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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° 22', 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 chemically active rays for photo- 
graphic purposes becomes 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, render this source of light valuable as a 
simple means of obtaining a given amount of illumination 
expressed in terms of our measurement of light 
The combustion of magnesium constitutes so definite and 
simple a source of light for the purpose of photochemical 
measurement, that the wide distribution of this metal becomes 
desirable. The application 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, accord- 
ing to a measurement we have made, as much light as 74 
stearine candles of which 5 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 burning for 10 hours, whereby about 201bs. of 
stearine is consumed, 72*2 grrns. of magnesium would be 
required. The magnesium wire 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 clockwork, and 
thus the end of the wire, guided by passing through a groove 
