97 
black sulphide gradually changes its colour, being converted into a 
reddish, dark coloured mass, which gives immediately the red solu- 
tion, if shaken with water. 
d. A piece of filter-paper soaked in the green colloidal solution, 
becomes very rapidly violet on being exposed to the air. If the 
oxygen of the air is first removed, no change of colour appears ; 
the precipitate generated in flocculating the green solution gives, 
however, automatically the reddish violet liquid, when exposed to the 
atmosphere. 
e. Whilst AwS,, precipitated from hot solutions is simply attacked 
by nitric acid (spec. grav.: 1,4) and oxydized to a brown solution, 
the greenish black sulphide is attacked by the same acid extremely 
vigorously, almost explosively: a red violet solution is formed, while, 
moreover, some sulphur is precipitated at the same time. The red- 
violet solution is, also after neutralisation of the acid in excess, slowly 
oxydized further, when in contact with the air; finally the solution 
becomes completely colourless, and the slightly acid liquid thus 
obtained appears to contain a sulphate. Not even a preliminary 
dilution of the red-violet liquid with water can prevent this oxydation 
to sulphate. The presence of mere traces of the unstable greenish 
black sulphide may be proved by this reaction of oxydizing the 
supernatant liquid by means of nitric acid’); and it is in this way, 
that we can demonstrate the fact, that the sulphides precipitated 
from ruthenium solutions by ammonium sulphide between 0° U. and 
boiling-temperature, are really mixtures of stable RuS, and the 
unstable greenish black sulphide, here described. It suffices to shake 
the precipitate simply with water, and to add strong nitric acid to 
this suspension: the red colour will then appear immediately. 
§ 3. Because the new sulphide appeared to lose its characteristic 
properties, if heated even to only 110° C., it was necessary, under 
exclusion of the oxygen of the air as much as possible, to prepare 
it always at dower temperatures; also it must be rigorcusly purified 
for the purpose of analysis. In the process of precipitation, free sulphur 
is moreover always formed, — a fact also noticeable?) in working 
with other ruthenium sulphides, — and therefore necessitating repeated 
') Already other investigators have occasionally had an opportunity to observe a 
pink coloration of the solutions obtained in their studies on ruthenium sulphides , 
without any attempt at an explanation of the said phenomenon, it was e.g. men- 
tioned by Antony and A. LucuHsssi, loco cit 30, (II), 540, (1900). 
2) C. Ciaus, Ann. der Chem. u. Pharm., 59, 245, (1846); U. Anrony and A. 
Lucuess!, Gazz. Chim. Ital. 30, (II), 539, (1900). 
7 
Proceedings Royal Acad. Amsterdam. Vol XXIII. 
