364 PEOFESSOE BimSEN AIST) DE. H. E. EOSCOE’S PHOTO-CHENnCAE PJISEAECEES. 
Series o£ Experiments IV. 
Time. 
Corrected 
observation. 
0 
102-1 
1 
104-2 
2 
107-3 
3 
113-5 
4 
125-0 
5 
135-4 
6 
145-9 
7 
159-3 
8 
172-5 
9 
185-6 
10 
198-8 
11 
211-9 
12 
226-1 
13 
237-2 
14 
251-4 
15 
266-1 
Action of the light 
in one minute. 
!Mean action from 
three observations. 
2-1 
3-1 
6-2 
11-5 
10-4 
10-5 
13-4d 
' 13-2 '> 
13-23 
13-1 
13-2'' 
13-1 
13-50 
14-2 
11-n 
14-2 
14-7J 
13-35 

The mean of the three values in the fourth column is 13‘36, ghing the following 
differences : — 
Pound. 
Mean. 
Difference. 
13-23 . . 
. 13-36 . 
. . •FO-13 
13-50 . . 
. 13-36 . 
. . -0-14 
13-35 . . 
. 13-36 . 
. . 4-0-01 
From these numbers, it is seen that by the combination of such a series of obseiTations. 
the small irregularities, arising chiefly from capillary resistance in the tube ss, are satis- 
factorily eliminated. 
In order to ascertain the limits of accuracy to which the indications of our instru- 
ment reached, it was necessary to determine, first, the length of time diu’ing which the 
electrolysis of a given volume of hydrochloric acid yields a gas of uniform composition ; 
and secondly, the circumstances which influence the establishment of the statical equih- 
brium between the free and absorbed gases in the liquids contained in the apparatus. 
For this pm’pose we required a source of light nEich should remain constant for a con- 
siderable length of time ; this we obtained by employing a coal-gas flame of constant 
dimensions issuing from Scott’s annular burner. A millimetre scale was etched upon 
the glass chimney of the lamp by hydrofluoric acid, and in order to avoid the errors 
arising from parallax, the height of the flame was read off on the scale through a tele- 
scope placed at the same height. 
The amount of the chemical rays emitted by any flame depends upon its dimensions 
and its luminous intensity. We eliminated all sources of error arising from -variations in 
the dimensions of the flame, by cutting off the greater part of the light nith a screen, 
and thus employing only a small portion of the most intensely luminous rays as the 
source of light. In order that the intensity of the flame should remain as much as 
possible constant during our experiments, we determined the height of the flame of the 
