PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 013 
The photometric observations with these columns of chlorine, all of which were 
expanded to a height of 77’6 millims. by dilution with air, gave — 
Series of Experiments X. 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
Exp. 5. 
Exp. 6. 
lo 
8-45 
10-93 
11-13 
11-10 
11-39 
, 10-65 
I 
2-53 
3-35 
4-39 
4-15 
5-16 
5-51 
h 
77-6 
77-6 
56-1 
56-1 
43-6 
32-2 
The values of calculated from these experiments, are — - 
No. of 
Expt. 
1. 179 millims. 
2. 184 millims. 
3. 179 millims. 
4. 167 millims. 
5. 172 millims. 
6. 165 millims. 
Mean . . . .174-3 millims. 
This mean value agrees very closely with that found from the experiments with chlorine 
undiluted with air, but it is seen that the variation in the separate values is not incon- 
siderable. The cause of this lies not only in the unavoidable errors of experiment, but 
more especially in the fact that the formula used in the calculation is, strictly speaking, 
only applicable to homogeneous light, whilst the chemically active rays emanating from 
our coal-gas flame w^ere certainly not all of the same degree of refrangibility. 
If the intensity Iq of the light, measured by our instrument, were composed of the 
components • • of various degrees of refrangibility, the formula I = IolO“*“ would 
become I = .. 
The values calculated according, to the first formula can therefore only give a mean value 
for the various chemical rays in our flame, and the numbers obtained must vary more 
considerably amongst themselves than if homogeneous light had been employed. The 
mean of all the determinations in Series VIII. and X. gives the value of the optical 
coefficient of extinction of chlorine at 0° C. and 0-76 pressure, for rays from a coal-gas 
flame, to be j y^.Q =Q'Q0578. 
If the light is not consumed in the act of photo-chemical change, the coefficient just 
found must remain unaltered when the chlorine and hydrogen mixture is employed ; 
but if, on the contrary, light is not only lost by the optical extinction, but also an 
amount of light proportional to the chemical action vanishes, experiment must give a 
larger value for the coefficient. In the first case the light merely liberates the chemical 
MDCCCLVII. 4 L 
