine is then run into the next tank (A), the top of which is on a level with the bottom of the first one. Across the top of this tank is stretched a frame of flannel, through which the nitroglycerine has to filter. This removes any solid matters, such as dirt or scum. Upon leaving this tank, it passes through a similar flannel frame across another tank (B), and is finally drawn off by a tap in the bottom of the tank into rubber buckets. The taps in these tanks are best made of vulcanite.