Physical Relations of Osmium. 297 
facility, Claus and others having already noticed the reducing 
effect of light upon salts of the platinum metals. <A portion of 
the crust of yellow acid from the sides of the tube was carefully 
examined for ruthenium, the various tests given by Claus, as 
well as that recently proposed by Dr. Gibbs, being made use of ; 
but no proof of the presence of this metal could be obtained. 
The properties of osmium and its compounds are very re- 
markable, and render it a matter of no little interest to trace the 
analogies of this rare substance and fix its place among the other 
elemenis. It is described in most chemical works along with pla- 
tinum and its associated metals,—mainly on the ground of com- 
munity of origin ; for in many respects it is unlike the platinum, 
palladium, rhodium, &c., with which it always occurs in nature. 
All these metals are commonly thought of as very infusible, of 
great density, very slightly affected by reagents, and very easily 
reduced from their compounds to the metallic state; when more 
closely examined they are found to differ from each other in 
many of their other properties. The arrangement by Claus of 
the platinum metals in three groups, each containing one metal 
of high and one of low atomic weight, viz. 
Platinum, Iridium, Osmium, 
Palladium, Rhodium, Ruthenium, 
has been alluded to above ; the two members of each group are 
more closely related to each other than to any of the rest. 
Osmium and ruthenium are clearly the most electro-negative of 
the series. Graham has inferred the isomorphism of platinum, 
alladium, iridium, and osmium, from the fact that their potassio- 
chlorides all crystallize in the form of the regular octahedron : 
the corresponding compound of ruthenium has since been added 
to the list, while that of rhodium is still unknown. The occur- 
rence of two salts under the same form, zn the regular system, of 
course does not of itself suffice to establish the relation of iso- 
morphism between them ; iridio-chloride of potassium seems how- 
binoxide) in crystals. He roasts the powder of platinum-residue in a stream 
of air drawn through a porcelain tube at a bright red heat ; osmic acid volati- 
lizes, and is said to carry with it mechanically the oxide of ruthenium, 
which deposits upon fragments of porcelain placed in the cooler part of the 
tube. But the oxide is in distinct crystals, and can therefore scarcely be 
conceived of as a powder borne along in a merely mechanical way by a 
stream of vapour; and, moreover, there is no reason for oxide of ruthenium 
only being so borne along, while other substances of no greater density re- 
main behind. Is it not more likely that a volatile and very easily reducible 
homologue of osmic acid is formed, and almost immediately afterwards 
decomposed, depositing the bimoxide of ruthenium ? 
