96 
$ 2. If now the flocculated solution is quietly left standing during 
a couple of days, its colour becomes gradually pale pink; and after 
standing somewhat longer, finally a more or less intense reddish 
violet solution is obtained, while at the bottom of the vessel a preci- 
pitate of finely divided sulphur has accumulated. An analogous 
phenomenon is observed, when one tries to subject the original, dark 
green solution to dialysis: also in that case a pink solution is finally 
obtained after the flocculation of the original green one. This new 
red solution, into which the original green liquid is transformed, 
also appears to be a colloidal solution: both the original and the 
red liquid exhibit the Tynparr-effect and on being examined with 
the ultra-microscope they both show the characteristic structure and 
the Brownian motion of true colloidal solutions. The stability of the 
red solution appears, however, to be much greater than that of the 
dark green solution mentioned before. 
Soon it became evident that for the change of the unstable green 
solution into the much stabler red solution, the presence of the free 
oxygen of the air is essential; that, in other words, an oxydation- 
process goes on, in which the greenish black precipitate originally 
formed is gradually dissolving under continuous absorption of oxygen, 
while a red colloidal solution is formed by it. This chain of 
events could be illustrated, leaving no doubt whatever about its 
truth, by the following series of experiments : 
a. Greenish black ruthenium-sulphide freshly precipitated at O° C. 
was first washed with icy-cold water, and subsequently dried 
after washing it with absolute alcohol and ether. Immediately it 
was mixed with water and shaken in a stoppered bottle; a suspen- 
sion is formed of an originally bluish hue, the upper layer of which 
is, however, already after one and a half hour converted into a 
pale pink liquid. After a day the colour turns reddish violet, while 
the quantity of the precipitate is gradually diminished, the longer 
the contact of the different substances lasts. Simultaneously a slight 
precipitate of sulphur is deposited on the bottom of the flask. 
b. At O° C. freshly precipitated greenish black sulphide, treated 
as described above, was vigorously shaken with water, and a conti- 
nuous current of pure air sucked through the liquid. Soon the 
solution turns reddish violet; after some days the original precipitate 
has completely disappeared, while some finely divided sulphur only 
remains, which can be easily removed by filtering. This is one of 
the best modes of preparing the red colloidal solutions. 
c. On being exposed to the air for a long time, the dry greenish- 
