Letts & Buaxe—The Carbonic Anhydride of the Atmosphere. 121 
As our immediate object at this stage was only to ascertain to 
what extent agreement among the results was possible, with varying 
times for absorption, no corrections for temperature, pressure, &c., 
were made. 
The receiving vessels were ordinary Winchester quart bottles 
of greenish-white glass, coated on the inside with paraffin wax. 
The absorbing solution was baryta water of the strength 1 c.c. = 
0-1 ec. CO, at N.T.P., the excess of which was afterwards titrated 
with dilute hydrochloric acid of corresponding strength, with 
phenol-phthalein as colour indicator. 
Wishing, as we have said, to test the extent of the accuracy 
and delicacy of Pettenkofer’s process, if carried out with all due 
precautions, we decided to experiment upon artificial mixtures 
of pure air and carbonic anhydride—the volume of the latter to 
correspond as nearly as possible with that usually present in 
“fresh ”’ air, z.e. 3 in 10,000. 
As regards the precautions we observed in the determina- 
tions, in addition to the specially prepared receiving vessels, we 
employed :— 
(1). A stock bottle for the baryta water, coated on its inner 
surface with paraffin wax, and connected with a measuring pipette ; 
the whole forming a closed system, into which only purified air 
could enter. 
(2). An apparatus for performing the titrations in vacuo, and 
for withdrawing the baryta water from the absorbing vessel in 
such a manner that access of carbonic anhydride from without 
was prevented. 
(3). Burettes, for running in the acid, of special construction 
and very carefully graduated. 
(4). Distilled water, free from all uncombined carbonic anhy- 
dride, for making up the standard solutions, and for washing out 
the receiving vessels. 
Before passing to the actual determinations we made of car- 
bonic anhydride both in artificial mixtures of that gas and purified 
air, and in ordinary air itself, it may be advisable to give some 
details regarding the above precautions. 
Receiving Vessels.—These, as a rule, were ordinary Winchester 
quart bottles of greenish-white glass, coated with paraffin wax on 
their inner surface; but occasionally we employed glass balloons 
