hing the plates with the fingers after they have been
cleaned. Sometimes, however, they are due to chemical defects in the
glass itself. In these cases, as a general thing, the discolorations
occur only after several days--a faultless mirror having been made at
first, and the browning subsequently developing slowly. The writer was
a student in the laboratory of Baron Liebig during the time that
distinguished chemist was carrying out the series of experiments which
resulted in devising a method of making silver mirrors commercially.
One of the greatest troubles with which he had to contend was this
browning--the cause for which was never fully cleared up by him. Some
years ago, the writer, having in his possession two mirrors made by
Liebig, and which had gradually become brown throughout, undertook an
examination of the deposit (which had been thoroughly protected from
extraneous influences by a strong film of varnish), and was surprised
to find that it consisted of a layer of silver sulphide. Without going
into detail, the source of the change was later found to lie within
the glass itself. In making glass to be used for mirrors, a
considerable portion of sodium sulphate is used, and in annealing,
this is partly reduced to sodium sulphide, which effloresces on the
surface of the glass. This efflorescence is, of course, removed on
cleaning the glass before silvering; but it is found that, in many
instances, on exposure of the mirror to the light for some time, a
further efflorescence occurs, and it is this which produces the
discoloration in cases such as we have cited. It has been suggested
that the tendency to subsequent efflorescence may be corrected by
boiling the plates, intended for silvering, for a couple of minutes,
in a 10 per cent. solution of sodium carbonate or bicarbonate. We have
no experience with the process, however.--National Druggist.

* * * * *

WILD AND DOMESTIC SHEEP IN THE BERLIN ZOOLOGICAL GARDEN.

As a rule, domestic animals are accorded very little space in
zoological gardens, but, although it is doubtless the first duty of
these popular institutions to show visitors animals which live in a
wild state in foreign lands, it is well, where there is sufficient
space and adequate means, to extend the limits of the collection so as
to include natives of our own woods and fields, thus enabling people
of a great city who are unfamiliar with nature to form an ide