Chemical Science. 179 



longing to platina ; but, as regards copper, a more curious one of 

 the same nature is shewn very easily by a common green wax 

 taper. These tapers are coloured with verdegris, and, when burnt, 

 the copper of the verdegris is reduced for a time in the wick. If 

 such a taper be lighted, and the flame then blown out, leaving the 

 wick glowing, combustion of the wax will still proceed, slowly in- 

 deed, but for hours and days together, until the whole of the wax is 

 burnt, or until the combustion has reached some part where it is 

 extinguished by the contact of neighbouring bodies. This does not 

 happen with white tapers, and hence they are safer for ordinary use. 



13. Carbazotic Acid and Carbazotate of Lead. — To obtain pure 

 carbazotic acid, the finest indigo is to be crushed and heated mode- 

 rately, with eight or ten times its weight of nitric acid : it dissolves, 

 effervescing, and producing much nitrous vapour. When the scum 

 has fallen, it is to be boiled, and fresh acid added, until no more 

 nitrous vapour is produced, which is ascertained by covering the 

 vessel from time to time with a capsule, and observing if red 

 fumes gather beneath it. Care being taken on this point, neither 

 resin nor artificial tannin is produced. When cold, hard, yellow 

 transparent crystals are formed, which, taken from the mother 

 liquor, are to be washed. When boiled with water, they dissolve, 

 and the few drops of oleaginous liquid on the surface are to be 

 removed by bibulous paper. Filtration and cooling then yield a 

 large quantity of brilliant yellow lamellar crystals. To obtain this 

 acid perfectly pure, it is to be redissolved in boiling water, satu- 

 rated with carbonate of potash and cooled ; the crystallized car- 

 bazotate of potash is to be washed and crystallized several times. 

 The first mother- water, mixed with cold water, gives a brown pre- 

 cipitate, which is to be washed, then put into boiling water, and 

 neutralized by carbonate of potash to give a fresh portion of the 

 carbazotate of potash. This salt, dissolved in boiling water, and 

 treated with sulphuric, nitric, or muriatic acid, gives, upon cooling, 

 very brilliant, clear yellow crystals, in plates mostly triangular, 

 which are the pure acid. 



Sometimes the indigo and nitric acid will give no crystals ; the 

 liquid is then to be evaporated, and the acid separated from the 

 residue in the manner above described. Much acid may also be 

 obtained from the liquid which floats over the precipitate, by eva- 

 porating it a certain degree. The residue is to be reboiled with 

 nitric acid, and then neutralized, &c, by potash, as before. Four 

 parts of the best indigo yield one part of carbazotic acid. 



The carbazotate of lead is readily formed from the pure acid and 

 carbonate of lead. It is a yellow precipitate, scarcely soluble in 

 water, and when dry, detonating strongly by heat or percussion. 

 It is proposed to be used in percussion guns, in place of fulminating 

 mercury ; but nothing is said in comparison of it with the improved 



N 2 



