perating 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
apparatus, as it is perfectly impenetrable by mercury, and is not
liable, like wood, to separate at the junctures, or to allow the mercury
to escape through chinks; neither does it run the risk of breaking, like
glass, stone-ware, or porcelain. Take a block of marble BCDE, Plate V.
Fig. 3. and 4. about two feet long, 15 or 18 inches broad, and ten
inches thick, and cause it to be hollowed out as at m n Fig. 5. about
four inches deep, as a reservoir for the mercury; and, to be able more
conveniently to fill the jars, cut the gutter T V, Fig. 3. 4. and 5. at
least four inches deeper; and, as this trench may sometimes prove
troublesome, it is made capable of being covered at pleasure by thin
boards, which slip into the grooves x y, Fig. 5. I have two marble
cisterns upon this construction, of different sizes, by which I can
always employ one of them as a r