known, the principal one being red. If heated between 230 degrees
and 260 degrees, away from air, the yellow variety changes to
red, which can be kept at all temperatures below 260 degrees.
Above that it changes back. Red P is not poisonous, ignites only
at a high temperature, and is not phosphorescent, like the
yellow. 206. Spontaneous Combustion of Phosphene, or Hydrogen
Phosphide, PH3.

Experiment 114.--Put into a 20cc.flask 1 g. P and 50cc.saturated
solution NaOH or KOH. Connect with the p.t. by a long d.t., as in
Figure 44, the end of which must be kept under water. Pour 3 or
4cc.of ether into the flask, to drive out the air. It is
necessary to exclude all air, as a dangerously explosive mixture
is formed with it. Heat the mixture, and as the gas passes over
and into the air, it takes fire spontaneously, and rings of smoke
successively rise. It will do no harm if, on taking away the
lamp, the water is drawn back into the flask; but in that case
the flask should be slightly lifted to prevent breakage by the
sudden rush of water. On no account let the air be drawn over.

The experiment has no practical value, but is an interesting
illustration of the spontaneous combustion of PH3 and of vortex
rings. What are the products of the combustion? An admixture of
another compound of P and H causes the combustion.

Chapter XL.

ARSENIC.

Examine metallic arsenic, realgar, orpiment, arsenopyrite,
arsenic trioxide, copper arsenite.

The compounds of arsenic are very poisonous if taken into the
system, and must be handled with care.

207. Separation. Experiment 115.--Draw out into two parts in the
Bunsen flame a piece of glass tubing 20cm long and 1 or 2cm in
diameter. Into the end of one of the ignition tubes thus formed,
when it is cool, put one-fourth of a gram of arsenic trioxide,
As2O3, using paper to transfer it. Now put into the tube a piece
of charcoal, and press it down to within 2 or 3cm of the AS2O3
(Fig. 45). Next heat the coal red-hot, and then at once heat the
As203. Continue this process till you see a metallic sublimate-
metallic mirror-on the tube above the coal. Break the tube and
examine the sublimate. It is As. Heat vaporizes the As2O;3.
Explain the chemical action. What is the agency of C in the
experiment? Of As2O3?  2 As2O3 + 3 C = ?

208. Tests.-Experiments 115 and 116 are used as tests for the
presence of arsenic.

Experiment 116.--Prepare a H generator, - a flask with a thistle-
tube and a philosopher's lamp tube (Fig. 46), put in some
granulated Zn, water, and HCl. Test the purity of the escaping
gas (Experiment 23), and when pure, light the jet of H. H is now
burning in air. To be sure that there is no As in the ingredients
used, hold the inside of a porcelain evaporating-dish directly
against the flame for a minute. If no silvery-white mirror is
found, the chemicals are free from As. Then pour through the
thistle-tube, while the lamp is still burning, 1cc.solution of
AS2O3 in HCl or H2O a bit of As2O3 not larger than a grain of
wheat in 10 cc. HCl.

See whether the color of the flame changes; then hold the
evaporating-dish once more in the flame, and notice a metallic
deposit of As. Set away the apparatus under the hood and leave
the light burning.

This experiment must not be performed unless all the cautions are
observed, since the gas in the flask (AsH3) is the most poisonous
known, and a single bubble of it inhaled is said to have killed
the discoverer. By confining the gas inside the flask there is no
danger.

Instead of using As2O3 solution, a little Paris green, wall paper
suspected of containing arsenic, green silk, or green paper
labels, etc., may be soaked in HCl, and tested.

209. Explanation.--The chemical changes are as follows: The
compounds of As, in this case As2O3, in presence of nascent H,
are immediately converted into the deadly hydrogen arsenide
(arsine, arseniuretted hydrogen), AsH3. As2O3 + 12 H = 2 AsH3 + 3
H2O. The AsH3 mixed with excess of H tends to escape and is
burned to As2O3 and H2O, and thus is rendered comparatively
harmless as it passes into the air. This is why the flame must be
burning when the arsenic compound is introduced. 2 AsH3 + 6 O =
As2O3 + 3 H2O.

In the combustion of AsH3, H burns at a lower point than As. The
introduction of a cold body like porcelain cools the flame below
the kindling-point of As, and this is deposited, while H burns,
in exactly the same way as lamp- black was collected in
Experiment 26.

210. Expert Analysis.--A modification of this experiment is
employed by experts to test for AS2O3 poisoning. The organs.--
stomach or liver--are cut into small pieces dissolved by nascent
Cl, or HClO, made from KC1O3 and HCl, and the solution is
introduced into a H generator, as above. AS2O3 preserves the
tissues it comes in contact with, for a long time, and the test
can be made years after death. All the chemicals must be pure,
since As is found in small quantities in most ores, and the Zn,
HCl, and H2SO4 of commerce are very likely to contain it. The
above is called Marsh's test, and is so delicate that a mere
trace of arsenic can be detected.

211. Properties and Occurrence.--As is a grayish white solid, of
metallic luster, while a few of its characters are non-metallic.
It is very widely distributed, being sometimes found native, and
sometimes combined, as AsS, realgar, As2S8, orpiment, and FeAsS,
arsenopyrite. Its chief source is the last, the fine powder of
which is strongly heated, when As separates and sublimes. It has
the odor of garlic, as may be observed by heating a little on
charcoal with the blow-pipe.

212. Atomic Volume.--As is peculiar in that its atomic volume, so
far as the volume can be determined, is only half that of the H
atom. Its vapor density is 150, which gives 300 for the molecular
weight, while its least combining or atomic weight is 75. 300,
the molecular weight = 75, the atomic weight =4, the number of
atoms in the molecule. All gaseous molecules being of the same
size, represented by two squares, the atomic volume of As must be
one-fourth of this size, represented by half of one square. Of
what other element is this true? 213. Uses of As2O3.-Arsenic is
used in shot-manufacture, for hardening the metal. Its most
important compound is As2O3, arsenic trioxide, called also
arsenious anhydride, arsenious acid, white arsenic, etc. So
poisonous is this that enough could be piled on a one-cent piece
to kill a dozen persons. Taken in too large quantities it acts as
an emetic. The antidote is ferric hydrate Fe2(OH)6 and a mustard
emetic, followed by oil or milk.

The vapor density of this compound shows that its symbol should
be As4O6, but the improper one, As2O3, is likely to remain in
use. Another oxide, As2O5, arsenic pentoxide, exists, but is less
important. Show how the respective acid formulae are obtained
from these anhydrides. See page 50.

AS2O3 is used in making Paris green; in many green coloring
materials, in which it exists as copper arsenite; in coloring
wall papers, and in fly and rat poisons. It is employed for
preserving skins, etc. Fashionable women sometimes eat it for the
purpose of beautifying the complexion, to which it imparts a
ghastly white, unhealthy hue. Mountaineers in some parts of
Europe eat it for the greater power of endurance which it is
supposed to give them. By beginning with small doses these
arsenic-eaters finally consume a considerable quantity of the
poison with apparent impunity; but as soon as the habit is
stopped, all the pangs of arsenic-poisoning set in. Wall paper
containing arsenic is said to be injurious to some people, while
apparently harmless to others.

Chapter XLI.

SILICON, SILICA, AND SILICATES.

214. Comparison of Si and C.--The element Si resembles carbon in
valence and in allotropic forms. It occurs in three forms like C,
a diamond form, a graphite, and an amorphous. C forms the basis
of the vegetable and animal world; Si, of the mineral. Most soils
and rocks, except limestone, are mainly compounds of O, Si, and
metals. While O is estimated to make up nearly one- half of the
known crust of the earth, Si constitutes fully a third. The two
are usually combined, as silica, SiO2, or silicates, SiO2
combined with metallic oxides. This affinity for O is so strong
that Si is not found uncombined, and is separated with great
difficulty and only at the highest temperatures. No special use
has yet been found for it, except as an alloy with Al. Its
compounds are very important.

215 Silica.--Examine some specimens of quartz, rock crystal,
white and colored sands, agate, jasper, flint, etc.; test their
hardness with a knife blade, and see whether they will scratch
glass. Notice that quartz crystals are hexagonal or six-sided
prisms, terminated by hexagonal pyramids. The coloring matters
are impurities, often Fe and Mn, if red or brown. When pure,
quartz is transparent as glass, infusible except in the oxy-
hydrogen blow- pipe, and harder than glass. Rock crystal is
massive Si02. Sand is generally either silica or silicates.

The common variety of Si02 is not soluble in water or in acids,
except HF. An amorphous variety is to some extent soluble in
water. Most geysers deposit the latter in successive layers about
their mouths. Agate, chalcedony, and opal have probably an origin
similar to this. A solution of this variety of SiO2 forms a
jelly-like masscolloid--which will not diffuse through a membrane
of parchment -dialyzer--when suspended in water. Crystalloids
will diffuse through such a membrane, if they are in solution.
This principle forms the basis of dialysis.

All substances are supposed to be either crystalloids, i.e.
susceptible of crystallization, or colloids-jelly-like masses.
HCl is the most diffusible in liquids of all known substances;
caramel is one of the least so. To separate the two, they would
be put into a dialyzer suspended in water, when HCl will diffuse
through into the water, and caramel will remain. As2O3, in cases
of suspected poisoning, was formerly separated from the stomach
in this way, as it is a crystalloid, whereas most of the other

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