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BRITISH PHARMACEUTICAL CONFERENCE. 
5 to 10 drops of a solution of moderate strength will be found quite sufficient; 
on the contrary, if a large quantity be used, it will entirely vitiate the ana- 
lysis ; whether under these conditions it has the power of precipitating gela¬ 
tine or not, I am scarcely prepared to say,—metallic salts generally do not 
precipitate gelatine, but, according to M. Naquet, alum wdll do so, if an alkali 
be likewise present, certainly in this case it causes more gelatine to be required 
tlian otherwise would be, and thus raises the percentage. I bought my own 
experience upon this point, having been once led into considerable error by 
the injudicious use of this agent. 
2. There is no occasion to remove the exhausted material from the liquor, 
but the analysis may at once be proceeded with in the same vessel, provided 
the solution has become perfectly cold. It is, however, convenient where 
several analyses are intended to be made of the same substance, to exhaust a 
larger quantity, to decant the liquor, and, after washing the dregs on a filter, 
to make the solution up to, say, 200 c. c., and then to take 50 c. c. for analysis 
with a pipette. 
3. It is useless to attempt to await the subsidence of the precipitate ; even 
should it subside readily, the supernatant fluid would be scarcely clear enough 
to test. A large number of filters, about 5 centimetres in diameter, must be pro¬ 
vided ; and, w hen it is required to test, a small quantity is to be run through a 
filter, and 2 drops received from it on to a black glass plate ; a drop of fil¬ 
tered gelatine solution is then placed in the centre of the other drop, in order 
to see if tannin still remains in the solution : if there is anv reaction, the fil- 
trate, filter and all, are to be returned to the beaker, and the operation re¬ 
peated over again, as often as necessary. 
It is essential to work in strong daylight, and neither at dusk nor with 
artificial light ; sometimes a few minutes will elapse before the above-men¬ 
tioned reaction, between the drops, will manifest itself; at all times it re¬ 
quires considerable patience. 
Although I intended this paper to be a purely practical one, it will scarcely 
be out of place if I allude briefly to some theoretical points which experience 
has brought under notice, respecting the division of the varieties of tannin 
into two great classes,—gallo- and mimotannic acids. I have already men¬ 
tioned that the cupric tannate precipitated from mimotannic acid, is capable 
of being readily distinguished from the same tannate precipitated from gallo¬ 
tannic acid, by the fact that the former is soluble, while the latter is insoluble 
in ammonia ; and this holds good w ith every substance that I have experi¬ 
mented upon. 
Again, another practical fact, which tends to the distinctive separation of 
these tannins, is that the leather produced by gallotannic acid is far superior 
in quality to that produced by mimotannic acid; and it is those substances 
alone which contain gallotannic acid that will produce upon leather that charac¬ 
teristic bloom, which is considered so important a guarantee both of the suc¬ 
cess of the operation and of the goodness of the product. Oak bark, sumach, 
valonia, divi-divi, myrobalans, etc., are all capable of producing this de¬ 
posit, but with catechu, larch bark, etc , it is entirely wanting. Of course 
1 am aware that it was well knowm that catechu, willow, hemlock (abies 
canadensis), etc., produced a leather inferior to oak bark, but I do not 
think it has been before attributed to the fact that these contained only 
mimotannic acid. 
That differences of opinion have existed respecting the relationship of these 
tannins, is shown, inasmuch as Berzelius, Geiger, Waltl, and'Cavalms, were 
all of opinion “ that tannin, from whatever source it may be obtained, is 
identical, and that the difference of the reactions with iron salt must be 
attributed to impurities which are accidentally present.” In opposition to 
