A very good method of rapidly determining the strength of the sulphuric acid is as follows:—­Weigh out in a small weighing bottle, as nearly as possible, 2.45 grms.  This is best done by running in 1.33 c.c. of the acid (1.33 x 1.84 = 2.447).  Wash into a large Erlenmeyer flask, carefully washing out the bottle, and also the stopper, &c.  Add a drop of phenol-phthalein solution and titrate, with a half normal solution of sodium hydrate (use a 100 c.c. burette).  Then if 2.45 grms. exactly have been taken, the readings on the burette will equal percentages of H_{2}SO_{4} (mono-hydrate) if not, calculate thus:—­2.444 grms. weighed, required 95.4 c.c.  NaOH.  Then—­

2.444 :  95.4 ::  2.45 :  x = 95.64 per cent.  H_{2}SO_{4}.

It has been proposed to free nitric acid from the oxides of nitrogen by blowing compressed air through it, and thus driving the gases in solution out.  The acid was contained in a closed lead tank, from which the escaping fumes were conducted into the chimney shaft, and on the bottom of which was a lead pipe, bent in the form of a circle, and pierced with holes, through which the compressed air was made to pass; but the process was not found to be of a very satisfactory nature, and it is certainly better not to allow the formation of these compounds in the manufacture of the acid in the first instance.  Another plan, however, is to heat the acid gently, and thus drive out the nitrous gases.  Both processes involve loss of nitric acid.

Having obtained nitric and sulphuric acids as pure as possible, the next operation is to mix them.  This is best done by weighing the carboys in which the acids are generally stored before the acids are drawn off into them from the condensers, and keeping their weights constantly attached to them by means of a label.  It is then a simple matter to weigh off as many carboys of acid as may be required for any number of mixings, and subtract the weights of the carboys.  The two acids should, after being weighed, be poured into a tank and mixed, and subsequently allowed to flow into an acid egg or montjus, to be afterwards forced up to the nitrating house in the danger area.  The montjus or acid egg is a strong cast-iron tank, of either an egg shape, or a cylinder with a round end.  If of the former shape, it would lie on its side, and upon the surface of the ground, and would have a manhole at one end, upon which a lid would be strongly bolted down; but if of the latter shape, the lid, of course, is upon the top, and the montjus itself is let into the ground.  In either case, the principle is the same.  One pipe, made of stout lead, goes to the bottom, and another just inside to convey the compressed air, the acids flowing away as the pressure is put on, just as blowing down one tube of an ordinary wash-bottle forces the water up the other tube to the jet.  The pressure necessarily will, of course, vary immensely, and will depend upon the height to which the acid has to be raised and the distance to be traversed.