rsenic is reduced, volatilized, and
condenses in the cool portion of the tube. By this method, the
smallest portion of a grain of the arsenical compound may be thus
examined with the greatest readiness. If the residue is now washed, by
which the soda is got rid of, the metallic arsenic may be obtained in
small spangles. If the compound examined be the sulphide of antimony,
the one-thousandth part can be readily detected, and hence this method
is admirably adapted to the examination of medicinal antimonial
compounds. The arsenites of silver and copper are reduced by the
formiate of soda to their metals, mixed with metallic arsenic. The
mercurial salts are all reduced with the metal plainly visible as a
bright silvery ring on the cool portion of the tube. The chloride and
nitrate of silver are completely reduced, and may be obtained after
working out the soda, as bright metallic spangles. The salts of
antimony and zinc are thus reduced; also the sulphate of cadmium. The
sublimate of the latter, although in appearance not unlike that of
arsenic, can easily be distinguished by its brighter color. It is, in
fact, the rich yellow of this sublimate which has led artists to adopt
it as one of their most valued pigments.

2. EXAMINATIONS IN THE OPEN TUBE.

The substance to be operated upon should be placed in the tube, about
half an inch from the end, and the flame applied at first very
cautiously, increasing gradually to the required temperature. The
tube, in all these _roasting_ operations, as they are termed, should
be held in an inclined position. The nearer perpendicular the tube is
held, the stronger is the draught of air that passes through it. If
but little heat is required in the open tube operation, the
spirit-lamp is the best method of applying the heat. But if a greater
temperature is required, then recourse must be had to the blowpipe.
Upon the angle of inclination of the tube depends the amount of air
that passes through it, and therefore, the rapidity of the draught
may be easily regulated at the will of the operator. The inclination
of the tube may, as a general rule, be about the angle represented in
Fig. 14.

[Illustration Fig. 14.]

The length of the tube must be about six inches, so that the portion
upon which the substance rested in a previous examination may be cut
off. The portion of the tube left will answer for several similar
operations.

When the substance is under examination, we should dev