116 
DE. AXDEEWS AXD PEOFESSOE TAIT OX THE 
§ 2 . 
The oxygen gas employed in the following experiments was prepared from fused 
chlorate of potash, and, to purify and dry it, was passed through two U-tubes ; the first 
containing fragments of marble moistened with a strong solution of caustic potash, the 
second, fragments of glass moistened with sulphuric acid. The potash tube was some- 
times suppressed. In order to remove every trace of nitrogen, the whole apparatus was 
placed in connexion with a good air-pump, and a vacuum produced to the extent of at 
least half an inch, while the gas was still being evolved from the fused chlorate (fig. 4). 
When the process of exhaustion was discontinued, the gas soon filled the apparatus, and 
was expelled through the mercury at the lower end of the long gauge. The operation 
of exhausting and refilling the vessel was performed three times in everj’ experiment. 
Supposing the connexions of the apparatus to have been perfectly air-tight, the nitrogen 
remaining, after this triple exhaustion, could not have amounted to more than i^j^th of 
the whole. This degree of accuracy was not, it is true, realized in practice, but the 
oxygen gas, prepared in this way, did not contain jQ^o g th of its volume of nitrogen. 
The connexion with the air-pump at a having been broken (the gas still continuing to 
pass freely over), the end of the tube was softened in a lamp and bent downwai'ds at an 
obtuse angle, so as to allow it to dip into sulphuric acid contained in a small dish, as 
shown in fig. 5. The current of gas was now arrested, by removing gradually the lamps 
from the chlorate of potash, so as to allow the apparatus to cool slowly. When the 
acid had ascended a short way in a c, the vessel was sealed hermetically at J, and, after 
the acid had ascended to about the point c, the vessel was removed, and placed in the 
upright position represented in figs. 1 and 2. It was sometimes necessary to expel a 
bubble or two of gas, in order that the column of acid in the siphon tube might be in a 
convenient position when the vessel was placed in the calorimeter. 
Previous to filling the vessels, they were always cleaned by means of boiling nitric 
acid, and subsequent washing with distilled water. They were afterwards carefully 
dried. To the success of several of the following experiments, this precaution was 
indispensable. 
An auxiliary vessel having been filled in the same manner, either with air or with 
oxygen, the two vessels were placed in the calorimeter (fig. 3), and the difference of the 
levels of the acid in each carefully read. In oui’ earlier experiments we generally used 
a cathetometer for this purpose, but latterly we found it more convenient and sufiiciently 
accurate to apply to the limbs of the siphon tubes a scale divided into millimetres. 
From the rapidity indeed with which the readings were thus made, the results were 
found to be fully as trustworthy as those obtained with the cathetometer. AMien quan- 
titative determinations were required, the temperature of the water in the calorimeter 
and the height of the barometer were carefully noted. 
After the levels were read, the free ends of the siphon tubes in both vessels were 
hermetically sealed. The primary vessel was then removed from the calorimeter and 
placed in connexion with the electrical machine, to be exposed to the action either of 
