or consumed at an irregular rate; and hence
the distance between them became greater or
less at certain intervals, destroying thereby
the equality of the light, which became more
or less intense as the carbon points approached
or receded from each other. To ensure a
proper condition of the light a regular
distance was essential: if the points became
too widely separated the flame expired;
if they were forced too near it deadened to a
heavy dull glow. Mechanical contrivances of
some ingenuity were tried to obviate this
difficulty, but without avail, and it was not
until Dr. Watson devised the beautiful method
now employed, by which the points of carbon
are made self-regulating, that a continuous
and steady light was obtained.

The electric light although triumphant as
an illuminator, was, at first, too costly in its
consumption of the raw materials of electricity
to make it available for ordinary purposes.
It may have been likened to some beautiful
animal, which was found to consume far more
food than it was worth. The electric animal
swallowed too much iron, zinc, copper, acids,
and salts, to pay for its work: it was not
content with eating away its carbon points, but,
like many a noble steed, "ate its head off."

Many plans were devised for cheapening
the production of electricity, and this was
partially compassed by the employment of
cheaper metals in combination with the
normal acids. The cheapest metals were
found to be iron, lead, and zinc, but still
the consumption of these with the chemicals
employed outstripped the value of the
electricity, and something more had to be
achieved. For the purposes of an electric
light it had been for some time ascertained
that constancy and intensity in the battery
employed were essentials: in other words,
unless the stream of electricity was both
regular and powerful, no effect would be
produced. A battery of cast-iron and zinc
arranged in such a way that the former is
separated from the latter by a porous
diaphragm of potter's biscuit-ware, the iron
being excited by a mixture of saltpetre and
sulphuric acid diluted by water, or by dilute
nitrous acid and the zinc acted on by dilute
sulphuric acid, affords great intensity. This
is known as the Maynooth battery.

The products of such a battery as the
above are, in addition to the electricity which
is turned to account, several salts which have
hitherto been thrown aside as valueless.
These were the articles known to chemists as
nitrate and sulphate of iron and sulphate of
zinc, the latter being the white vitriol of
commerce. The actual value of any of these
salts is so trifiling, and the demand for them
so limited, that the residuary liquor of the
Maynooth battery containing them may, for
all practical purposes, be called worthless. It
was evident that if this waste solution of the
metals and acids could be turned to profitable
account, the cost of the electricity would be
proportionately reduced. To this object,
therefore, Dr. Watson directed all his energies.

The result of countless delicate and
painstaking experiments has been the conversion
of the hitherto refuse liquor of the Maynooth
battery into articles of considerable
commercial value. It was known that certain
salts of iron and lead—that is to say,
combinations of acids with those metals—
precipitated in the form of salts, when mixed
with certain chemicals, produced a number
of beautiful pigments of great delicacy and
purity. This was seized on as a means of
employing to a profit the waste liquor of the
battery, and the result showed that the plan
of producing light and colour from the same
elementary bodies was perfectly practicable.
In this way the cast-iron and zinc
apparatus of Maynooth was converted into a
chromatic battery.

This process is as simple as it is beautiful.
In the iron and zinc battery, nitric and
sulphuric acids are employed in a diluted form,
the ordinary resulting waste of which are
solutions of nitrate of iron and sulphate of
zinc. Instead of these residuary liquors
being thrown aside as undeserving of care, they
are removed separately from the chromatic
battery, and, having been brought to a certain
heat by means of steam, are blended with a
solution of prussiate of potash, which, with
the iron liquor, throws down a splendid blue
pigment—Prussian blue, in fact, of great
purity—whilst with the zinc liquor it
precipitates a fine ultramarine blue.

After some agitation the colouring matter
is allowed to subside, the clear liquor is drawn
off, and finally the heavy deposition of blue is
removed from the bottom of the vats and
placed on cloth stretchers, whereon the
moisture is allowed to drain from it. Subsequent
pressure, and a final gradual drying
in carefully heated chambers completes the
process, and the result is a pigment suitable
for employment in the fine arts, for house
decoration or paper-colouring. It is difficult
to conceive a deeper or more ethereal blue
than the rich yet delicate ultramarine of the
chromatic battery. Equally gorgeous are the
electric reds produced by boiling the zinc
yellow with lime in varying proportions,
according to the depth of colour required.
By a combination of these zinc yellows with
the iron blues, a series of greens are produced
of an infinity of shades, and which have the
property of standing high temperatures
without injury.

Yellows of great delicacy, ranging from
pale lemon to a bright orange yellow, are
produced by treating the waste liquor of the
lead and nitric acid compartments of the
battery with chromate of potash, which is, in
plainer language, a salt composed of potash
and chromic acid.

If, instead of the chromate, prussiate of
potash be added to the residuum of the lead
and zinc battery, a delicate white pigment