ance which produces light and fire, and which all bodies in nature
are so eager to obtain?

MRS. B.

Most of the metallic oxyds are poisonous, and derive this property from
their union with oxygen. The white lead, so much used in paint, owes its
pernicious effects to oxygen. In general, oxygen, in a concrete state,
appears to be particularly destructive in its effects on flesh or any
animal matter; and those oxyds are most caustic that have an acrid
burning taste, which proceeds from the metal having but a slight
affinity for oxygen, and therefore easily yielding it to the flesh,
which it corrodes and destroys.

EMILY.

What is the meaning of the word _caustic_, which you have just used?

MRS. B.

It expresses that property which some bodies possess, of disorganizing
and destroying animal matter, by operating a kind of combustion, or at
least a chemical decomposition. You must often have heard of caustic
used to burn warts, or other animal excrescences; most of these bodies
owe their destructive power to the oxygen with which they are combined.
The common caustic, called _lunar caustic_, is a compound formed by the
union of nitric acid and silver; and it is supposed to owe its caustic
qualities to the oxygen contained in the nitric acid.

CAROLINE.

But, pray, are not acids still more caustic than oxyds, as they contain
a greater proportion of oxygen?

MRS. B.

Some of the acids are; but the caustic property of a body depends not
only upon the quantity of oxygen which it contains, but also upon its
slight affinity for that principle, and the consequent facility with
which it yields it.

EMILY.

Is not this destructive property of oxygen accounted for?

MRS. B.

It proceeds probably from the strong attraction of oxygen for hydrogen;
for if the one rapidly absorb the other from the animal fibre,
a disorganisation of the substance must ensue.

EMILY.

Caustics are, then, very properly said to burn the flesh, since the
combination of oxygen and hydrogen is an actual combustion.

CAROLINE.

Now, I think, this effect would be more properly termed an oxydation, as
there is no disengagement of light and heat.

MRS. B.

But there really is a sensation of heat produced by the action of
caustics.

EMILY.

If oxygen is so caustic, why does not that which is contained in the
atmosphere burn us?

MRS. B.

Because it is in a gaseous state, and has a greater attraction for its
electricity than for t