tures as coal-tar
fractions, petroleum and the "wash" of the spirit industry, is very
important, and many types of apparatus are employed in order to effect
a separation of the vapours. The earliest form, invented by C. B.
Mansfield to facilitate the fractionation of paraffin and coal-tar
distillates, consisted in having a pipe leading from the inclined
delivery tube of the still to the still again, so that any vapour
which condensed in the delivery tube was returned to the still. Of
really effective columns Coupier's was one of the earliest. The
vapours rising from the still traverse a tall vertical column, and are
then conveyed through a series of bulbs placed in a bath kept at the
boiling-point of the most volatile constituent. The more volatile
vapours pass over to the condensing plant, while the less volatile
ones condense in the bulbs and are returned to the column at varying
heights by means of connecting tubes. The French column is similar in
action. The Coffey still is one of the most effective and is employed
in the spirit, ammonia, coal-tar and other industries. It consists of
a vertical column divided into a number of sections by horizontal
plates, which are perforated so that the ascending vapours have to
traverse a layer of liquid. Above this "separator" is a reflux
condenser, termed the "cooler," maintained at the correct temperature
so that only the more volatile component passes to the receiver. The
success of the operation chiefly depends upon the proper management of
the cooler.

8. _Commercial Distillation of Water._--Distilled water, i.e. water
free from salts and to some extent of the dissolved gases which are
always present in natural waters, is of indispensable value in many
operations both of scientific and industrial chemistry. The apparatus
and process for distilling ordinary water are very simple. The body of
the still is made of copper, with a head and worm, or condensing
apparatus, either of copper or tin. The still is usually fed
continuously by the heated water from the condenser. The first portion
of the distillate brings over the gases dissolved in the water,
ammonia and other volatile impurities, and is consequently rejected;
scarcely two-fifths of the entire quantity of water can be safely used
as pure distilled water.

Apparatus for the economic production of a potable water from
sea-water is of vital importa