520 
PIIAEMACEUTICAL MEETING. 
introduce into it 150 to 200 c. c. of a 2 or 3 per cent, caustic soda, or 3 to 6 per 
cent, potash solution, and then drop in about 1 gramme of tannin, loosely wrapped 
in paper, close the bottle instantly, and open the pinch cock just for a moment 
to equalize the pressure between the external and internal air. The absorption 
of oxygen commences immediately, and is hastened by strongly shaking the 
bottle, which must be wrapped in a cloth, to avoid raising the temperature by 
the warmth of the hand. 
To ascertain the quantity of oxygen absorbed, weigh about 250 c. c. of dis¬ 
tilled water into a beaker, and dip the end of the exterior glass tube into it; on 
opening the pinchcock, the water, owing to the vacuum in the interior, will flow 
into the bottle, at the same time taking care that the fluid in the two vessels is 
kept at the same level, so as to avoid unequal pressure. The experiment is 
ended, when, after repeated shakings, no more water runs from the beaker into 
the bottle, and the difference in weight of the water in the beaker, in grammes, 
gives the amount of oxygen absorbed in cubic centimetres, which can be cor¬ 
rected for the standard temperature and pressure. 
The next thing to find out is, how many c. c. of oxygen are equivalent to a defi¬ 
nite quantity of tannic acid: now Mittenzwey says that 1 grm. of tannin will 
absorb the amount of oxygen as '1 grm. of gallic acid, viz. 175 c. c. at 20° C., 
and in twelve experiments the greatest variation observed was 2 c. c. with tannic, 
and 1’2 c. c. with gallic acid. Of course the whole value of the process depends 
upon the accuracy of these determinations, and with the hope of verifying them, 
if possible, I instituted a series of experiments having that end in view, and the 
result of those experiments is, that I am quite assured that 175 c. c. is not the 
correct volume absorbed by 1 grm. of tannin at 20° C. 
After repeated trials, I found that 1 grm. of commercial tannin absorbs, with 
considerable uniformity, 225 c. c. of oxygen at the above temperature, and the 
following are the numbers upon which this assertion is based;— 
•783 
grm. at 10° C. absorbed 169-2 
C. ' 
Calculated for 
1 grm. at 20° C. 
c. = 224-1. 
•671 
^3 
3 3 3 3 
145-8 
33 
= 225-4. 
•694 
33 
13-85° C. „ 
152-8 
33 
= 224-0 
•801 
33 
3 3 3 3 
175-9 
33 
= 224-61. 
•821 
33 
14° C. „ 
181-4 
33 
= 225-9. 
•732 
33 
13-5° C. „ 
161-6 
33 
= 226-1. 
The greatest variation being 2T c. c. 
Several points must be particularly attended to in order to ensure a correct 
analysis, which, though simple in themseves, if neglected are certain to vitiate 
the result. 
1. Ensure that the bottle is accurately closed, else as soon as a slight vacuum 
is produced inside, the pressure of the external air forces some through the pores 
of the cork; to obviate it, brush the upper half of the cork over with sealing- 
wax, and before shaking lute it round with linseed-meal. 
2. The temperature of the air with which the bottle is filled must be carefully 
noted, and when finished must be brought to the same temperature, before cal¬ 
culating the volume. 
3. Sufficient length of time must be allowed for the tannin to absorb as much 
oxygen as it will; and if five or six operations be conducted at once, this may 
be readily done without inconvenience. 
4. The bottle must be large enough to hold more oxygen than will be required; 
1 grm. of tannin may be used in a litre and a half bottle, but not more. 
5. The volume must be reduced to some standard temperature. 
Although I have given 225 as the number of c. c. obtained in these experi¬ 
ments, there is no doubt that the theoretical volume is 231 c. c. at 20° C.; com- 
