PEOCESSES FOE THE ESTIMATION OF TANNIN. 
517 
Marriage's method * advances us no further in this direction; it consists in 
precipitating the tannin by means of solution of ammonio-sulphate of copper; 
but this salt of copper precipitates many vegetable infusions containing no 
tannin, and further, I could never succeed in ascertaining the end of the opera¬ 
tion ; although Mr. Marriage states that the colour towards the termination of 
the reaction almost vanishes, certainly I never found it to do so, for if there 
was any alteration it appeared to be in the opposite direction. 
Pelouze and Stephens simultaneously proposed to estimate tannic acid by 
combining it directly with cortical tissue, as in tanning. For this purpose small 
strips of the best ox-hides, shaved thin, are washed in water, thoroughly dried 
and weighed, they are then soaked in water till they become soft and porous, 
when they are transferred to the solution to be analysed; the increase in weight, 
caused by absorption of the tannin, after having been brought to the same state 
of dryness as before, gives the proportion of tannin present. 
This, though apparently an unexceptionable method, fails to give satisfactory 
results, for in the first place the operation has to extend over a lengthened 
period, as the acid is very slowly absorbed; and, secondly, owing to that length 
of time, the infusion is apt to become partially converted into gallic acid; more¬ 
over the skin takes up small quantities of other bodies, such as colouring matter, 
etc., which tend more or less to affect its weight. 
Hammer's process .^—The foregoing method has been modified by M. Hammer, 
who estimates the strength of a solution by ascertaining the specific gravity 
before and after the removing the tannin. Having first noticed the density, 
precipitate the tannin by agitation with rasped animal hide, filter, and take its 
density again; the difference between these two specific gravities shows how much 
is due to the tannin present. By means, then, of a table which he has prepared, 
an approximation to the percentage may be obtained. 
These six preceding processes, from the various objections stated, I have found 
not to work well; that is, they do not possess that accuracy which, combined 
with a certain amount of simplicity, is necessary to the success of the analysis. 
We now come to three other methods, which I believe, as far as our knowledge 
at present carries us, are the only processes by which astringent bodies can be 
practically analysed. 
The Gelatine j^rocess .—This well-known method may be very properly divided 
into two parts, viz. the old, and, 1 may say, the now obsolete mode, and its 
more recent modification by Mulder. Tlie whole consists in precipitating the 
tannin solution by means of a volumetric solution of gelatine. 
Sir Humphry Davy used to analyse with gelatine gravimetrically, filtering 
off the precipitate, drying it, and reckoning § of the whole as pure tannin; this 
has, however, been shown to be both tedious and incorrect, as the solution re¬ 
fuses to filter, and the first • portions precipitated contain by far a larger pro¬ 
portion of tannin than do those which fall towards the end. 
As it is also perfectly impossible to work by the old volumetric method, we 
will at once proceed to Mulder’s excellent improvement upon it; he says, “ after 
many trials, I found that the addition of a small quantity of alum to the solu¬ 
tion of gelatine, furnished a suitable means to enable tannin to be precipitated 
without any difficulty from any fluids containing it, so that it may be deter¬ 
mined with the greatest exactitude. The separation of the precipitate takes 
place so readily and completely that in the course of a few minutes the fluid 
over the precipitate becomes quite limpid, and consequently may be immediately 
tested for any possible residue of tannin.” 
To prepare the gelatine solution, dissolve in a litre of water 3 grammes of 
■* Pliarm. Jouni. 1862, Vol. III. p. 509. 
f Joiu’n. de Cliimie Med., and Pharm. Jouni. 1862, Yol. III. p. 433* 
