The Sun.
London, England, 1864-02-23 00:00:00

Interesting Photo-Chemical Discoveries,

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ferary
At a recent meeting of the Mane Lester _
and Philosophical Society, Professor Roscoe exhi-
bited the light emitted by burning a portion of a
fine spcinsen of pure magnesium wire Imm. in
diameter and 10ft. long, whieh had been manufac-
tured by Mr. Sonstadt. Professor Bunsen and the
speaker had examined the photo-chemical action of
the sun compared with that of a terrestrial source
of light, and for the purpose of this comparison they
chose the light evolved by the combustion of mag-
nesium wire. They showed that a burning surface
of magnesium wire which, seen from a point at the
sea's level, has an apparent magnitude equal to
that of the sun, effects on that point the same
chemical action as the sun would do when shining
from a cloudless sky at a height of 9 deg. 53 min.
above the horizon; On comparing the chemi-
cal with the visible brightness of these two
sources of light, it was found that the brightness of
the sun's disc, as measured by the eye when the
sun's zenith distance was 67 deg. 22 min. is 524'7
times as great as that of the burning magnesium
wire, whilst at the same zenith-distance the chemical
brightness of the sun is only 36 6 times as great.
Hence the value of this light as a source of the che-
mically active rays for photographic purposes be-
comes at once apparent. The extract from the
memoir referred to is as follows :—" The steady and
equable light evolved by magnesium wire burning
in the air, and the immense chemical action thus
produced, rendered this source of light valuable as
a simple means of obtaining a given amount of illu-
mination expressed in terms of our measurement of
light. * * The combustion of magnesium constitutes
so definite end simple a source of light for the purpose
of photochemical measurement, that the wide distri-
bution of this metal becomes desirable". The appli-
cation of this metal as a source of light may even
become of technical importance. A burning mag-
nesium wire of the thickness 0.297 millimetre
evolves, according to a measurement we have made,
es much light as 7 stearine candles, of which five go
to the pound. If this light lasted one minute, 0'987
metre of wire, weighing 0.1204 grm. would be burnt.
In order to produce a light equal to 74 candles burn-
ing for ten hours, whereby about 201 b of stearine is
consumed, 72.2 gems. of magnesium would be re-
quired. The magnesium wire can be easily pre-
pared by forcing out the metal from a heated steel
press having a fine opening at bottom ; this wire
might be rolled up in coils on a spindle,which could
be made to revolve by clockwork, and thus the
end of the wire, guided by passing through a
groove or between rollers, could be continually
pushed forward into a gas or spirit-lamp flame,
in which it would burn." Professor Roscoe stated
that great credit was due to Mr. Sondstadt for the
able manner in which he had brought the difficult
subject of the metallurgy of magnesium into the
present satisfactory position, and expressed his
opinion that, even for photographic purposes, the
application of the metal will prove most important.
Mr. Brothers, Mr. Parry, and other photographers
present, corroborated Dr. Roscoe's opinion respect-
ing the value of such a source of light for photo-
graphy. Since the meeting Mr. Brothers made an
experiment upon the magnesium light, which be
reports as follows :—" The result of an experiment
I have just tried is that in 50 seconds with the
magnesium light I have obtained a good negative
copy of an engraving, the copy being made in a
darkened room. Another copy was made in the
usual way in daylight, and in 50 seconds the result
was about equal to the negative taken by the arti-
ficial light. The sun was shining, but there was a
good deal of fog in the atmosphere."