to make perfectly comparative dyeing experiments it is best
to use porcelain dye-pots (these may be bought from most dealers in
chemical apparatus), and to heat these in a water-bath arrangement.

The simplest arrangement is sketched in figure 33; it consists of a
copper bath measuring 15 inches long by 10-1/2 inches broad and (p. 213)
6-1/2 inches deep; this is covered by a lid on which are six apertures
to take the porcelain dye-baths. The bath is heated by two round
gas-boiling burners of the type already referred to.

The copper bath is filled with water which, on being heated to the
boil by the gas burners, heat up the dye-liquors in the dye-pots. The
temperature in the dye-pots under such conditions can never reach the
boiling point; where it is desirable, as in some cases of wool
mordanting and dyeing that it should be so high, then there should be
added to the water in the copper bath a quantity of calcium chloride,
which forms a solution that has a much higher boiling point than that
of water, and so the dye-liquors in the dye-pots may be heated up to
the boil.

An objection might be raised that with such an apparatus the
temperature in every part of the bath may not be uniform, and so the
temperature of the dye-liquors in the pots might vary also, and
differences of temperature often have a considerable influence on the
shade of the colour which is being dyed. This is a minor objection,
which is more academic in its origin than of practical importance. To
obviate it Mr. William Marshall, of the Rochdale Technical School, has
devised a circular form of dye-bath, in which the temperature in every
part can be kept quite uniform.

The dyeing laboratories of Technical Schools and Colleges are
generally provided with a more elaborate set of dyeing appliances.
These in the latest constructed consist of a copper bath supported on
a hollow pair of trunnions, so that it can be turned over if needed.
Into the bath are firmly fixed three earthenware or porcelain
dye-pots; steam for heating can be sent through the trunnions. After
the dyeing tests have been made the apparatus can be turned over and
the contents of the dye-pots emptied into a sink which is provided for
the purpose.

Many other pieces of apparatus have been devised and made for the (p. 214)
purpose of carrying on dyeing experiments on the small scale, but it
will not be needful to describe these in detail. After all no more
efficient apparatus can