Phosphorite

3721

Photography

and as a stage in the preparation of other
compounds of phosphorus.

Phosphorite, an impure massive form of
apatite, mined for the calcium phosphate it
contains, and forming one of the chief
sources of phosphorus and its compounds,
and of artificial manures.

Phosphorous Acid, HnPOc, is formed by
acting on phosphorous trichloride with wa-
ter or hydrochloric acid, and can be obtained
crystalline by evaporating the solution. It
acts as a powerful reducing agent.

Phosphorus, P, 31, an element, which,
though never found free in nature, is widely
distributed in combination. Thus, it is an
essential ingredient of the protoplasm of all
living cells, and is present in well-marked
amounts in nervous tissue and bones. Phos-
phorus is also widely distributed in min-
erals. Ordinary phosphorus is a waxy solid
that is colorless when pure, and forms bril-
liant, highly refractive crystals when sub-
limed in a vacuum, though usually it is
more or less colored pale yellow or buff from
the action of light or the presence of im-
purities. Phosphorus is most marked chem-
ically by the readiness with which it is oxi-
dized: thus, it glows and gives off fumes
of a garlic odor when exposed to the air,
and the action which thus takes place often
warms it up sufficiently to cause inflamma-
tion, which takes place a few degrees above
its melting point, and with exceptional readi-
ness if the phosphorus is in the finely-divided
state obtained by evaporating its solution in
carbon disulphide. When set on fire in the
open air, or oxygen, it burns with a dazzling
white light. Ordinary phosohorus is very
poisonous, even small doses causing gastro-
intestinal irritation; and though this may
pass off, it is followed by fatty degeneration
and internal haemorrhage, which is usually
fatal. Burns caused by phosphorus are very
troublesome to heal. Ordinary phosphorus
is mainly employed in match manufacture.
Other uses of common phosphorus are for
the manufacture of vermin-killer, the pre-
paration of phosphor-bronze, to a small ex-
tent in medicine, and in the preparation of
organic compounds.

Photius (c. 820—c. 891),patriarch of Con-
stantinople. Photius, through a council held
at Constantinople, effected the temporary
withdrawal of the Eastern from the Western
Church.

Photochemistry deals with those chemical
changes that are brought about or acceler-

ated by the action of light—e. g. those that
determine the growth of green plants, are
instrumental in the act of vision, in the
bleaching of colors, and are 'the basis of the
various photographic processes. Most photo-
chemical actions are primarily decomposi-
tions: thus, with chlorine and hydrogen
mixtures, from experiments on which most
of the exact photochemical data are derived
the first step in the action is probably the
disintegration of the chlorine and hydrogen
molecules into atoms, which are then free
to combine to form hydrogen chloride. With
elements such as yellow phosphorus, which
is converted to the red variety, and in the
formation of the latent photographic image,
the nature of the action is not so clear, but
probably indicates a molecular rearrange-
ment. The presence of water and oxygen,
and the formation of hydrogen peroxide (the
production of which in sunlight has been
shown to hinder putrefaction, and has prob-
ably much to do with bleaching), have very
important bearings on the chemical action
of light.

Photoengraving. See Process Work.

Photography is the art of preparing per-
manent representations of objects by means
of the light they emit or transmit. The first
photographs produced in the camera were
made by Daguerre and Niepce (c. 1839),
who sensitized a polished silver plate with
the fumes of iodine, exposed it in the cam-
era, developed it by means of mercury vapor,
and fixed the resulting image by dissolving
the unacted-on iodide with potassium cyan-
ide. The next advance was made hi 1841, by
Fox Talbot, who invented the 'calotype5
process. In 1864 Bolton and Sayc'e introduced
collodion emulsion. A still further advance
was made by Bolton in 1874, when he intro-
duced a washed collodion emulsion. This ad-
vance not only gave much higher speed, but
the raw emulsion and plates coated there-
with had much greater keeping power and it
did much to popularize photography. The
gelatino-bromide dry plate was invented in
1871 by Maddox, and greatly improved in
1878 by Bennett. The camera in which the
image is impressed on the sensitive surface is
a light-tight box in which the plate is fixed
in such a position that an image of the ob-
ject to be photographed is projected on to
it by a lens or pinhole. Development is gen-
erally carried out in a 'dark room* lighted
by rays that do not appreciably affect the
plate—such as red light for ordinary plates—