802 
MR. BRODIE ON THE CONDITION OF CERTAIN ELEMENTS 
change takes place* by the decomposition of water, and that the oxygen which is 
formed is the result of the combination of the element of the water with the element 
of the peroxide, or whether we consider the iodine to be transferred directly to the 
barium, and the whole of the oxygen to come from the peroxide itself. Whatever 
theory accounts for this reaction, the same theory, I will venture to assert, will ex- 
plain the decomposition of the metallic oxides. It remains only to show why it is 
that this decomposition with iodine comes before us as a chemical reaction of the 
simplest nature, and that in the other experiments the decomposition is apparently 
so variable. The reason I believe to be this, that in these latter experiments the 
peroxide is simultaneously decomposed in two ways, between which we must discri- 
minate, by the action of contact and by the chemical action of the oxide or chloride. 
The decomposition by platinum is a pure contact action, in which the peroxide loses 
only one equivalent of oxygen. The decomposition with iodine is a pure chemical 
action, in which two equivalents of oxygen are given off. The action with metallic 
oxides is an action compounded of these two, and therefore it is that the loss, as I have 
already pointed out, ever lies between these two limits. There are two causes by 
either or by both of which this decomposition may be effected ; the one, the metallic 
silver or other substance which is the result of the reduction ; the other, the very 
oxide or chloride or peroxide itself which is reduced. Certain oxides which are not 
themselves reduced, can unquestionably act in this manner ; and it is highly probable 
therefore that those which are reduced, and which belong to the same class of bodies, 
have the same property. On this view therefore the law of the formation of the 
oxygen is ever one and the same, and so far as relates to the chemical action of the 
peroxide of barium on the metallic oxide this is ever definite. The fact therefore 
which is expressed by the series of ratios is by no means any different forms of com- 
bination of the particles of the oxygen with each other, but simply the way in which the 
decomposition of the peroxide of barium is distributed between these different modes of 
action, and the relative velocity with which these two modes of decomposition take 
place. If, for example, the ratio of the loss of oxygen from the peroxide of barium to 
that from the metallic oxide, be as 4 : 1, three parts of the peroxide of barium will have 
been decomposed by the contact and one by the chemical action ; if the ratio be as 2 : 1, 
the decomposition will have taken place in equal proportions between the two. All 
the phenomena are perfectly consistent with this view. As a fact, we know that the 
contact action proceeds far more rapidly at a high than at a low temperature ; we 
need not therefore be surprised if the relative velocity of this to the chemical action 
* I have made the experiment of heating the dry peroxide with oxide of silver until the latter was reduced, 
but the loss of oxygen was only that of the silver. It is however remarkable, that when it is heated in a similar 
manner with chloride of silver, the chloride is reduced and the two equivalents of oxygen of the peroxide 
evolved. This decomposition takes place also with baryta, hut not with lime ; and I cannot but suspect that it 
is connected with the peculiar property of baryta of taking up a second equivalent of oxygen which does not 
belong to lime. It is moreover extremely difficult to prepare a baryta quite free from peroxide and water, so 
as to make a truly comparative experiment. 
