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Page 35
_Selenium and Tellurium._--Both these bodies are readily and
completely reduced by the current either in acid or alkaline
solutions. Selenium is thrown down at first of a fine brownish red,
which gradually becomes darker. The deposit of tellurium is of a
bluish black color. If the current is feeble, the deposit of selenium
is moderately compact; that of tellurium is always loose, and it often
floats on the liquid. A strong current precipitates both as powders.
The positive pole is coated during electrolysis with a film of a dark
color in case of selenium, but of a lemon yellow with tellurium. As in
case of arsenic and antimony, the hydrogen evolved at the negative
pole combines with the reduced substances, forming hydrogen, selenide,
or telluride, which remain in part in solution in the liquid. The
reduced metal separates out at the anode in a friable
condition.--_Zeitschrift fur Analytische Chemie, and Chemical News._
* * * * *
THE ELECTRO-CHEMICAL EQUIVALENT OF SILVER.
A very careful and important determination of the electrochemical
equivalent of silver has been made at the observatory of the Physical
Institute of W�rzbourg, and the results are that an ampere current
flowing for a second, or a coulomb of electricity deposits 1.1183
milligrammes of silver or 0.3281 milligramme of copper, and decomposes
0.09328 milligramme of water, a result agreeing closely with that of
Lord Rayleigh recently communicated to the Physical Society. An ampere
therefore deposits 4.0259 grammes of silver per hour; Kohlrausch's
value is 4.0824, a value hitherto accepted universally. This value is
so useful in measuring electric currents with accuracy, and free from
the disturbances of magnetism, etc., that it is eminently satisfactory
to find the German value agree with that of Lord Rayleigh, which will
probably be adopted by English electricians.
* * * * *
A NEW STANDARD LIGHT.
Herr Hefner-Alteneck has suggested a new standard light for
photometric purposes, which promises to be very simple and effective
in operation. The light is produced by an open flame of amyl-acetate
burning from a wick of cotton fiber which fills a tube of German
silver 1 in. long and 316 mils. internal diameter; the external
diameter being 324 mils. The flame is 1.58 in. high from top to
bottom; and it should be lighted at least ten minutes before using the
light for testing. A cylindrical glass chimney surrounds it to ward
off air currents. About 2 per cent. of the light is absorbed by the
glass. The power of the flame is that of a standard English candle;
and experiments have shown that amyl acetate, which besides is not
expensive, is the best fuel for steadiness and brilliance. Neither the
substitution of commercial amyl-acetate for pure nor the use of a wick
of cotton thread for loose cotton fiber alters the illuminating power;
but the wick should be trimmed square across the mouth of the tube,
for if it project and droop the illuminating power is increased.
* * * * *
[NATURE.]
DR. FEUSSNER'S NEW POLARIZING PRISM.
In a recent number of the _Zeitschrift fur Instrumentenkunde_ (iv.,
42-50, February, 1884), Dr. K. Feussner of Karlsruhe has given a
detailed description of a polarizing prism lately devised by him,
which presents several points of novelty, and for which certain
advantages are claimed. The paper also contains an account, although
not an exhaustive one, of the various polarizing prisms which have
from time to time been constructed by means of different combinations
of Iceland spar. The literature of this subject is scattered and
somewhat difficult of access, and moreover only a small part of it has
hitherto been translated into English; and it would appear therefore
that a brief abstract of the paper may not be without service to those
among the readers of _Nature_ who may be unacquainted with the
original memoirs, or who may not have the necessary references at
hand.
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