DR. DALTON ON THE CONSTITUTION OF THE ATMOSPHERE. 357 
It is best not to rely too much upon hydrogen taken from a bottle half filled with 
water. 
2. Oxygen gas, and others, will show carbonic acid by sending them up through 
a narrow eudiometer tube filled with lime-w T ater, provided the acid gas amounts to 
\ per cent, of the original ; but it does not show any carbonic acid in this way in 
atmospheric air, though the acid is always present to the amount perhaps of 10 j 0 th 
part. The proportion of pure oxygen in any sample containing from 90 to 100 per 
cent, of that gas, may be found either by hydrogen gas or nitrous gas ; and if great 
accuracy is required, I recommend testing it both ways, as has already been men- 
tioned under the head nitrous gas. 
3. The gradual deterioration of oxygen, hydrogen, nitrous gas, common air, &c., 
when by use the phial becames or f filled with trough water, is a circumstance 
by no means to be overlooked. The entrance of water that has been sometime 
stagnant in the cistern, though preserved carefully from any material impurities, 
always affects the remaining air, though the phial be w’ell corked and immersed in a 
cup of water. The cause is obvious to those acquainted with the laws that regulate 
the absorption of gases by water. The common air in the water (the quantity of 
which varies much as to the oxygen part) is continually either making its escape into 
the incumbent air of the phial, or this last air is entering the water, so that the de- 
gree of purity is continually changing in a small degree. This renders it necessary 
to test the actual state of this gas after it has been some time in the phial, before we 
recommence the use of it. A phial of air may be pure at first, and only 90 per cent, 
at its conclusion. I have known samples of common air kept in bottles at first con- 
taining 21 per cent, of oxygen, and after some months a small residue was found to 
contain only 19 per cent. 
4. It may not be improper here to relate some unpublished results which I formerly 
obtained when experimenting on subjects here discussed. In my memoranda for 
1816, I find that I took water well boiled (supposed i of an hour or more) and then 
poured it gently into a Florence flask, filling it up into the narrowest part of the 
neck, and left it so, exposed to the atmosphere for three days without any agitation. 
At the end of this, 2700 grains of water imbibed 49 grain measures of atmospheric 
air by agitation, which is about -f- of a full share ; hence -j- of a full share must have 
been, both the air that was left in after boiling, and that acquired from the atmo- 
sphere in three days by absorption from the small exposed surface. 
Water boiled in a kettle for three or four minutes, then suddenly cooled and trans- 
ferred without agitation into a bottle containing 2700 grains, and then agitated with 
atmospheric air, imbibed 32 measures, which are about half a charge ; whence it may 
be inferred that water boiled for three or four minutes loses about half of its air. 
I boiled a kettle full of water for a quarter of an hour ; let it stand a day or two 
to cool, then transferred it carefully by a siphon into a cylindric jar of 8 inches dia- 
meter and 10 inches deep; afterwards drew off daily by a siphon 2700 grain mea- 
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