AT THE MOMENT OF CHEMICAL CHANGE. 
775 
It was therefore the quantitative relation of the decomposing substances which 
seemed to me the essential point to be determined, and which experiment alone could 
decide ; for it was quite possible (and indeed had been taken for granted) that the 
action varied according to some other law. It might have none of the characters of 
a chemical change ; it might, for example, vary directly with the acting masses, or 
with the temperature alone, or be a function, so to say, of so many variables that the 
true law of action would be altogether hidden. 
Preparation of the Peroxide of Barium. 
The baryta used in the following experiments was prepared by the ignition of the 
pure nitrate in a crucible of fine white earthenware. This was protected externally 
by a common earthenware crucible, and the space between the two filled up with 
crucible dust. The nitrate was thrown gradually into the crucible, one portion being 
thoroughly decomposed before the addition of the next. Baryta, as commonly pre- 
pared, contains large quantities of peroxide, as may readily be ascertained by dis- 
solving in hydrochloric acid and adding bichromate of potash to the solution ; and a 
very long and strong ignition is necessary to drive off the last portion of oxygen. 
The purest baryta is not porous, but perfectly compact, very heavy, and of the crystal- 
line grain of fine marble. The baryta thus prepared contains traces only of matter 
from the crucible, the adhering particles of which are to be carefully broken off, and 
hardly any impurities, but a little water, and a very little carbonic acid. If an ordinary 
Hessian crucible be used for its preparation, as was the case with the baryta used in 
some of the earlier of my experiments, the baryta will be very impure. 
To prepare the peroxide, oxygen was passed over the baryta, broken into pieces of 
the size of a small pea and placed in a combustion-tube, which was heated in an 
ordinary charcoal trough. The oxygen wa« made from peroxide of manganese and 
carefully freed from carbonic acid, and dried before entering the tube. The gas was 
contained in a gas-holder so as to regulate the current, and was passed over the per- 
oxide, after all absorption had ceased, until the tube was cool. The temperature 
necessary for the absorption is a low red heat, which is not to be exceeded, as a portion 
of oxygen is given off at a higher temperature. The operation may be distinctly fol- 
lowed through the tube, the baryta glowing when the stream of gas comes in contact 
with it. The absorption is both rapid and complete; and I have repeatedly made the 
experiment of attaching to the end of the tube, where the oxygen not absorbed would 
pass out, a small tube dipping into mercury, and have found that the experiment can 
readily be so regulated that a considerable stream of gas will enter at one end of the 
tube and not a bubble pass out at the other ; and indeed the absorption was often so 
rapid that the gas was absorbed faster than it entered, and the mercury rose several 
inches in the tube ; the whole oxygen therefore is absorbed. 
The peroxide presents after the experiment a mass of uniform texture and appear- 
ance, and is very white, the baryta itself being of a darker colour. A little of the 
