8 Macgregor Skene. 
were quite similar, except that throughout the growth was less 
vigorous. 
To determine the most suitable concentration of ammonium 
sulphate, cultures with different concentrations of that salt were 
employed, namely:—0:0-01%: 0-05%: 0-1%: 0-25%: 0-5%. 
In the first and last of these no growth took place: the other 
solutions all showed a more or less satisfactory development, but 
in that containing 0-1% the growth was best. 
The conclusion which may be drawn from these experiments 
is that ammonium sulphate is the most favourable source of 
nitrogen for the purple sulphur bacteria: but that, as in mixed 
cultures a decomposition of certain organic compounds with 
formation of ammonium takes place, such compounds permit of a 
more or less satisfactory growth, so long as they do not otherwise 
exert a harmful influence. Such compounds are albumen, peptone 
and asparagine. 
Source of Carbon. 
The question of the source of carbon is much more difficult to 
decide. Pure cultures are even more desirable here than in dealing 
with the source of nitrogen. As I was unable to obtain these, I 
attempted, by means of comparative cultures, to give at all events 
a probable solution of the problem. Using 0-15% ammonium sulphate 
as the source of nitrogen I infected cultures containing the 
following substances:— 
Potassium formate. 
Potassium tartrate. 
Calcium formate. 
Calcium lactate. 
Calcium butyrate. 
Mannite. 
Dextrose. 
Maltose. 
Control without organic compound. 
Of each of these four different sets of cultures were carried 
out:— 
(1) Organic compound 0-25% + amnion, sulph. 
(2) „ „ 0-125% -f- amnion, sulph. 
(3) „ „ 0-25% + full Lieske soln. 
(4) „ ,, 0-125% + full Lieske soln. 
Of each of these either two or three cultures were infected. As 
the experiment was commenced in March, when the conditions are 
