ratus fig.
15, I place it inverted in a bason containing either quicksilver, or any
other fluid substance by which I chuse to have the air confined. I then,
by the help of the air pump, drive out as much of the air as I think
convenient, admitting the quicksilver, &c. to it, as at _a_, and
putting a brass ball on the end of the wire, I take the sparks or shocks
upon it, and thereby transmit them through the air to the liquor in the
tube.

To take the electric sparks in any kind of fluid, as oil, &c. I use the
same apparatus described above, and having poured into the tube as much
of the fluid as I conjecture I can make the electric spark pass through,
I fill the rest with quicksilver; and placing it inverted in a bason of
quicksilver, I take the sparks as before.

If air be generated very fast by this process, I use a tube that is
narrow at the top, and grows wider below, as fig. 17, that the
quicksilver may not recede too soon beyond the striking distance.

Sometimes I have used a different apparatus for this purpose,
represented fig. 18. Taking a pretty wide glass tube, hermetically
sealed at the upper-end, and open below, at about an inch, or at what
distance I think convenient from the top, I get two holes made in it,
opposite to each other. Through these I put two wires, and fastening
them with warm cement, I fix them at what distance I please from each
other. Between these wires I take the sparks, and the bubbles of air
rise, as they are formed, to the top of the tube.

PART I.

_Experiments and Observations made in, and before the year 1772._

In writing upon the subject of _different kinds of air_, I find myself
at a loss for proper _terms_, by which to distinguish them, those which
have hitherto obtained being by no means sufficiently characteristic, or
distinct. The only terms in common use are, _fixed air_, _mephitic_, and
_inflammable_. The last, indeed, sufficiently characterizes and
distinguishes that kind of air which takes fire, and explodes on the
approach of flame; but it might have been termed _fixed_ with as much
propriety as that to which Dr. Black and others have given that
denomination, since it is originally part of some solid substance, and
exists in an unelastic state.

All these newly discovered kinds of air may also be called _factitious_;
and if, with others, we use the term _fixable_, it is still obvious to
remark, that it is applicable to them all; since they are all capabl