AT THE MOMENT OF CHEMICAL CHANGE. 
799 
ratios between which the whole action is comprised, either of which may be almost 
indefinitely approached. These are the limit of least reduction, in which the ratio of 
the loss from the peroxide of barium to that from the metallic oxide is infinite, or as 
1:0; and the limit of greatest reduction, which is the ratio of equality, in which the 
two substances would lose an equal amount of oxygen. This limit also, in no one 
experiment with any silver compound, have I ever exceeded or reached. The first 
limit is that in which the action would be a pure reduction by contact, sucli as 
takes place when platinum or carbon are the substances which cause the decompo- 
sition. The other would be a purely chemical action, such as may be seen in an 
experiment to which I will now proceed, and which, although perhaps not at first 
sight as striking as the reduction of the metallic oxides, is, philosophically considered, 
quite as remarkable, being indeed the same experiment, but detached from circum- 
stances which give in the other case the apparent differences to the reaction. 
Action of Iodine on the Peroxide of Barium. 
It occurred to me, that if it were for the reasons I have stated, that the iodide and 
chloride of silver decompose the peroxide of barium with the evolution of the oxygen 
it contains, iodine and chlorine should produce a similar effect. Indeed this was a 
very critical and important experiment; for we know the action of chlorine and 
iodine on baryta ; and on the view which is usually taken of this decomposition and 
of other analogous changes, namely, that the iodine takes away the barium from the 
oxygen because of its superior affinity for the metal, and that the oxygen which is 
thus liberated combines with the iodine because it is in a nascent condition, and 
thus converts it into iodous or iodic acid ; by acting on iodine with the peroxide of 
barium, we ought to have just twice as much iodic acid formed as in the other case ; 
nor was there any apparent reason why the oxygen from the peroxide of barium 
should not oxidize as well as the oxygen from the baryta. The same iodine, the same 
barium, and the same oxygen are supposed to be present ; and if the iodine acted at 
all on the peroxide, the oxygen would still be nascent. Considered however from that 
theoretical point of view which I have given in the early part of this paper, there was 
no reason to expect this result. I have there suggested (p. 767), that in the action of 
iodine upon baryta, the combination of iodine and oxygen takes place solely on ac- 
count of the polar relation in which the particles are placed under the peculiar cir- 
cumstances of the action. Each particle in the change being alternately positive and 
negative, thus ; — 
-| 1 _ — 
I I BaO=IBa-f-IO. 
V ) 
I at once determined the action to a limit which it could not exceed. When small quantities of the bichromate 
are employed the result is entirely different, and a very small quantity of the bichromate decomposes a very 
great excess of the peroxide. The reaction which I have given (p. 769) is to a certain extent hypothetical, it 
is the limit of greatest reduction. 
