Chemical Analysis of the Atmosphere. 283 
(by pressing_ it first through a tube containing moist rete ete 
finding the second tube altered in weight. ae 
_ The increase of weight of the hygromettic tube, in causing the 
discharge of 12972.5 grammes of water, is found, under ordinary 
circumstances, included within the limits of sixty to one hundred 
milligrammes, an amount apparently considerable enough to be but 
little influenced by errors of observation. On the 26th of Dec. 
1830, during the finest weather, at a temperature of 16° cent. estill 
Fah.) 1 obtained thirteen milligrammes from the same volume of air. 
Although this method of estimating the moisture of the atmosphere 
always requires an experiment, and for which common hygrometers 
cannot be substituted for the instrument now described, it appears to 
me that this is the only one which can give a direct result, and that 
it must on that account be useful in regulating others. I think never- 
theless, it may be used successfully in travelling. A tin vessel, of 
convenient dimensions, (from eight to ten thousand cubic centime- 
ters) would be easily transported, as well as tubes containing amian- 
thus moistened with sulphuric acid, and weighed beforehand, the ba- 
rometer and thermometer always making a pwd of se ae of 
a scientific traveller. 
To those who could not chadetieatly bisa a veneel: expressly 
adapted to this purpose, I would propose the use of a common bot- 
tle, thus—fill the bottle A, (Fig. 2.) with water, which may be drawn 
off by the syphon ab, and measured by means of a vessel of known 
capacity. ‘To the tube c adapt the hygrometric tube in the common 
manner. With this apparatus, which may be constructed wherever 
bottles can be found, and a small portable measuring glass, with the 
weighed tubes, it is easy to make experiments of sufficient exactness. 
Care must of course be taken to guard against the influence of hu- 
midity arising from the flow of the water, the decanting from the 
measuring glass into the bottle, &c. which will never be difficult. 
With a fixed apparatus, it would be advisable to perform the experi 
ment near a window, through an opening of which the hygrometric 
tube should pass into the open air. 
Il. Determination of Carbonic Acid. 
The valuation of the carbonic acid in the atmosphere has been 
less studied than that of the other elements, either because it pre- 
Sents greater obstacles or is judged to be of less interest. All that 
is Reey known, we owe to Theod. de Saussure, whose ee. 
