Chemical Analysis of the Atmosphere. 285 
If the lime is too dry, it absorbs the carbonic acid very imperfectly ; 
if too moist, it is difficult to make it pass the bend of the tube. By 
a few trials the right point may be easily obtained. 
2. The sulphuric acid included in the end of the tube, is phiced 
inthe same manner as in the determination of moisture. From fifty 
to sixty drops are always sufficient. ‘The effect of this acid is evi- 
- The lime in passing from the state of hydrate to that of ear- 
bonate, abandons its water, which would cause a diminution of weight, 
as well as the portion of water (still greater) carried along by the cur- 
rent of air passing through the tube. It is in order to retain this wa- 
ter that the hygrometric portion of the tube becomes necessary. 
_3. The air before it enters into the tube containing the lime, ought 
to pass through a common hygrometric tube, in order to deposit its 
water, which, without this precaution, would be confounded with the 
carbonic acid. 
4. All the weighings ought to be done by substitution, that is to say, 
in taring (en tarant) a known weight,—for example, .2 of a gramme, 
with the tubes, and on the same plate of the balance, and in deducting 
from this weight, that required to be substituted for it after the oper- 
ation,—the difference indicating the increase of weight of the tube 
produced by the operation. We should never neglect to wipe the 
tubes. immediately before weighing, since the moisture which they — 
attract from the air, may, in a certain time, occasion an error of —/ 
milligrammes, as I have ascertained by direct experiment. 
5. The volume of air operated upon ought to be large, in ouller 
that the increase of weight of the lime may not be too small. In 
drawing 12972.5 grammes of water from the vessel A, I have ob- 
tained, in ordinary circumstances, an increase of seven to nine milli- 
grammes. It is therefore preferable to operate upon a volume of 
twice this size, which may be done without difficulty in the connate 
of fifty minutes. 
~ 6. To calculate the proportion of carbonic acid in the air. sstheeet 
We set out with the specific gravity of this gas, as determined by Ber- 
Zelius and Dulong, and which gives for 1.97978 grammes of carbo- 
nie acid its volume equal to one thousand cubic centimetres at 0° 
and at 0.76 of the barometric column, (=29.52 in.) The volume 
found. by calculation is reduced. to the volume which it would have 
at t the temperature of the vessel A and the prevailing height of the 
r, and from the sum representing the volume of analysed 
tr, Wwe obtain, by the rule of proportion, the volume of carbonic acid 
mM a given volume of air, e. g. in ten thousand parts. The reduction 
