2 
Macgregor Skene. 
have those sulphur bacteria which possess a red pigment—the so- 
called Purple Sulphur Bacteria. 
These are found in stagnating water, but are not very common. 
Since, however, they sometimes occur in such masses that they 
colour the water or the mud bright red, it is not surprising that 
they were early discovered and described; see Morren (1841), Cohn 
(1875), Ray Lankester (1873,1876), Warming (1875). But it was only 
in the year 1888 that Winogradsky recognised the importance 
of the sulphur inclusions for their metabolism, and so laid a sure 
foundation for further investigations. At the same time he cleared 
up in essentials the systematics of this difficult group. 
In order to study the life-histories of the different purple 
bacteria the Russian investigator devised a method by means of 
which he could keep the organisms in question under microscopical 
control for a considerable length of time. He placed small colonies 
in a drop of sulphuretted hydrogen water on a slide, and covered 
them with a slip, under which lay a few glass splinters to prevent 
crushing. If care was taken to prevent evaporation, and to add 
fresh sulphuretted hydrogen water once or twice every day, the 
bacteria throve for months, and multiplied considerably. 
In spite of many endeavours Winogradsky was unable to obtain 
pure cultures, and therefore was obliged to draw his conclusions as 
to the nutrition of the bacteria from the results of his drop cultures. 
The chief points of his discussion may be summarised as follows:— 
1. Hydrogen sulphide is absolutely necessary for a good 
growth : it is oxidised first to sulphur, then to sulphuric acid : the 
sulphur globules which one sees in the interior of the cells are the 
intermediate products of this oxidation : they grow in size if the 
supply of hydrogen sulphide is abundant, in absence of this gas they 
grow smaller and finally disappear. 
2. In order to carry out this oxidation the bacteria require 
oxygen, but only in small quantities. In natural conditions they 
are always accompanied by minute green bacteria: these are capable 
of photosynthesis, and the oxygen so liberated is utilised by the 
sulphur bacteria, which are thus to be regarded as microaerophil. 
3. The bacteria require organic food substances, but can 
flourish with quite small quantities: thus Winogradsky used for 
his cultures Strassburg tap water to which he added 0-005—0-01% 
of calcium butyrate, or of sodium acetate: with peptone and meat 
extract he could obtain no better results. This remarkable fact is 
to be explained (a) by the very slow growth of the bacteria, which 
