OF ARTS AND SCIENCES. 353 



large numbers, and soon after the mass becomes pasty, and begins to 

 puff up, while the evolution of fumes slackens ; at this point the lamp 

 is to be removed. The product, if charring was avoided in the early- 

 part of the process (there is little danger of it later), is of a reddish 

 brown color, and almost completely solid. The loss in weight should 

 be about 7 grams, and the whole operation takes little more than 

 fifteen minutes. After the residue is cold, it is dissolved in water, and 

 treated with an excess of baric carbonate to remove sulphuric acid, 

 when the greater part of the colored impurity is carried down by the 

 baric sulphate, so that the filtrate is usually of a pale wine-yellow. 

 The precipitate should be boiled out with water once or twice, and 

 the washings added to the filtrate, which, after converting any barium 

 salt into potassic trisulphonate by potassic sulphate, is evaporated until 

 its surftice after cooling is covered with little crystals, when it usually 

 deposits fine crystals after a short time, which can be increased in 

 size by the spontaneous evaporation of part of the mother liquor, and 

 purified by recrystallization from boiling water. The mother liquors, 

 when sufficiently concentrated, solidify to a brownish pasty mass of 

 a finely granular consistency, filled with larger crystals of potassic 

 benzoltrisulphonate, which can be separated mechanically by scraping 

 with a flat-pointed platinum stirrer the semi-liquid finer-grained im- 

 purity through the holes in a perforated platinum cone placed in a 

 funnel. The larger crystals left behind are then purified by crystalli- 

 zation from water. The yield from this process can be raised by 

 careful work to 44 per cent of the theoretical yield, and it is possible 

 to increase it still further by boiling the precipitate formed by baric 

 carbonate with a solution of potassic carbonate, as the baric benzol- 

 trisulphonate is very sparingly soluble in water. We cannot recom- 

 mend this treatment, however, as the potassic benzol trisulphonate 

 extracted in this way is contaminated to such an extent with the col- 

 ored impurities carried down by the precipitate of barium salts, that 

 it is very hard to obtain white and pure crystals from it; in fact, it is 

 easier to prepare fresh from the benzolmetadisulphonate. 



The benzoltrisulphonic acid can also be prepared from benzol with- 

 out isolating the lower sulphonic acids, and this method is sometimes 

 convenient when large quantities of the substance are desired ; but it 

 is decidedly inferior to that just described, for two reasons, first, be- 

 cause the yield is very much smaller, and, second, a certain amount of 

 benzolparadisulphonate is formed, which it is not easy to separate from 

 the trisulphonate by crystallization. For this purpose 100 (or more) 

 grams of benzol are dissolved in the same volume of common sulphuric 

 VOL. xxn. (n. s. XIV.) 23 



