Dr. Henry’s Experiments on Ammonia , &c. 43^ 
The avidity with which ammonia retains moisture, and 
again absorbs it when artificially dried, is very remarkable. 
A confined quantity of common air may be completely desic- 
cated, in the space of a few minutes, by pure potash, or by 
muriate of lime ; so that no ice shall appear in the inner sur- 
face of the containingvessel, when exposed to a cold of — 26° 
of Fahrenheit. But ammonia requires exposure during 
some hours to potash, to stand the test even of o° Fahrenheit ; 
and a single transfer of the dried gas, through the mercury of 
a trough in ordinary use, again communicates moisture to it. 
Muriatic acid gas, freed merely from visible moisture, deposits 
no water at the temperature of 2 6° Fahrenheit. This is 
probably owing to its strong affinity for water ; for electricity, 
after the full action of muriate of lime, evolves, as I have 
lately ascertained, about its bulk of hydrogen gas, the 
recent muriatic acid gas giving about -i-th after the same 
treatment.* 
have been generated by electricity. But though this supposition may explain the non- 
appearance of visible moisture , it does not account for the inefficiency of a powerful 
cooling cause to discover traces of watery vapour : for this is a test which renders 
apparent very minute quantities of water in gases. 
* In a course of experiments, which I have described in the Philosophical Trans- 
actions for 1800, it appeared that muriatic acid gas, after being dried by muriate of 
lime, gave nearly as much hydrogen by electrization, as gas which had not been thus 
exposed. I was not however aware, at that time, of the extreme caution necessary in 
experiments of this kind ; and was satisfied with transferring the acid gas from a large 
vessel, in which it had been dried, into the electrizing tube, a mode of proceeding 
which I now find to be quite inadmissible. The action of muriate of lime, which has 
undergone fusion, on muriatic acid gas, is rendered very sensible, when considerable 
quantities are used, by the evolution of much heat, and by a diminution of the volume 
of the gas. Ammonia, also, is contracted in bulk by dry caustic potash. Muriate 
of lime cannot be employed for its desiccation, since this substance rapidly absorbs 
