composed, forming 1 eq. of Prussian
blue and 6 eq. of the potassa salt, which remains in solution, or
3K_{2}Cfy + 2(Fe_{2}O_{3}3NO_{3}) = Fe_{4}Cfy_{3} + 6(KO,NO_{5}). It
forms a bulky precipitate of an intense blue, is quite insoluble in water
or weak acids, with the exception of oxalic acid, with which it gives a
deep blue liquid, occasionally used as blue ink.

Ferridcyanide of potassium, added to a salt of the sesquioxide of iron,
yields no precipitate, but merely darkens the reddish-brown solution;
with protoxide of iron it gives a blue precipitate, containing
Fe_{3}Cfdy, which is of a brighter tint than that of Prussian blue, and
is known by the name of Turnbull's blue. Hence, the ferridcyanide of
potassium is as excellent a test for protoxide of iron as the yellow
ferrocyanide is for the sesquioxide.--_E., Photo. Times_.

* * * * *

FOUCAULT'S APPARATUS FOR MANUFACTURING ILLUMINATING GAS AND
HYDROGEN.

The illuminating gas and hydrogen apparatus, illustrated herewith, is
adapted to all cases in which it is desirable to manufacture gas upon a
small scale.

Through the use solely of oil or water, it produces illuminating gas or
pure hydrogen for all the applications that may be required of them. It
consists of three parts, viz., of a vaporizer, A, which converts the
liquids into gas; of a distributer, B, which contains and distributes the
liquids to be converted into gas, and of a regulator, C, which
automatically regulates the flow of the liquids in proportion as they are
used.

[Illustration: FIG. 1.--FOUCAULT'S GAS APPARATUS.]

In the vaporizer Mr. Foucault, the inventor of the apparatus, obtains a
perfectly regular combustion through the use of a central column, 15,
charged with fuel, closed at the upper part, open beneath, and entering a
furnace that is fed by it with regularity, the zone of combustion not
being able to extend beyond the level of the draught. The grate, 16, is
capable of revolving upon its axis in order to separate the cinders. It
also oscillates, and is provided with jaws for crushing the fuel; and it
may likewise be lowered so as to let the fire drop into the ash-pan when
it is desired to stop operations.

The vaporizer, properly so called, is not placed directly over the fire,
and for this reason the production of a spheroidal state of the liquid is
avoided. It consists of a vessel, 44, into which the liquid is led by a
pipe, 43. The cast-iro