333 
A COLOURLESS VARNISH. 
of this light is exceedingly rich in violet and ultra-violet rays, due partly to the incande¬ 
scent vapour of magnesium, and partly to the intensely-heated magnesia formed by the 
combustion. Professor Bunsen and the speaker, in 1859, determined the chemically ac¬ 
tive power possessed by this light, and compared it with that of the sun ; and they sug¬ 
gested the application of this light for the purpose of photography. They shoived* 
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 if shining from a cloudless sky at a height of 9° 53 
above the horizon. On comparing 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, is 524*7 times 
as great as that of burning magnesium-wire when the sun’s zenith distance is G7° 22 '; 
whilst at the same zenith distance, the sun’s chemical brightness is only 3G*G times as 
great. Hence the value of this light as a source of the chemically active rays for pho¬ 
tographic purposes becomes at once apparent. 
Professor Bunsen and the speaker state in the memoir above referred to, that, “ the 
steady and equable light evolved by magnesium wire, burning in the air, and the im¬ 
mense chemical action thus produced, render this source of light valuable as a simple 
means of obtaining a given amount of chemical illumination, and that the combustion 
of this metal constitutes so definite and simple a source of light for the purpose of 
photo-chemical measurement, that the wide distribution of magnesium becomes desira¬ 
ble. The application of this metal as a source of light may even become of technical 
importance. A burning magnesium-wire of the thickness of 0*297 millimetre, evolves, 
according to the measurement we have made, as much light as 74 stearine candles of 
which five go to the pound. If this light lasted one minute, 0*987 metre of wire, weigh¬ 
ing 0*120 grammes, would be burnt. In order to produce a light equal to 74 candles 
burning for ten hours, whereby about 20 lb. of stearine are consumed, 72*2 grammes (2^ 
ounces) of magnesium would be required. The magnesium w r ire can be easily prepared 
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 clock¬ 
work ; 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.” 
It afforded the speaker great pleasure to state that the foregoing suggestion had now 
been actually carried out. Mr. Edward Sonstadt has succeeded in preparing magnesium 
on the large scale, and great credit is due to this gentleman for the able manner in 
which he has brought the difficult subject of the metallurgy of magnesium to its present 
very satisfactory position. 
Some fine specimens of crude and distilled magnesium, weighing 3 lbs., were ex¬ 
hibited as manufactured by Mr. Sonstadt’s process, by Messrs. Mellor and Co., of Man¬ 
chester. 
The wire is now to be had at the comparatively low rate of 3c?. per foot ;f and half 
an inch of the wire evolves, on burning, light enough to transfer a positive image to a 
dry collodion plate; whilst, by the combustion of 10 grains, a perfect photographic por¬ 
trait may be taken, so that the speaker believed that for photographic purposes alone the 
magnesium light will prove most important. The photo-chemical power of the light 
was illustrated by taking a portrait during the discourse. In doing this the speaker was 
aided by Mr. Brothers, photographer, of Manchester, who was the first to use the light 
for portraiture. 
H. E. E. 
A COLOUKLESS TARNISH. 
At the time the process of varnish-making by Lulling was laid before the Society of 
Arts, Mr. Eield put in a claim, when both the processes and products were found to 
answer the intended purpose, and the claimants were awarded twenty guineas each. 
Mr. Field describes his process as follows:—Six ounces of shellac, coarsely powdered. 
* Phil. Trans. 1859, p. 920. 
f From Messrs. Johnson and Matthey, of Hatton Garden. 
