PEOrESSOE BIJNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 885 
distance of 1 metre ; and we determine experimentally for each instrument the number 
of such units (n) which correspond to one division on the observation-tube. By multi- 
plying the observed number of divisions (f) by the number of photometric units (n) 
equal to one division, the observations are reduced to a comparable standard, whose unit 
is taken to be the amount of action which a standard flame at the distance of one 
metre efiects on the normal mixture of chlorine and hydrogen in one minute, when the 
depth of insolated gas is so small that the alteration of the value of the coeflicient of 
absorption for difierent sources of light becomes inappreciable. 
JVis coXl this unit ci chcmicoX unit oj^ light, and ten thousand of thoso units, ons chofnical 
degree of light. 
In the following experiments we have always employed the same insolation-vessel, but 
two obser\ation-tubes. On one of the observation-tubes, or scales, which we shall 
henceforward caU “ the narrow,” or “No. 1 scale,” one division corresponded to 0*6612 ; 
on the wide, or “No. 2. scale,” one division corresponded to ^^=2•249. One division on 
the narrow scale had a capacity of 0*7642 cubic millimetres; one division on the wide 
scale contained 2*598 cubic millimetres. 
A few examples will best explain the use of this measure of light. 
1. By Chemical Illumination we signify that amount of chemically active light which 
falls perpendicularly on a plane surface. If the insolation-vessel of our instrument be 
supposed to form a part of this plane, the number of divisions on the scale which the 
index has passed in one minute, multiplied by n, expresses the number of units of light 
to which that plane is illuminated. As an example of this kind, we will determine, for 
tw*o difierent sources of light, the distance at which they must be placed, in order that a 
giien surface may be illuminated with one degree of light. We chose as the sources of 
light flames of carbonic oxide and coal-gas, which were each fed with gas at the rate of 
4 105 cub. cent., at 0 and 0'“*76 per second. The carbonic oxide issued fro m the large 
platinum burner, the coal-gas from an ordinary burner, and both gases were expelled 
from the burners under a pressure but very slightly exceeding that of the atmosphere. 
The rays from both flames had to pass through two plates of mica and a water-screen ; the 
narrow scale. No. 1, was employed in these experiments. The carbonic oxide flame, placed 
at a distance of 0 1/6 from the insolation-vessel, produced an action of 7*68 divisions in 
one minute. This corresponds to Kw. 7*68 = 10*15 degrees of light. As the amount of 
illumination is inversely proportional to the square of the distance from the source of 
light to the illuminated surface, the distance {r) at which the source of light must be 
placed in order that the surface may be illuminated one degree, is found to be 
r=x/0*176M0*15=0*5607 metre. 
The flame of coal-gas, burnt under the same conditions, gave at a distance of 0™‘216 
from the insolation-vessel an action of 13*98 divisions in the minute, corresponding to 
K.w. 13*98 = 18*48 degrees of light. This flame accordingly efiects a chemical illumi- 
nation of one degree at a distance equal to 
r=x/fi2l^T8T8=0*9287 metre. 
6 A 
MDCCCLIX. 
