The mixed acids having been forced up to the danger area, and to a level higher than the position of the nitrating house, should, before being used, be allowed to cool, and leaden tanks of sufficient capacity to hold at least enough acid for four or five nitrations should be placed in a wooden house upon a level at least 6 or 7 feet above the nitrating house.  In this house also should be a smaller lead tank, holding, when filled to a certain mark, just enough of the mixed acids for one nitration.  The object of this tank is, that as soon as the man in charge knows that the last nitration is finished, he refills this smaller tank (which contains just enough of the mixed acids), and allows its contents to flow down into the nitrating house and into the nitrator, ready for the next nitration.  The nitration is usually conducted in a vessel constructed of lead, some 4 feet wide at the bottom, and rather less at the top, and about 4 feet or so high.  The size, of course, depends upon the volume of the charge it is intended to nitrate at one operation, but it is always better that the tank should be only two-thirds full.  A good charge is 16 cwt. of the mixed acids, in the proportion of three to five; that is, 6 cwt. of nitric acid, and 10 cwt. of sulphuric acid, and 247 lbs. of glycerine.

Upon reference to the equation showing the formation of nitro-glycerine, it will be seen that for every 1 lb. of glycerine 2.47 lbs. of nitro-glycerine should be furnished,[A] but in practice the yield is only a little over 2 lbs., the loss being accounted for by the unavoidable formation of some of the lower nitrate of glycerine (the mono-nitrate), which afterward dissolves in the washing waters.  The lead tank (Fig. 5) is generally cased in woodwork, with a platform in front for the man in charge of the nitrating to stand upon, and whence to work the various taps.  The top of the tank is closed in with a dome of lead, in which is a small glass window, through which the progress of the nitrating operation can be watched.  From the top of this dome is a tube of lead which is carried up through the roof of the building.  It serves as a chimney to carry off the acid fumes which are given off during the nitration.  The interior of this tank contains at least three concentric spirals of at least 1-inch lead pipe, through which water can be made to flow during the whole operation of nitrating.  Another lead pipe is carried through the dome of the tank, as far as the bottom, where it is bent round in the form of a circle.  Through this pipe, which is pierced with small holes, about 1 inch apart, compressed air is forced at a pressure of about 60 lbs. in order to keep the liquids in a state of constant agitation during the whole period of nitration.  There must also be a rather wide pipe, of say 2 inches internal diameter, carried through the dome of the tank, which will serve to carry the mixed acid to be used in the operation into the tank.  There is still another pipe to go through the dome, viz., one to carry the glycerine into the tank.  This need not be a large bore pipe, as the glycerine is generally added to the mixed acids in a thin stream (an injector is often used).