PEOFESSOE BUTS'SEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 385 
(which we shall fully describe in the following communication), by means of which 
columns of sensitive gas of various lengths could be exposed to the action of a constant 
source of light. In the first experiment the depth of the insolated column of gas was 
154’0 miUims., and in the second 81’0 millims. 
The first vertical column of the following Table gives the times of observation from 
the beginning of the insolation, the second and third contain the corresponding chemical 
action for each depth of gas, measured by the quantity of hydrochloric acid formed every 
thu’ty seconds. 
Series of Experiments III. 
Time in minutes. 
Exp . 1. 
Exp . 2. 
154’0 millims. 
Sl’O millims. 
0-0 
0-0 
0*0 
0-5 
0-9 
1-0 
1-0 
1-8 
2-4 
1-5 
3-9 
5-1 
2-0 
5-5 
6-7 
2-5 
6-4 
8-2* 
3-0 
7-7 
8-0 
3-5 
8-8 
8-2 
4-0 
? 
8-6 
4-5 
? 
8-0 
5-0 
lO - l * 
8-2 
5*5 
10-3 
8-6 
6-0 
11-4 
8-0 
6-5 
10*4 
7-0 
11-2 
7-5 
10-5 
Mean action 
10-7 
8-2 
The mean maximum action in the long cylinder was 10*7, in the shorter 8’2. In the 
longer column the induction was completed in four and a half minutes, in the column 
which was only half as long, in two minutes. It is also seen that the induction increases 
more rapidly in the smaller than in the larger volume of gas. Hence we may conclude 
that the photo-chemical induction is retarded when, other circumstances remaining the 
same, the mass of insolated gas is increased. The two curves, Plate XIX. fig. I., clearly 
show this relation ; the abscissae denote the times of observation, and the ordinates the 
corresponding action measured by the amount of hydrochloric acid formed in thirty 
seconds. Curvn No. I represents the increased action of the column 154 millims., and 
curve No. 2 of column 81 millims. 
The decrease of the rate of induction with the mass of the gas appears at first sight 
inexplicable. In the first experiment the insolation was conducted under exactly the 
same conditions, in cylinders of unequal length, hut of equal diameter. The uppermost 
layer of gas in both cylinders was therefore exposed to the light under precisely the same 
circumstances, but in the larger cylinder a second layer of gas existed below, which was 
not the case in the smaller cylinder, and still the smaller cylinder gave a greater amount 
of action than the larger. Series of Experiments II. A., Exp. 1, showed that the amount 
MDCCCLVII. 3 E 
