PROCESSES FOR THE ESTIMATION OF TANNIN. 
521 
mercial tannin never contains more than 96 to 97 per cent., and that is the 
reason why the numbers do not approach with greater exactitude, but could we 
in any way procure chemically pure tanuin to work upou, we should doubtless 
find that it would absorb exactly 231-1 c. c. 
These results are remarkably corroborated by the labours of Buchner,* who 
some time since investigated the whole series of tannates and gallates, and 
amongst them specially examined the bodies produced, by the oxidation of 
tannin in alkaline solutions. By direct separation and analysis of the body so 
produced, he found that 3 equiralents of tannic anhydride, CjgHj(,Og, absorbed 
24 equivalents of oxygen, carbonic anhydride being simultaneously produced, 
the composition of the resulting product being Cj.HigOjj:— 
3 (C„H.<,0,) + O., = 3 (C..H.„0„) + 9 COj. 
V ^ ^ 
Tannoxylic anhydride. 
Now if in Buchner’s hands 3 at. of tannin consumed 24 at. of O, by free 
exposure to the air, there is no reason why it should not do the same under the 
present circumstances, and if so 1 grm. of the above composition would absorb 
225-5 c. c. at 20° C. Bub the now universally received formula for tannin is 
CgyllgaOjy, aiid, therefore, from its somewhat lower atomic weight 1 grm. would 
absorb 231-1 c. c., at the same temperature and pressure. We see at once the 
similarity existing between these numbers and those obtained in the foregoing- 
experiments. 
I think this is sufficient to prove that 175, the volume given by Mittenzwey, 
should be superseded by 231; at the same time bearing in mind to whom the 
credit for the introduction of the process is due. There is one slight drawback 
in the fact that it reckons everything that will absorb oxygen as tannic acid, 
so that if gallic acid be present the tannin in a fresh sample must be precipitated 
by albumen or gelatine, the gallic acid separately determined, and then deducted 
from the total amount; in all other respects the process answers well, and in 
accuracy of result, in giving a minimum of trouble, and in rapidity of execution 
is excelled by none, and it may, without hesitation be adopted by any one who 
may have occasion to make an analysis of an astringent substance. 
Such are the various processes which have, from time to time, been published 
for the estimation of tannin; and although I am of opinion that its analysis 
may be effected in other ways, such perhaps as a tannate of an alkaloid, or by 
oxidation with plumbic peroxide, I have forborne to bring them forward on 
the present occasion, because as yet they are not found to possess any decided 
advantages over the processes already known, and further, because 1 am aware 
that in practice as well as in theory it is useless to supplant one process or 
hypothesis by the substi'ution of another, without affording a more satisfactory 
explanation of questions at issue. 
Baker Street, Portman Square, W. 
The Chairman thanked Mr. Watts for his paper, but said, at that late 
hour it would be impossible to enter upon a discussion of it. 
Mr. Hemingway laid two bottles on the table, containing tincture of 
hemlock, to which reference had been made by Hr. Harley in a communi¬ 
cation on that subject. He said he was authorized by Hr. Harley to state 
that he hoped at the next meeting of the Society to make a further com¬ 
munication on the preparations of hemlock, relating to those of the, fresh 
plant. 
* Annalen der Chem. und Pharm. b. 53, p. 175-369. 
VOL. VIII. 2 M 
