282 Chemical Analysis of the Aimosphere. 
ments, the half of this volume might be sufficient, there are cases in 
which this quantity becomes necessary. 
_.2. The discharged water is received ita large bottle B, measur- 
ed very exactly and marked on its neck with a file. The bottle which — 
I use contains 12972.5 cubic centimetres. By filling it twice, draw 
off nearly the whole volume contained in 
_8. The hygrometric tube is eleven or eralve inches long and as 
three lines to three and a half in diameter, internally. It has two 
enlargements near the two ends, destined to receive the hygroscopic 
liquid, when the tube is not in a perfectly horizontal position. 
4. Common sulphuric acid serves as a hygrometric substance. A 
quantity of amianthus, sufficient to line the sides of the tube, is ‘in- 
troduced, which is moistened by letting about fifty drops of acid fall 
into it. It is easy to moisten the fae uniformly throughout t the 
length of the tube, by suitable inclinations. The tube, thus prepar- 
ed and stopped at the two ends, is weighed in a balance which indi- 
cates milligrammes, (two hundredth part (nearly) of a grain.) 
The flowing of the water must be carefully regulated. If too 
rapid, a part of the vapor might escape the action of the sulphuric 
acid. If on the contrary it be too slow, time would be lost unneces~ 
sarily, and the air would be acted upon during an interval in which 
some change might take place in the constitution of the atmosphere. 
Experience has taught me that thirteen thousand grammes of water™ 
may be drawn from the vessel A in an interval of ten minutes, with- 
out the risk of any inconvenience. 
6. In calculating the result furnished by the operation, the neces 
sary corrections must be made for barometric pressure, temperature 
' of the air which enters the vessel A by taking that of the discharg- 
ing water, the temperature of the free air the moisture of which we 
are trying, and the tension of the water contained in the vessel- 
These reductions are too well known to philosophers to need further» 
explanation. 
Although in the use of this apparatus, there is no difficulty in fol 
lowing exactly the dimensions and manipulations described, it will be 
well for the student to adapt a second tube, containing sulphurie acid, 
between the hygrometric tube and the vessel A. If the weight of 
this latter tube is not altered by the passage of the air requisite to the 
operation, the arrangement may be deemed satisfactory. I have sent 
through the tube the required volume of air, saturated with moisture 
* Between three and four gallons. 
