|ONO_{2} and that of the mono-nitrate of glycerine, C_{3}H_{5}|OH |OH |ONO_{2} and of the tri-nitrate or (nitro-glycerine), C_{3}H_{5}|ONO_{2} |ONO_{2} that is, the three hydrogens of the semi-molecules of hydroxyl in the glycerine have been replaced by the NO_{2} group. In the manufacture upon the large scale, a mixture of three parts by weight of nitric acid and five parts of sulphuric acid are used. From the above equation it will be seen that every 1 lb. of glycerol should give 2.47 lbs. of nitro-glycerol ((227+1)/92 = 2.47), but in practice the yield is only about 2 lbs. to 2.22, the loss being accounted for by the unavoidable formation of some of the lower nitrate, which dissolves in water, and is thus washed away, and partly perhaps to the presence of a little water (or other non-nitrable matter) in the glycerine, but chiefly to the former, which is due to the acids having become too weak. CHAPTER II. _MANUFACTURE OF NITRO-GLYCERINE._ Properties of Nitro-Glycerine--Manufacture of Nitro-Glycerine--Nitration-- The Nathan Nitrator--Separation--Filtering and Washing--The Waste Acids-- Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and Gun-Cotton. ~Properties of Nitro-Glycerine.~--Nitro-glycerol is a heavy oily liquid of specific gravity 1.6 at 15 deg. C., and when quite pure is colourless. The commercial product is a pale straw yellow, but varies much according to the purity of the materials used in its manufacture. It is insoluble in water, crystallises at 10.5 deg. C., but different commercial samples behave very differently in this respect, and minute impurities prevent or delay crystallisation. Solid nitro-glycerol[A] melts at about 12 deg. C., but requires to be exposed to this temperature for some time before melting. The specific gravity of the solid form is 1.735 at +10 deg. C.; it contracts one-twelfth of its volume in solidifying. Beckerheim[B] gives the specific heat as 0.4248 between the temperatures of 9.5 deg. and 9.8 deg. C., and L. de Bruyn gives the boiling point as above 200 deg.. [Footnote A: Di-nitro-mono chlorhydrin, when added to nitro-glycerine up to 20 per cent., is said to prevent its freezing.] [Footnote B: _Isb., Chem. Tech._, 22, 481-487. 1876.]