PROFESSOE BENSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 375 
for photo-chemical measurements. These variations of the flame can be best prevented 
by allowing the gas to issue from a small air-chamber, and by surrounding the flame 
with a screen so that all lateral currents of air are completely excluded. 
We have found that by means of the arrangement represented in Plate XVIII. flg. 5, a 
flame of coal gas could be obtained of extremely constant dimensions and luminous power. 
xV-V, flg. 5, is a blackened tin box in the interior of which the gas-flame I burns. The 
bottom of the box is perforated to admit of an upward current of air, and the sliding 
front of the box is furnished with a screen of water, «, contained between two parallel 
plates of glass. The gas streams from the dehvery-tube b into the small air-chamber c, 
and issues from the platinum burner with a pressure of about 1 millim. of water, as 
is seen by the small manometer g attached to the air-chamber. The dimensions of 
the flame are kept constant by regulating the supply of gas, so that the highest point of 
the flame always touches the thin platinum wire attached to a moveable ring fitting on 
to the dfrided tube A, upon which the height of the flame can be read off. In order to 
place the flame at any wished-for distance from the insolation-vessel, the tube h carrying 
the gas-burner can be moved through a holder, k, at the back of the box, and the 
distance read off on an etched millimetre scale. When the distance of the insolation- 
vessel from the flame has been once measured for a known position of the flame, a simple 
observation on the divided tube b gives the required distance. When the box was in 
use the top was covered on each side of the flame, leaving only a space of about two 
inches open. The dimensions of the flame burning in the box thus adjusted, when 
observed through the opening of the screen, /, flg. 4, were seen to remain perfectly con- 
stant, so much so that the light appeared to be emitted from a solid body. 
We have employed this flame, which for the sake of brevity we shall call the standard 
flame, in the first place for the piu’pose of determining the constancy of the indications 
of the instrument at different times and under various chcumstances. For this purpose, 
the chemical intensity of the hght proceeding fr’om standard gas-flames of the same 
heights was repeatedly measured at various intervals during sixteen days in the month of 
June 1856. In all these measurements, each one of which is the mean of ten inde- 
pendent observations, the height of the standard flame was 42 '2 millims., and the 
distance between the flame and insolation-vessel was 216’0 millims. 
Series of Experiments X. 
Time of the ob- 
servations. 
Action in 
one minute. 
Deviation from 
mean value. 
11th June ... 
14-00 
-fO-01 
12th „ ... 
14-26 
-1-0-35 
13th „ ... 
13-80 
-0-11 
19th „ ... 
13-83 
-0-08 
21st „ 
13-88 
— 0-03 
26th „ ... 
13-71 
-0-19 
Mean value ... 
13-91 
