296 Prof, J. W. Mallet on the Chemical and 
through various shades of colour—yellow, orange-yellow, brown, 
green, and at last blue; he attributed these tints to the succes- 
sive formation of sulphates of the binoxide, sesquioxide, and blue 
oxide: but may not the first step in the reduction have been 
osmious acid, giving the yellow colour ? 
Another and altogether different view of the nature of the vola~ 
tile yellow substance above described was suggested as possible 
by some remarks of Claus, in a recent paper ‘‘ On the Tendency 
to Reduction of Salts of Iridium” (Ann. der Chem. und Pharm. 
August 1858, p. 129). This author has shown that the platinum 
metals fall naturally into three groups, in each of which are con- 
tained two metals resembling in general habit and relations each 
other more closely than members of the remaining groups. Pla- 
tinum and palladium constitute the first of these pairs, iridium 
and rhodium the second, osmium and ruthenium the third. The 
atomic weight of the first-mentioned member of each pair is 
higher than (nearly double) that of the second. 
In the paper quoted, Claus remarks that the metal of lower 
atomic weight in each of these groups is much more easily 
reduced than the other from superior to inferior grades of com- 
bination with chlorine: thus the bichloride of palladium is re- 
duced with much greater ease to protochloride than is the corre- 
sponding compound of platinum ; and for the same reason, pro- 
bably, the bichloride of rhodium is not known, but only the ses- 
quichloride, while both salts of iridium can be easily obtained. 
On this same principle Claus explains the fact that no oxide of 
ruthenium homologous with osmic acid has been obtained, while 
he gives the following reasons for suspecting the existence of 
such an oxide :—“ This opinion is based upon the fact, that in 
my preparation of compounds of ruthenium, which can be ob- 
tained only by energetic processes of oxidation, the material 
worked upon, notwithstanding all my care and economy, gra- 
dually diminished, and yet I have never succeeded in collecting 
a volatile product. Once only, when I had fused ruthenium 
perfectly free from osmium, with caustic potash and nitre, dis- 
solved the mass in water, and decomposed it with nitric acid, I 
observed a peculiar odour, quite distinct from that of osmic or 
nitrous acid ; and afterwards, having covered the beaker, which 
was smeared on the edge with tallow, with a plate of glass, I 
remarked an unmistakeable blackening of the tallow, caused by 
the reduction of a volatile metallic compound.” 
It seemed possible that the volatile yellow substance to which 
the present paper refers might have been an acid oxide of ruthe- 
nium*—RuO*, RuO*, or RuO°—and reducible with extreme 
* If such a compound exist, an explanation may be found for the process 
by which Fremy has obtained a lower oxide of ruthenium (probably the 
