12 Macgregor Skene. 
It was found possible to grow the bacteria in flasks containing 
at the bottom a layer of agar 1 cm. thick, containing the chalk, on 
to which a layer of the culture solution was poured. Under these 
conditions separate colonies were formed on the surface of the 
agar : but at the same time the autotrophic bacillus grew completely 
over the jelly, and so rendered an isolation impossible. The 
experiment proves at least that the failure to grow in solid media 
is not due to difficulty in neutralisation. 
3. There only remains the possibility of an unknown action 
of the solid medium as such. This view is supported by the fact 
that in two cases a slight growth was observed in solid media: the 
first was in gelatine at a spot where other organisms had caused 
liquefaction : the second in silica, where liquefaction had taken place 
as the result of jarring. The attempt to isolate the bacteria by 
chemotatic methods proved equally unsuccessful. It was made 
with fresh-water Chromatium material. It is not difficult to entice 
the Chromatia into a capillary by means of a solution of hydrogen 
sulphide of suitable strength, but unfortunately they are invariably 
accompanied by masses of two other organisms—a slender bacillus, 
and a long spirillum (cp. Lidforss, 1912). 
Reaction of the Liquid. 
As to this, we may note that it must be slightly alkaline or at 
least neutral. This is more necessary even than with ordinary 
bacteria, because the metabolism of the sulphur bacteria entails the 
constant production of fairly large quantities of sulphuric acid : this 
must be neutralised or growth is inhibited. This is clearly shown 
by the fact that cultures not supplied with chalk show no develop¬ 
ment. The chalk cannot be replaced by magnesium carbonate. 
Ill— The Relation of the Purple Sulphur Bacteria to 
Hydrogen Sulphide. 
In spite of the fine reasoning of Winogradsky, Molisch and 
Nadson have cast doubts on the necessity of hydrogen sulphide for 
the sulphur bacteria : the latter says definitely that it only serves 
as a protection against oxygen. As the question could not be 
regarded as settled it was considered advisable to carry out cultures 
designed to investigate the relation of the bacteria to the gas in 
presence of which they always occur in nature. If flasks are in¬ 
fected with sulphur bacteria, and some are placed in the air, others 
under the bell-jars with hydrogen sulphide, it is found that only the 
latter show a (strong) development: the former show absolutely 
