DR. A. SCOTT ON THE COMPOSITION OF WATER BY VOLUME. 
545 
The palladium was used in the form of thin sheets and was ignited in a current of 
pure air for some time to a full red heat, to remove all traces of greasy matter which 
it might have acquired during the rolling. 
The potassium chlorate was very carefully recrystallized and fused in a platinum 
dish before use, and in the later experiments a small quantity of pure sodium 
hydrate made from the metal was added, to prevent, if possible, the evolution of small 
quantities of chlorine. 
Very few experiments were made with mercuric oxide as the temperature required 
for its decomposition was rather high for soft German glass to stand with safety. 
Two samples were used, the first of which at first gave off some carbon dioxide, so a 
sample was prepared with great care from the metal, by first converting it into 
mercurous chloride, then into mercuric chloride, and finally into the oxide which, 
after thorough washing, was moistened with pure soda solution and dried. The 
soda efiectually retained any carbon dioxide in this sample, but the repeated fractures 
of the tubes containing the mercuric oxide led me to abandon its use. Silver oxide 
seems to be by far the most convenient source of pure oxygen, and has been used 
in all the later experiments. It was made at first by precipitating silver nitrate 
solution by means of pure barium hydrate, washing free from barium nitrate, and then 
drying rapidly on the water bath. The first sample prepared had absorbed so much 
carbon dioxide from the air during its drying that it rendered worthless several 
experiments. The second sample was, therefore, prepared by the same reaction, but 
was very carefully washed with thoroughly boiled water in closed flasks, and dried, 
in vacuo, in vessels with ground glass stoppers, free from all grease. Thinking that 
nitrates might still adhere to the precipitate as Stas showed they were so prone to do, 
a quantity of silver sulphate was precipitated wdth barium hydrate, so that a mixture 
of silver oxide and barium sulphate was obtained and used after drying as above, and 
worked extremely well. The silver oxide used in the last experiments, and which 
has given gas in which no impurity, however small, can be detected, was prepared 
from silver sulphate and excess of pure potassium hydrate, and after washing, giving 
another treatment with some fairly dilute potassium hydrate solution by boiling for 
half an hour or so, again washing with wmter well boiled and just coloured with 
^ potassium permanganate, till free from sulphates, then allowed to settle, and the clear 
liquid decanted and a little pure soda added (as was done always with the silver 
oxide) and the oxide dried, in vacuo, at 110° C. 
Fig. 1 show^s the apparatus as used in the later series of experiments, and is drawn 
to scale as regards the size of the principal parts ; the tubes are merely elongated 
I so as to show their connection clearly. The scale F, wdiich is 1 metre in length, 
indicates the size of the various parts. 
The constant volume is contained between a mark on the narrow tube above B, 
and a similar mark about the middle of the tube D. The internal diameter of these 
tubes, which are of strong Sprengel tubing, is almost exactly 2 millims., as they 
MDUCCXCIII.—A. 4 A 
