REGNAULT’S HYGROMETRICAL RESEARCHES. 619 
riment, The tube No. 1 had absorbed 1°235 grm. of water; the 
tube No. 2 had not absorbed any. Thus the first tube had com- 
pletely dried the air. 
I made a second experiment, which will appear more conclu- 
sive than the last. Before the counterpoised tube, No. 1, I at- 
tached a tube filled with moistened sponge, and in front of this 
latter tube, three U-shaped tubes, filled with pumice-stone moist- 
ened with sulphuric acid, each 1 metre long; the third of these 
large tubes was immersed in a mixture of ice and chloride of 
calcium ; the air consequently arrived perfectly dry in the tube 
containing the wet sponge; there it took up a fresh quantity of 
moisture, which it deposited in the drying counterpoised tubes. 
The tube with wet sponge lost in this experiment . 0°767 gr. 
Whe drying-tube, No. 1, gained . 2°. 2. 6 «(O°767 
wee tue, No.2; rained 2° SS EO 
These experiments prove most evidently that the first drying- 
tube suffices, notwithstanding its small dimensions, to dry the 
air completely. The tube No, 2 is only confirmatory, and as 
such it is well to preserve it. 
I may observe in passing, that by multiplying the apparatus 
intended to absorb gases or vapours, in the hope of obtaining a 
more complete absorption, much greater errors are committed 
in the weighings than those which it is sought to avoid. In 
fact, when the volume of the absorbing apparatus is consider- 
able, we cannot neglect in the weighings the changes which su- 
pervene in the nature of the external air during the interval be- 
tween the two weighings. Now, these changes cannot be deter- 
mined with any precision. The least difference between the 
temperature of the external air and that of the apparatus at 
the moment of weighing,—a difference which it is impossible 
to avoid,—occasions a perceptible error. Lastly, the vitreous, 
eminently hygroscopic surface of the apparatus may be covered 
with an unequal quantity of humidity in the two weighings. 
In experiments which require great accuracy, the experimenter 
should seek to reduce the apparatus to the smallest possible 
dimensions, instead of increasing them, as there is at the present 
day a tendency todo. The multiplication of the apparatus causes 
moreover too great resistances to the passage of the gas, and it 
becomes impossible to answer for the equality of pressure in the 
different parts of the apparatus. 
