Mr. Davy on a Combination 
156 
act on nitrous gas or muriatic acid, or carbonic oxide, or sul- 
phureous gasses, when they have been carefully dried. It is 
the substance which I employed in all the experiments on 
the combinations of oxymuriatic gas, described in my last 
two papers. 
The gas produced by the action of muriatic acid on the 
salts which have been called hyperoxymuriates, on the con- 
trary, differs very much in its properties, according as the 
manner in which it is prepared and collected is different. 
When much acid is employed to a small quantity of salt, 
and the gas is collected over water, the water becomes tinged 
of a lemon colour ; but the gas collected is the same as that 
procured from manganese. 
When the gas is collected over mercury, and is procured 
from a weak acid, and from a great excess of salt, by a low 
heat, its colour is a dense tint of brilliant yellow green, and it 
possesses properties entirely different from the gas collected 
over water. 
It sometimes explodes during the time of its transfer from 
one vessel to another, producing heat and light, with an ex- 
pansion of volume ; and it may be always made to explode 
by a very gentle heat, often by that of the hand.* 
* My brother, Mr. J. Davy, from whom I receive constant and able assistance in 
all my chemical enquiries, had several times observed explosion, in transferring 
the gas from hyperoxymuriate of potash, over mercury, and he was inclined to 
attribute the phenomenon to the combustion of a thin film of mercury, in contact 
with a globule of gas. I several times endeavoured to produce the effect, but without 
success, till an add was employed for the preparation of the gas, so diluted as not to 
afford it without the assistance of heat The change of colour and expansion of vo- 
lume, when the effect took place, immediately convinced me, that it was owing to a 
decomposition of the gas. 
