440 A Comparison of the Old and New Theories [Junez, 
deprived of water, difficultly soluble, and after ignition completely 
insoluble. In this case a new combustion takes place between the 
oxygen and chromium already combined ; that is to say, anew 
electro-chemical discharge, by which the elements not only coms 
bine more intimately, but the oxygen has lost its former properties, 
or its former electro-chemical polarization has been exchanged for 
a complete electro-chemical indifference.* It is clear that if the 
oxide of chromium at this temperature were gaseous, the produc- 
tion of fire would cause it to explode, without the ingredients 
undergoing any new combination with another body (perhaps even a 
separation), and without the oxide of chromium ceasing to be the 
same compound, and in the same proportions as before. If we 
- eould obtain chromic acid free from water, and in a separate state, 
probably when exposed to a higher temperature it would exhibit 
the same appearance of fire and separation cf oxygen as takes place 
with euchlorine in the same circumstances. 
Edmund Davy found that when a neutral solution of platinum 
was precipitated by hydro-sulphuret of potash, and the precipitate 
dried in air deprived of oxygen, a black compound of sulphur was 
obtained, which, when heated out of the contact of air, gave out 
sulphur, and some sulphureted hydrogen gas, while a combustion 
similar to that in the formation of the metallic sulphurets appeared, 
and common sulphuret of platinum remained behind. In this case 
the very same phenomenon is observable asin euchlorine. The 
platinum combines at a low temperature loosely with a greater pro- 
portion of sulpbur than it can retain in a higher temperature. When 
the compound is heated there is produced fire, because the platinum 
combines more intimately with a portion of the sulphur, and another 
portion which cannot be retained at that temperature is disengaged. 
I have found that when we heat the oxide of rhodium obtained 
from the soda-muriate of rhodium, water first comes over; and on 
increasing the temperature, combustion takes place, oxygen gas is 
suddenly disengaged, and a sub-oxide of rhodium remains behind. 
Here again we have the same appearance as with euchlorine. 
Rhodium has this in common with the base of muriatic acid that 
its first and third oxides are salifiable bases, while the second pos- 
sesses the characters of a super-oxide, giving out oxymuriatic acid 
when digested with muriatic acid, and forming salts with no acid, 
but in some measure combining itself with the bases. ‘The per-. 
oxide, on the contrary, is a well marked salifiable base ; but its 
oxygen is less intimately combined. Jt cannot be obtained from 
* If we adopt the theory, and. placg 
the electre-chemical properties of bodies 
in the electro-chemical polarity of their 
sniallest parts, the first combination may D CB A 
be produced by the discharge of the two 
poles, B, C. The compound is still 
polar by the electricity of A and D, 
that is, it possesses those properties 
which it loses by the discharge of A, D, because the body then becomes indifferent, 
