102 
h. A concentrated violet solution turns blue on addition of a dilute 
acid; but after neutralizing with a base, the red-violet colour is 
restored; etc. 
SUMMARY. 
In the above we were able to demonstrate, that the product 
of the precipitation of a salt of tetravalent ruthenium by 
ammonium sulphide differs with the temperature: at 100° C., 
brownish black RwS, is formed besides free sulphur, but at 0° C. 
greenish black RuS, is formed, which has the character of an 
irreversible colloidal substance, and which in the presence of 
ammonium sulphide in excess, gives a beautiful green, but unstable 
colloidal solution. At intermediate temperatures mixtures of both 
sulphides are formed besides free sulphur. 
The dark green sulphide and the green colloidal solution of Rw5S,, 
the particles of which are negatively charged, rapidly absorbs free 
oxygen, and is transformed into a reddish violet solution of the 
reversible colloidal rwtheniwn-pyrosulphite: RuwS,O,,, the particles 
of which bear a positive electrostatic charge. This salt is, in its turn, 
changed by oxygen (air, nitric acid, permanganate) into rutheniwm 
sulphate and free sulphuric acid. The properties and reactions of 
these different products were investigated on general lines. 
Laboratory for Inorganic and Physical 
Chemistry of the University. 
Groningen, May 1920. 
