[Footnote A:  Comptes Rendus, 75; and Desortiaux, “Traite sur la Poudre,” 684-686.]

Nitro-glycerine is now generally made by adding the glycerine to a mixture of sulphuric and nitric acids.  The sulphuric acid, however, takes no part in the reaction, but is absolutely necessary to combine with the water that is formed by the decomposition, and thus to keep up the strength of the nitric acid, otherwise lower nitrates of glycerine would be formed that are soluble in water, and which would be lost in the subsequent process of washing to which the nitro-compound is subjected, in order to remove the excess of acids, the retention of which in the nitro-glycerol is very dangerous.  Nitro-glycerol, which was formerly considered to be a nitro-substitution compound of glycerol, was thought to be formed thus—­

C_{3}H_{8}O_{3} + 3HNO_{3} = C{3}H_{5}(NO_{2})_{3}O_{3} + 3H_{2}O;

but more recent researches rather point to its being regarded as a nitric ether of glycerol, or glycerine, and to its being formed thus—­

C_{3}H_{8}O_{3} + 3 HNO_{3} = C{3}H_{5}(NO_{3})_{3} + 3H_{2}O.
             92 227

|OH
The formula of glycerine is C_{3}H_{8}O_{8}, or C_{3}H_{5}|OH
|OH

|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that is, the three hydrogens of the semi-molecules of hydroxyl in the glycerine have been replaced by the NO_{2} group.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