Red mercury oxide (which is formed as mercury rust by heating
mercury in air) is decomposed like copper carbonate (only by heating
more slowly and at a somewhat higher temperature), with the formation
of the peculiar gas, oxygen. For this purpose the mercury oxide is
placed in a glass tube or retort,19 to which a gas delivery tube is
attached by means of a cork. This tube is bent downwards, as shown
in the drawing (Fig. 1). The open end of the gas delivery tube is immersed
in a vessel filled with water, called a pneumatic trough.20 When
the gas begins to be evolved in the retort it is obliged, having no other
outlet, to escape through the gas delivery tube into the water in the
pneumatic trough, and therefore its evolution will be rendered visible
by the bubbles coming from this tube. In heating the retort containing
the mercury oxide, the air contained in the apparatus is first partly
expelled, owing to its expansion by heat, and then the peculiar gas
called ‘oxygen’ is evolved, and may be easily collected as it comes off.
For this purpose a vessel (an ordinary cylinder, as in the drawing) is
filled quite full with water and its mouth closed; it is then inverted
and placed in this position under the water in the trough; the mouth
is then opened. The cylinder will remain full of water—that is, the
water will remain at a higher level in it than in the surrounding vessel,
owing to the atmospheric pressure. The atmosphere presses on the
surface of the water in the trough, and prevents the water from flowing
out of the cylinder. The mouth of the cylinder is placed over the
end of the gas delivery tube,21 and the bubbles issuing from it will rise
into the cylinder and displace the water contained in it. Gases are
generally collected in this manner. When a sufficient quantity of gas
has accumulated in the cylinder it can be clearly shown that it is not
air, but another gas which is distinguished by its capacity for vigorously
supporting combustion. In order to show this, the cylinder is closed,
under water, and removed from the bath; its mouth is then turned upwards,
and a smouldering taper plunged into it. As is well known, a
smouldering taper will be extinguished in air, but in the gas which
is given off from red mercury oxide it burns clearly and vigorously,
showing the property possessed by this gas for supporting combustion
more energetically than air, and thus enabling it to be distinguished
from the latter. It may be observed in this experiment that, besides
the formation of oxygen, metallic mercury is formed, which, volatilising
at the high temperature required for the reaction, condenses on the
cooler parts of the retort as a mirror or in globules. Thus two substances,
mercury and oxygen, are obtained by heating red mercury
oxide. In this reaction, from one substance, two new substances are
produced—that is, a decomposition has taken place. The means of
collecting and investigating gases were known before Lavoisier's time,
but he first showed the real part they played in the processes of many
chemical changes which before his era were either wrongly understood
(as will be afterwards explained) or were not explained at all, but only
observed in their superficial aspects. This experiment on red mercury
oxide has a special significance in the history of chemistry contemporary
with Lavoisier, because the oxygen gas which is here evolved is
contained in the atmosphere, and plays a most important part in
nature, especially in the respiration of animals, in combustion in air,
and in the formation of rusts or scoriæ (earths, as they were then
called) from metals—that is, of earthy substances, like the ores from
which metals are extracted.
In order to illustrate by experiment one more example of chemical change and the application of the law of the indestructibility of matter, we will consider the reaction between common table salt and lunar caustic, which is well known from its use in cauterising wounds. By taking a clear solution of each and mixing them together, it will at once be observed that a solid white substance is