AT THE MOMENT OF CHEMICAL CHANGE. 
777 
Before the study of these decompositions was possible, it was necessary to have 
some ready and accurate method of determining the amount of oxygen in the various 
preparations of the peroxide, which could never be obtained absolutely pure and cor- 
responding to a theoretical formula, and of which I cannot indeed find that any satis- 
factory analysis has yet been made. 
After various experiments 1 selected two methods, which combined the advantage 
of great facility of execution with perfect agreement in their results. One of these 
methods depends on the chemical change which takes place when the peroxide of 
barium is brought in contact with an acid solution of chromic acid, when, as I have 
before mentioned, both the peroxide and the chromic acid are decomposed with evo- 
lution of gas. Supposing this to be a definite and constant reaction, we have, it is 
evident, in the evolved gas, a measure of the oxygen in the peroxide, from which, if 
the reaction were known, the latter might be calculated. For this, however, it was 
necessary first to discover the nature of the decomposition. The other method de- 
pends on the decomposition of the acid solution of the peroxide by finely-divided 
platinum or carbon, in which case the oxygen evolved is plainly half the total oxygen 
in the peroxide itself. For the determination of the oxygen in the preparation of a 
small quantity of the peroxide, it was quite practicable to weigh the baryta before 
and after the absorption of the gas ; and although this plan could not be applied to 
those larger quantities of the substance which it was desirable to prepare at once, 
yet, as there could be no doubt as to the general accuracy of the determination if the 
experiment were conducted with care, I availed myself of it for determining the re- 
action in question with chromic acid, and for a general control over the methods. 
The baryta was placed in a platinum tube about 8 inches long and half an inch wide, 
and the whole experiment conducted precisely in the manner already described in 
the preparation of the peroxide. The tube was first weighed empty, the ends being 
closed with dry corks, again with the baryta, and again after the experiment. The 
peroxide was then rapidly pounded and used for the other determinations. A glass 
tube cannot be used for this experiment, as the glass is always slightly acted upon, 
where in contact with the baryta, which, although of little consequence in an experi- 
ment on a large scale, as causing an impurity in the peroxide, yet gives rise to a dis- 
crepancy between the amount of oxygen calculated from the absorption and the per- 
centage of oxygen deduced from the other experiments. 
Two experiments, conducted in this manner, gave the following results : — 
A. 12'581 grms. of baryta increased in weight 0‘952 grm., corresponding to an 
increase in weight of 7*53 on 100 parts. 
B. 15T80 grms. of baryta increased in weight Till grm., corresponding to an 
increase in weight of 7’318 on 100 parts. 
Hence the two preparations of peroxide of barium contained respectively — 
Preparation A, 7'03 per cent. ; preparation B, 6‘81 per cent, of oxygen in addition 
to the oxygen of the baryta. 
5 G 
MDCCCL. 
