504

DYEING AND CALICO PRINTING.

liquidare put in abeakerwith 1000 c.c. of water and io of hydrochloric (muriatic)acid. The beaker is then set in a white porcelain basin, or a soup-plate, andthe chloride of lime liquor is carefully added, with constant stirring, till thegreen colour disappears, and is replaced by a faint yellow. The quantity ofchloride of lime solution required to produce this result is then read off andnoted. A fresh quantity of the indigo solution is then mixed with hydrochloricacid water exactly as above; 20 c.c. of the solution of tannin are then added,and the chloride of lime liquor is dropped in as before till the green tint finallydisappears. From the number of degrees of the chloride of lime solutionrequired to produce this effedt we subtract the number found as above forthe indigo solution alone. The remainder is the amount of the chloride oflime'solution, representing 0*020 grm. of tannin. It is advisable that about100 c.c. of the chloride of lime liquor may correspond to 0*2 grm. of tannin.In determining the value of a sample of sumac, myrobalans, &c., we take5 grms., suitably ground up ; boil it for thirty to forty-five minutes with £ litreof pure water, let it cool, transfer the whole to a litre flask, make up withwater to the exadt bulk of 1 litre, and shake up. When it has settled, 30 c.c.of the clear liquid are taken out with a pipette for the adtual analysis. Thisquantity is mixed with the indigo solution, water, and hydrochloric acid asabove, and the standard chloride of lime-liquor is then dropped in from theburette till the green colour disappears. If this process is carefully performedthe results are slightly too high.

Hammer takes the specific gravity of a solution of tannin, removes thetannin, and takes the specific gravity again. From the loss apparent on thissecond determination the amount of tannin present is calculated by means ofthe accompanying table. (See next page).

The specific gravity of the solution to be tested is taken either with anordinary specific gravity bottle or a very fine hydrometer. For withdrawingthe tannin from its solutions the material used is raw hide. A piece of this,cleaned and prepared as if for tanning, is well washed, dried at a gentleheat till its weight becomes constant, rasped to powder, and preserved in thatstate in a well-stoppered bottle. When wanted for the performance of ananalysis an amount of this, not less than four times the supposed quantity oftannin, is weighed out, softened by steeping in water, well squeezed in a finelinen cloth, so as to remove any superfluous moisture which would otherwisedilute the solution, and then digested for a short time in the liquid to betested, with frequent shaking. In this manner the whole of the tannin maybe withdrawn from a liquid. In using this method the sumac, galls, &c.,must first be powdered, rasped, or otherwise well divided, boiled with water,and exhausted in a displacement-apparatus. 1 part of the substance shouldbe completely extracted with 10 to 12 parts of water. The extract is madeup with water to some known weight, well shaken up to secure homogeneousmixture, and the specific gravity of the liquid ascertained. A portion of thesolution, rather more than sufficient for the determination of the specificgravity, is weighed off, and digested with an excess of hide-raspings, withfrequent agitation, as explained above. The liquid is then filtered throughfine linen into a hydrometer-tube or specific-gravity bottle, and its specificgravity determined once more. From the loss the amount of tannin may becalculated.