ompelled to part with the latter,
when the temperature is further increased. I have put some oxyd of
manganese into a retort, which is an earthen vessel with a bent neck,
such as you see here. (PLATE VII. Fig. 2.) --The retort containing the
manganese you cannot see, as I have enclosed it in this furnace, where
it is now red-hot. But, in order to make you sensible of the escape of
the gas, which is itself invisible, I have connected the neck of the
retort with this bent tube, the extremity of which is immersed in this
vessel of water. (PLATE VII. Fig. 3.) --Do you see the bubbles of air
rise through the water?

CAROLINE.

Perfectly. This, then, is pure oxygen gas; what a pity it should be
lost! Could you not preserve it?

MRS. B.

We shall collect it in this receiver. --For this purpose, you observe,
I first fill it with water, in order to exclude the atmospherical air;
and then place it over the bubbles that issue from the retort, so as to
make them rise through the water to the upper part of the receiver.

EMILY.

The bubbles of oxygen gas rise, I suppose, from their specific levity?

MRS. B.

Yes; for though oxygen forms rather a heavy gas, it is light compared to
water. You see how it gradually displaces the water from the receiver.
It is now full of gas, and I may leave it inverted in water on this
shelf, where I can keep the gas as long as I choose, for future
experiments. This apparatus (which is indispensable in all experiments
in which gases are concerned) is called a water-bath.

CAROLINE.

It is a very clever contrivance, indeed; equally simple and useful. How
convenient the shelf is for the receiver to rest upon under water, and
the holes in it for the gas to pass into the receiver! I long to make
some experiments with this apparatus.

MRS. B.

I shall try your skill that way, when you have a little more experience.
I am now going to show you an experiment, which proves, in a very
striking manner, how essential oxygen is to combustion. You will see
that iron itself will burn in this gas, in the most rapid and brilliant
manner.

CAROLINE.

Really! I did not know that it was possible to burn iron.

EMILY.

Iron is a simple body, and you know, Caroline, that all simple bodies
are naturally positive, and therefore must have an affinity for oxygen.

MRS. B.

Iron will, however, not burn in atmospherical air without a very great
elevation of temperature; but it is eminently combustible in