AT THE MOMENT OF CHEMICAL CHANGE. 
801 
the quantities of the substances taken and the loss ; in the second the comparison is 
made between this loss and the oxygen of the peroxide. 
Table XIII.— A. 
Peroxide of 
barium Q. 
Iodine. 
Loss of oxygen. 
1. 
1‘094 grms. 
30-242 grms. 
0-187 grm. 
2. 
0-973 
16-082 
0-164 
3. 
0-843 
11-633 
0-140 
4. 
1-409 
15-5485 
0-238 
5. 
1-845 
17*8225 
0-315 
6. 
2-0675 
17-130 
0-358 
7. 
0-7995 
5-5325 
0-140 
8. 
1-9364 
10-6495 
0-331 
9. 
1-2405 
5-277 
0-212 
10. 
1-275 
3-517 
0-215 
Table XIII.— B. 
Oxygen in the 
peroxide Q. 
Loss of oxygen. 
Oxygen in the 
peroxide. 
Loss of oxygen. 
1. 
17*09 
199*1 
2. 
16-85 
196*3 
3. 
16-60 
193-4 
4. 
16-89 
196-8 
5. 
6. 
8-58 
17*06 
17*31 
100 
198-8 
201-7 
7. 
17*51 
204-0 
8. 
17*09 
199*1 
9. 
17*08 
199-0 
10. 
16-86 
196*5 
The mean of these experiments gives the ratio of the oxygen in the peroxide com- 
pared with the loss, as 8‘58 : 17‘03, or as 100: 198’4. They clearly prove that the 
reaction is as I have stated. The quantity of iodine taken varies from 2 to 20 equiva- 
lents. The number of these experiments may appear superfluous, but there were some 
circumstances connected with the reaction when a smaller quantity of iodine was em- 
ployed, which caused me particularly to inquire whether any excess of iodine would 
alter its nature. It is plain that this cannot be done. I have also ascertained the 
fact, that with a perfectly pure peroxide an equivalent of iodine always decomposes 
exactly an equivalent of the peroxide, so that if an excess of the peroxide be taken it 
remains undecomposed. In this important point this reaction differs from the other 
experiments I have given, in which very small quantities of the substance decom- 
pose the whole of the peroxide present. With iodine this will not take place. 
The decomposition of the chloride of silver by the peroxide of barium is a true 
link between these experiments with iodine and the reduction of the metallic oxides. 
In whatever rational form we may express these facts, the facts themselves are the 
same. We cannot see the mode in which the action is effected, nor for the present 
argument is it at all important whether we consider, as we reasonably may, that the 
MDCCCL. 5 K 
