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.

At this stage, a sample should be taken to the laboratory and tested.  If the sample will not pass the tests, which is often the case, the charge must be rewashed for one hour, or some other time, according to the judgment of the chemist in charge.  In the case of an obstinate charge, it is of much more avail to wash a large number of times with small quantities of water, and for a short time, than to use a lot of water and wash for half an hour.  Plenty of compressed air should be used, as the compound nitric ethers which are formed are thus got rid of.  As five or six charges are often in this house at one time, it is necessary to have as many tanks arranged in tiers, otherwise one or two refractory charges would stop the nitrating house and the rest of the nitro-glycerine plant.  The chief causes of the washed material not passing the heat test are, either that the acids were not clean, or they contained objectionable impurities, or more frequently, the quality of the glycerine used.  The glycerine used for making nitro-glycerine should conform to the following tests, some of which, however, are of greater importance than others.  The glycerine should—­

1.  Have minimum specific gravity at 15 deg.  C. of 1.261.

2.  Should nitrify well.

3.  Separation should be sharp within half an hour, without the separation of flocculent matter, nor should any white flocculent matter (due to fatty acids) be formed when the nitrated glycerine is thrown into water and neutralised with carbonate of soda.

4.  Should be free from lime and chlorine, and contain only traces of arsenic, sulphuric acid, &c.

5.  Should not leave more than 0.25 per cent. of inorganic and organic residue together when evaporated in a platinum dish without ebullition (about 160 deg.  C.) or partial decomposition.