PEOFESSOE BimSEN AM) DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
397 
Maxima. 
Difference from Mean. 
No. 1 . . 
. 28-0 
-fO-6 
No. 2 . . 
. 27-0 
-0-4 
No. 3 . . 
. 26-8 
-0-9 
No. 4 . . 
. 27-1 
-0-3 
No. 5 . . 
. 27-2 
-0-2 
No. 6 . . 
. 28-4 
+1-0 
No. 7 . . 
. 27-4 
0-0 
No. 8 . . 
. 27-5 
4-0-1 
No. 9 . . 
. 28-1 
-fO-7 
No. 10 . . 
. 26-3 
-1-1 
Mean. 
. 27-4 
The curves (Plate XXII. fig. VI.) clearly show the diminution of the resistance to com- 
bination on darkening. The abscissae denote the duration of the insolation, the ordinates 
the chemical action corresponding to these exposures, measured by the volume of hydi’o- 
chloric acid formed in one minute. The curves with odd numbers proceeding from the 
bottom upwards, represent the progression of the induction for gas rendered continually 
more pure by longer saturation ; those with even numbers show the increase of the induc- 
tion for the same gas, which was allowed to stand for some time in the dark. It is seen 
that the lower curves representing the action of the freshly prepared gas. Nos. 1, 3, 5, 7 
and 9, continually rise with increased purity of the gas, and approach the curves which 
show the constant action of the gas which has stood in the dark, until, in Nos. 9 and 
10, the two curves difier only by an amount which falls within the observational errors. 
The perfectly pure chlorine and hydrogen mixture acts therefore very differently from 
that which contains even the most minute trace of impurity. The resistance to combi- 
nation of the first gas does not undergo any change when left to itself in the dark ; in 
the second case this resistance is diminished under similar circumstances, resembling a 
wire, which, having been lengthened by a suspended weight, returns gradually to its 
original dimensions on removing the weight. 
There is thus a normal resistance to combination w belonging to the pure gaseous 
mixture, and this resistance does not alter by standing, but only on exposure to light ; 
and there is also an increased resistance to combination w-\-n (belonging to an impure 
gas), which returns to the normal resistance w by simply standing in the dark. It appeared 
possible that this increased resistance might depend upon the natmre of the decomposing 
poles ; experiment, however, showed that the same phenomenon was observed whether 
poles of carbon or platinum were employed. 
An explanation of the laws of photo-chemical induction derived from the experiments 
already communicated, may easily be found in the assumption that the chlorine or 
hydrogen, or both gases, undergo upon exposure to light an allotropic modification, in 
which they possess more than the ordinary combining power, or in other words, that both 
