SOME ASTRINGENT SUBSTANCES, ETC. 229 



proposed by Mr. Warrington, and as many persons to whom it is of interest 

 may probably be unacquainted with the details of the operation, we will 

 proceed to describe it. 



In the manufacture of leather, the hides, after being softened, are, as is 

 well known, steeped in a decoction of bark, by which means the gelatine 

 and fibrine become chemically united with tannic acid, and the peculiar 

 properties of leather are conferred on the hide, — advantage is taken of this 

 fact in Warrington's process, — an aqueous solution of gelatine being used 

 as the precipitant of the tannic acid extracted from the bark by water. 



The method is volumetric, and very easy of execution. To prepare 

 the test solution, 30 grs. of gelatine (the long fibrous isinglass is found to 

 be very pure and pretty uniform) are dissolved in about 8,000 grs. of hot 

 water, boiled for ten minutes, and allowed to cool ; 10 grs. of powdered 

 alum are added, the solution is agitated and allowed to stand until it has 

 become clear, and then made up to 10,000 grs. This solution cannot be 

 used with advantage if kept longer than two days, as the gelatine is very 

 liable to decompose. 



The solution is standardized by ascertaining the quantity required to 

 precipitate a known weight of pure tannic acid ; we have found about 5 grs 

 to be the most convenient quantity of the tannine to employ. 



The addition of alum to the gelatine solution was proposed by Miiller j 

 it insures a more speedy and perfect separation of the precipitated tannate 

 of gelatine, and without the use of this precaution, Mulligan and Dowling 

 found it impossible to obtain even an approximate estimate of the amount 

 of tannine in the bark solutions of the tan-yards. 



The point at which all the tannic acid was completely precipitated was 

 ascertained in the following manner, which is a modification of that pro- 

 posed by Warrington : — A glass tube, about 10 inches in length and § in 

 internal diameter, is selected (a piece of combustion tube is very con- 

 venient) and closed at one end with a firm plug of fine washed sponge j 

 the solution is allowed to stand for a few moments in order to give the 

 precipitate time to settle down, a portion of the comparatively clear super- 

 natant liquid is then poured into the glass tube, and filtered into a test- 

 tube, to this a drop or two of the gelatine solution is added to see whether 

 there be any precipitate or not ; and great accuracy can be obtained by 

 comparing this with a similar tube containing an equal quantity of distilled 

 water, to which a similar quantity of gelatine solution is added (of course 

 if there be still a precipitate, the solution removed, and the water must 

 be returned to the vessel, which may be a Phillips, or any other tall 

 glass). This comparison will be found very convenient in the examination 

 of the solutions obtained from substances rich in tannine, where only a 

 small quantity of the material has been employed, and the solution 

 becomes, from the difficulty of extraction, necessarily dilute, any little 

 inaccuracy becoming then much magnified from the high multiplication 

 required in calculating out the percentages. We have adopted this down- 



