886 PEOEESSOE BimSEN AXD DE. H. E. EOSCOE’S PHOTO-CHEMICAX EESEAECHE3. 
In like manner it is found that when the carbonic oxide flame is removed to a distance 
of 0“-3349, and the coal-gas flame to a distance of 0“-4309, the surface is chemically 
illuminated by 5 units of light. 
2. The chemical illuminating power, or the chemical intensity of various soui-ces of 
light which may be considered as luminous points, is measured by the chemical action 
effected by the rays emanating from such sources in equal times and at equal distances. 
As, however, in the case of such sources of light, the chemical illuminating power is 
directly proportional to the square of the distances at which an equal amount of chemical 
illumination is obtained, and as the standard flame produces an illumination of one unit 
of light at a distance of 1 metre, we have, in order to compare the chemical iHummating 
power of any luminous source of the land described, with the unit of chemical light of 
the standard flame, only to take the square of the distance at which the luminous som-ce 
in question produces the illumination of one unit of light. If we carry out this calcu- 
lation for the three flames already mentioned, we find that the chemical intensities of 
the standard flame, the carbonic oxide flame of the above dimensions, and the coal-gas 
flame, are respectively represented by the numbers 
1, 0-718, and 1-972. 
It is of some interest to compare the chemical with the visual* illuminating power of 
these different flames. For this purpose we have compared the risual illumination 
produced by the carbonic oxide flame burnmg under the prescribed conditions, and 
issuing at the rate of 6-032 cub. cent, per second, -with that of the flame of coal-gas 
above described. Although these photometric measurements could not be made very 
exactly, owmg to the different colour of the two flames, we were nevertheless able to 
assure ourselves that a coal-gas flame, burning at the rate of 4-105 per second, produced 
a visual illumination of at least 160 times as great an intensity as the carbonic oxide 
flame, burning at the rate of 6-032 cub. cent. 
The chemical illuminating powers of flames of carbonic oxide issuing at the rate of 
6 cub. cent, 6-032 cub. cent., and 4-105 cub. cent, in the second, are found from 
formula (2.) to be in the relation of 21-34,28-28, and 15-32. As in one and the same 
source of light the chemical and visual intensities are proportional, the visual intensities 
of these three flames are to the visual intensity of the coal-gas flame, issuing at the rate 
of 4-105 cub. cent, as 21-34, 28-28, and 15-32, to 42-42. Hence we have 
Visual Chemical 
intensity. intensity. 
Elaine of carbonic oxide issuing at rate of 5 cub. cent, per second .... 1-000 I'OOO 
Elame of carbonic oxide issuing at rate of 4-105 cub. cent, per second . . 0-718 0*718 
Elaine of coal-gas issuing at rate of 4-105 cub. cent, per second . . 198-800 1-972 
From this we see that the visual illuminating power of the above coal-gas flame is 
nearly 200 times as large as that of the normal carbonic oxide flame, whereas the che- 
mical illuminating power of the same is not double that of the normal flame ; and hence 
* By “visual” illuminating power we intend to signify the luminous intensity as measui-ed by its effect 
on tbe eye. 
