Physical Relations of Osmium. 303 
sesquioxide united to ammonia, which, dissolved in hydrochloric 
acid, yields a brown compound, supposed to be the sesqui- 
chloride of osmium andammonium. Neither of these, however, 
can be-crystallized, nor has the constitution assigned to either 
been supported by an analysis. The so-called ammonio-sesqui- 
oxide detonates when heated (sometimes with much violence, as 
I have noticed in remoying by heat the deposit of this substance 
which forms on the end of a retort-neck during the distillation 
of osmic acid into a receiver containing ammonia), and hence is 
probably analogous to fulminating platinum, containing perhaps 
the binoxide of osmium. The binoxide itself is a feeble base, 
the characteristic colour of whose salts in solution is yellow, as is 
the case with the corresponding compounds of iridium. Similar 
remarks apply to the binoxide of ruthenium—probably the body 
obtained, as we have shown, by Fremy in crystals, The teroxide 
of osmium is the body supposed to have been isolated in the ex- 
periment described at the beginning of this paper. — Its position 
as a feeble acid, capable, however, under some circumstances, of 
playing the part of a base, its fusibility and volatility (greater 
apparently than those of osmic acid, as nitrous acid is more 
fusible and volatile than hyponitric), its probable crystallization 
in octahedrons of the regular system, in which arsenious acid 
and teroxide of antimony are also found, all tend to indicate 
homology with the other teroxides of the arsenic group. The 
general relations of ruthenic acid, so far as these are known, 
place it im a similar position. Just as we find hyponitrie acid 
(NO*) and antimonious acid (SbO*) to be the most stable of the 
higher oxides of nitrogen and antimony, so the well-known osmic 
acid (OsO*) seems to be the grade of oxidation which osmium 
most readily assumes and retains when not in contact with bases. 
OsO? and OsO? (the latter as described by Fremy) seem scarcely 
capable of existing in the separate state ; when set free from their 
salts they soon pass into OsO*; while it may as well be doubted 
that the latter ever exists as a distinct acid in combination with 
bases, as that NO? or SbO* does so. No so-called osmiate has 
ever been analysed ; the saturating capacity of the acid, if it be 
such, is unknown; when free and in solution in water, it has no 
acid reaction ; it does not displace carbonic acid from the carbo- 
nates, and it is itself expelled by heat from most of its supposed 
compounds, and is separated in part by water even from potash 
and soda, Nocompound of OsO*with a base has been obtained 
in crystals, while Fremy states that he has crystallized the alka- 
line salts of both OsO® and OsO*®, RuO4 and RuO® are as yet 
unknown. 
The tendency throughout the whole arsenic group is mani- 
festly to the production of the acid compounds MO? and MO; 
