i6 Macgregor Skene. 
bacteria. When I was cultivating marine species I was unable to 
observe these green bacteria in a single case. Quite recently, 
however, I have had at my disposal very fine fresh-water material 
of Chromatium Okenii (for which my best thanks are due to Dr. 
O. V. Darbishire), and in this the green bacteria certainly do occur 
with great regularity. They are very small—about 1‘7/x long and 
0'5/x broad, and bright green : they occur in little spiny clumps, 
which cannot be mistaken for such forms as Stichococcus. If a 
Chromatium settles down it always does so beside one of these little 
green masses. It is therefore probable that Winogradsky’s views 
are correct. 
Amongst the salt-water forms I have never seen these green 
bacteria : instead, however, there is a frequent development of 
Chlorella and of several blue-green algae. In my flask cultures 
these did not as a rule make an appearance—such flasks as showed 
a development of green algae had of course to be rejected. 
In nature, then, the purple sulphur bacteria obtain their supply 
of oxygen from green assimilating organisms in their neighbourhood. 
It is probable that in my flask cultures the method of supplying 
sulphuretted hydrogen, solved at the same time the problem of 
oxygen supply. In the bell-jars there is always present a large 
excess of oxygen, and as I generally waited until all traces of 
sulphuretted hydrogen had disappeared before renewing the supply, 
there must have been in the solution a constant varying between 
sulphuretted hydrogen and oxygen content. Under these conditions 
the bacteria certainly developed splendidly. It is also of interest, 
that when test-tubes were used instead of flasks, and the layer of 
liquid had a depth of 5 to 6 cms. instead of 1 to 2 cms., the development 
was in the first place rather slower, and, in the second took place 
on the walls of the test-tube near the surface of the liquid—not 
on the sediment at the bottom. I imagine that this is to be referred 
to the less easy penetration of oxygen to the lower layers. 
That the bacteria are certainly not anaerobic is clear from the 
fact that they can remain alive for several days in a liquid exposed 
freely to the air. 
The views of Molisch and Winogradsky, that a certain small 
quantity of oxygen is necessary, would seem to be well founded. 
VI. —Summary. 
The results of the foregoing experiments may be summarised 
as follows 
1. The attempts to obtain pure cultures of purple sulphur 
bacteria have been without success. 
