BOEACIC ACID 



429 



upper basin A B, the weak solution is run out, through the channel a into tho second 

 basin c D. Here it takes up another dose of boracic acid, and after 24 hours is run 

 off into tho next lower basin E F. After having in this way traversed six or eight of 

 the lagoons, tho solution acquires a specific gravity of 1'007, and contains about one- 

 half per cent, of boracic acid. This solution passes from tho last basin a H (fig. 171) 

 into a scries of clarifiers and evaporating vessels, represented in section in fig. 171 



and in plan in fig. 111. In the large square brick vessel i, called a vasco, the 

 solution deposits much of the mud which it holds in suspension. After subsidence of 

 a great part of its mechanical impurities, the comparatively clear liquor is drawn off 

 into other settling tanks J, K, and thence into a long succession of square leaden 

 evaporating pans, of which half-a-dozen are represented by i, M, N, o, P, Q. The heat 

 for effecting this evaporation was formerly obtained by the combustion of wood, but 

 Count Larderel's capital improvement an improvement without which tho Tuscan 

 soffioni could never have been profitably utilized consisting in dispensing altogether 

 with the use of artificial fuel, and conducting the evaporation by means of the natural 

 heat of the volcanic emanations. Accordingly, jets of steam issuing from tho ground 

 are introduced through flues under the evaporating pans, and conducted successively 

 from the lowest to the highest of the series. The solution during its passage through 

 tho system of evaporating vessels, which lasts about 62 hours, gradually becomes 

 concentrated, and by the time it reaches the last vessel is sufficiently strong to be 

 passed to the crystallising tubs, s 8. Fig. 173 represents an improved form of 



evaporating apparatus, in which the liquid from tho vasco B passes to a shallow 

 boiler, c, whence it runs slowly over an inclined table of sheet lead, D E, about 150 

 feet long, and having its surface corrugated so as to form a series of channels through 

 which the solution flows. During its passage, tho solution slowly evaporates, by the 

 heat of the soffioni introduced below, and 

 finally reaches the vessel F in a sufficiently 

 concentrated form. 



The concentrated solution, obtained by 

 either of these methods of evaporation, is 

 mixed with some of the mother-liquor of tho 

 pans, and set to crystallise in the round tubs 

 s s (figs. 171, 172, 174), each having a capacity 

 of about 8 cubic feet, and being made of wood, 

 lined with lead. The small crystals on re- 

 moval are drained in baskets J, at tho top of 

 the tub, and while still moist are spread out 

 Jn a layer on tho floor c c of tho drying 

 chamber (fig. 174), which is heated by stoam circulating in a space included between 

 this floor and another floor below. 



