———_ ee eee 
ON MURIATIC AcID Gas, &e. 289 
the combination of the gases in a close vessel, and heated with- 
out exposure to the air, not. the slightest trace of water ap- 
pears, even when the experiment is performed on a sa 
scale. 
The reverse of this I was able to demonstrate by farther ex- 
perimental investigations. It was shewn, that the salt absorbs 
no moisture from the air in the common state of dryness and 
temperature in which the experiment is performed: when 
weighed immediately on its formation, in an exhausted vessel, 
it gains no weight from exposure, but remains the same after 
a number of hours ; and when exposed to the air in the freest 
manner, it remains, after many days, perfectly dry. It was far- 
ther shewn, that when the other circumstances of the experi- 
ment are the same, it yields no larger portion of water when it 
has been exposed to the air, than it does without this previous 
exposure. And, lastly, it was proved, that when the salt has 
been formed, and is heated without the air having been admit- 
ted, water is obtained from it. This last result was even at 
length admitted by those who had advanced the opposite as- 
sertion, in an experiment performed with a view to determine 
the fact. The quantity of water was indeed less than what is 
procured in the other mode; but this was obviously owing to 
the circumstances of the experiment being unfavourable to its 
expulsion,—more particularly to the difficulty of applying a re- 
gulated temperature to a thin crust of salt, so as to separate 
the water without volatilising the salt itself,—and to the effect 
arising from the whole internal surface of a large vessel being 
encrusted with the salt, so that if the’ heat is locally applied, 
the aqueous vapour expelled from one part is in a great mea- 
sure condensed and absorbed at another, or if the heat is ap- 
plied equally, is retained in the elastic form, and, as it is cool- 
ed, is equally condensed.. Accordingly, when the experiment 
Vou. VIII. P. II. Oo . was 
