PEOFESSOE BTJNSEN AJTO DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 379 
and the pale flame produced by chloride of antimony are, on the contrary, so rich in che- 
mically active rays, that the action begins as soon as the saturated coke is placed on the 
tube ; and in one case it reached the amount of twenty-seven divisions in one minute. 
At the end of this Part we have to consider the important question regarding the 
influence which the changes in atmospheric temperature exert on the indications of the 
instrument. For this purpose we have conducted four series of observations on the 17th, 
18th, 19th, and 20th of September, 1856, in a room the temperature of which was kept 
constant within half a degree Celsius, during the filling and observation with the appa- 
ratus. The volume represented by one degree on the observation-tube was 3-4 times as 
large as in Series X. The distance between the flame, of the same height as in the former 
experiments, and the insolation-vessel was O^TSGb. In order to compare the results 
thus obtained with those of Series X., it was only necessary to multiply the observations 
with the factor ^ ^ ^ =0T358. 
u-2162 
The observed values are placed in the following Table. The first vertical column con- 
tains the constant temperatm’es at which the apparatus was filled and the observations 
made ; the second, the action produced by the constant standard flame every minute, 
expressed in degrees of the instrument (a mean of eight actual observations) ; the third, 
the mean of these actions, and the fom’th their differences. 
Series of Experiments XII. 
From the irregularity of the signs of the differences in the fourth column, we may con- 
clude that the indications of the instrument remain unaffected by changes of temperature 
between 18° C. and 27° C., any alteration which takes place falling within the limits of 
observ'ational errors. 
As the barometric pressure varied in the foregoing experiments from 0“-746 to 0“'760 
without the differences between the indications becoming perceptibly larger, the influ- 
ence of the changes in the atmospheric pressure can be overlooked, except in cases in 
which greater accuracy, as in the reduction to absolute measure, is required. In a later 
Part, when treating of the relation of the chemical action to the density of the chlo- 
3 D 2 
