PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 399 
the light must entirely depend upon photo-chemical action affecting only the increasing 
attractions of the chemically active molecules. 
We have often had occasion, in the course of this research, to show that the curves 
which represent the increase of the chemical action for a constant source of light have 
all a maximum of the increasing action, which is rendered visible by a flexure in the 
curve. It appeared of interest to examine whether this property of the induction-curves 
arises from the general mode of action of affinity, or whether the light plays a necessary 
part therein. We have therefore carried out some experiments upon idio-chemical in- 
duction ; that is, upon reactions which occur altogether under the influence of chemical 
attractions, without the assistance of heat, light or other outward forces. For this pur- 
pose a very dilute aqueous solution of bromine with tartaric acid was employed. This 
mixture, when allowed to stand in the dark at a constant temperature, undergoes a very 
slow decomposition, the free bromine gradually decomposing the organic acid and form- 
ing hydrobromic acid. The quantity of bromine contained in equal volumes of this 
solution was determined at various times after mixing To Tj . . . T„ by the iodometric 
method. The amount of bromine thus found, B^, subtracted from that originally con- 
tained, Bo, gave the amount of free bromine (Bo — BJ which had entered into combina- 
tion from the beginning of the experiment up to the time of the analysis. If a repre- 
sent the weight of iodine contained in one burette division of the standard solution, n 
the number of measures of sulphurous acid required to reduce the free iodine, t the 
number of burette degrees necessary to decompose one measure of sulphurous acid, and 
t the number of burette degrees requfred to neutralize the excess of sulphurous acid, 
and Br and I the atomic weights of bromine and iodine, the amount of bromine, B, con- 
tained in a volume of the liquid under examination is found from the equation 
B= — j — 
Having thus determined the quantity of bromine present in the same volume of liquid 
for the times To T, ...T„, the quantity of bromine which has entered into combination 
from one volume of the liquid in the times Ti T 2 , &c., is found from the equations 
g g g g 
-^ 5 — &c. The liquids employed in Experiment 1, Series XIII. contained, to 
I gramme of bromine, IT39 grms. crystallized tartaric acid and 648-9 grms. water; in 
Experiment 2, to I grm. bromine, 1-654 grm. tartaric acid and 706-5 grms. water; in 
Experiment 3, to I grm. bromine, 6-6166 grms. tartaric acid and 1410-2 grms. water. 
The value of a in all the analyses was 0-0022593. The atomic weights of bromine and 
iodine employed in the calculations were 500-00 and 794-37. The following arrange- 
ment of the experimental results gives, in the first vertical column, the duration of the 
spontaneous decomposition m hours. To Tj, &c. ; the second, third and fourth columns, 
the elements of analysis ; and the fifth, the values Bo Bj . . . ; and the last, the values of 
— - g— ‘ . . . , or the loss of bromine for the successive times Tj T 2 . . . T„. 
