1819.] Muriatic Acid Gas. 97 
who stated that the salt absorbs water in this manner to a very 
considerable extent ; that it is only from the salt in this state 
that water can be procured, and that whenit is formed from the 
combination of the gases in a close vessel, and heated without 
exposure to the air, not the slightest trace of water appears, even 
when the experiment is performed on a large scale. 
The reverse of this I was able to demonstrate by further expe- 
rimental investigations. It was shown that the salt absorbs no 
moisture from the air in the common state of dryness and tem- 
perature 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 dave: perfectly dry. It was further shown, 
that when the other circumstances of the experiment 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 admitted, water is 
obtained from it. This last result was even at length admitted 
by those who had advanced the opposite assertion, m an expe- 
riment 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 expe- 
riment being unfavourable to its expulsion, more particularly to 
the difficulty of applying a regulated temperature to a thin crust 
of salt, so as to separate the water without volatilizing the salt 
itself, and to the oHect 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 measure condensed and absorbed at another ; or if the 
heat is applied equally, is retamed in the elastic form, and, as it 
is cooled, is equally condensed. Accordingly, when the experi- 
ment was repeated, obviating these sources of error as far as 
possible, the water obtained was in larger quantity. Andasno 
fallacy belongs to the conducting the experiment in the more 
favourable mode in which it was first performed (the assertion of 
the absorption of water from the air being altogether unfounded), 
the quantity procured in that mode is to be regarded as the real 
result.* 
The argument was maintained that the water might be derived 
from hygrometric vapour in the gases submitted to experiment. 
This it was easy torefute. Dr. Henry had shown that ammonia, 
after exposure to potash,and muriatic acid after exposure to muriate 
of lime, retain no trace of vapour whatever ; and these precautions 
had been very carefully observed. The assertion was brought 
forward too only to account for the minute quantity of water 
* NichoJson’s Journal, xxxii, 186, &c,; xxxiv. 271. 
