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atmospherical air, combines with the oxygen, and undergoes this slow
combustion. But the same effect does not take place in oxygen gas,
because it is not capable of dissolving phosphorus; it is therefore
necessary, in this case, that heat should be applied to effect that
division of particles, which, in the former instance, is produced by the
nitrogen.

EMILY.

I have seen letters written with phosphorus, which are invisible by
day-light, but may be read in the dark by their own light. They look as
if they were written with fire; yet they do not seem to burn.

MRS. B.

But they do really burn; for it is by their slow combustion that the
light is emitted; and phosphorus acid is the result of this combustion.

Phosphorus is sometimes used as a test to estimate the purity of
atmospherical air. For this purpose, it is burnt in a graduated tube,
called an _Eudiometer_ (PLATE XI. fig. 2.), and from the quantity of air
which the phosphorus absorbs, the proportion of oxygen in the air
examined is deduced; for the phosphorus will absorb all the oxygen, and
the nitrogen alone will remain.

EMILY.

And the more oxygen is contained in the atmosphere, the purer,
I suppose, it is esteemed?

MRS. B.

Certainly. Phosphorus, when melted, combines with a great variety of
substances. With sulphur it forms a compound so extremely combustible,
that it immediately takes fire on coming in contact with the air. It is
with this composition that phosphoric matches are prepared, which kindle
as soon as they are taken out of their case and are exposed to the air.

EMILY.

I have a box of these curious matches; but I have observed, that in very
cold weather, they will not take fire without being previously rubbed.

MRS. B.

By rubbing them you raise their temperature; for, you know, friction is
one of the means of extricating heat.

EMILY.

Will phosphorus combine with hydrogen gas, as sulphur does?

MRS. B.

Yes; and the compound gas which results from this combination has a
smell still more fetid than the sulphuretted hydrogen; it resembles that
of garlic.

The _phosphoretted hydrogen gas_ has this remarkable peculiarity, that
it takes fire spontaneously in the atmosphere, at any temperature. It is
thus, probably, that are produced those transient flames, or flashes of
light, called by the vulgar _Will-of-the Whisp_, or more properly
_Ignes-fatui_, which are often seen in church-yards, and places where
the putrefactio