part, and, being turned with their mouths downwards, are afterwards set
upon the shelf ABCD, as shown Plate X. Fig. 1. F. The upper parts of the
sides of the cistern above the level of the shelf are called the _rim_
or _borders_.
The cistern ought to be filled with water, so as to stand at least an
inch and a half deep upon the shelf, and it should be of such dimensions
as to admit of at least one foot of water in every direction in the
well. This size is sufficient for ordinary occasions; but it is often
convenient, and even necessary, to have more room; I would therefore
advise such as intend to employ themselves usefully in chemical
experiments, to have this apparatus made of considerable magnitude,
where their place of operating will allow. The well of my principal
cistern holds four cubical feet of water, and its shelf has a surface of
fourteen square feet; yet, in spite of this size, which I at first
thought immoderate, I am often straitened for room.
In laboratories, where a considerable number of experiments are
performed, it is necessary to have several lesser cisterns, besides the
large one, which may be called the _general magazine_; and even some
portable ones, which may be moved when necessary, near a furnace, or
wherever they may be wanted. There are likewise some operations which
dirty the water of the apparatus, and therefore require to be carried
on in cisterns by themselves.
It were doubtless considerably cheaper to use cisterns, or iron-bound
tubs, of wood simply dove-tailed, instead of being lined with lead or
copper; and in my first experiments I used them made in that way; but I
soon discovered their inconvenience. If the water be not always kept at
the same level, such of the dovetails as are left dry shrink, and, when
more water is added, it escapes through the joints, and runs out.
We employ cristal jars or bell glasses, Pl. V. Fig. 9. A. for containing
the gasses in this apparatus; and, for transporting these, when full of
gas, from one cistern to another, or for keeping them in reserve when
the cistern is too full, we make use of a flat dish BC, surrounded by a
standing up rim or border, with two handles DE for carrying it by.
After several trials of different materials, I have found marble the
best substance for constructing the mercurial pneumato-chemical
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