422 
THOMPSON YATES LABORATORIES REPORT 
of reaction. Otherwise the reaction most favourable for the formation of sulphide 
in ten per cent, peptone is much more acid to phenolphthalein than the most favour- 
able reaction in two per cent, peptone ; being about + 25 in the former, and + 1 to 
+ 16 in the latter. Now, if these two reactions be interchanged in the two media, 
i.e., the ten per cent, be reduced to + 1 and the two per cent, be increased to + 25, 
the sulphicie forming power is almost destroyed in the latter, and much reduced in 
former. On the other hand, with litmus, the solution of two per cent, peptone in 
water, which is very favourable for sulphide production, is slightly alkaline ; whereas 
the ten per cent, solution, nearly equally alkaline to litmus, requires the addition of 
about 2 c.cm. normal potassium hydrate per litre to permit of the most rapid produc- 
tion of sulphide. That is, the point of best reaction is much more alkaline to litmus 
in ten per cent, peptone than in two per cent. 
A similar difference between the two indicators is illustrated by the fact that some 
of the bacteria described in the appended table in two per cent, ferro-peptone medium, 
while forming sulphide freely, induce a marked increase of acidity to phenolphthalein, 
but not to litmus. Thus, organism No. 1, after three days incubation, had induced 
an acidity to phenolphthalein of + 35; No. 2 and No. 9 an acidity of +30; and 
No. 7 of + 25. But in all cases the medium was still alkaline to litmus. The con- 
trols of ferro-peptone water showed acidity of a little over + 10 to phenolphthalein. 
It may be that the acids of the peptone, and those formed by the bacteria, are 
more compatible with their activity than hydrochloric acid. The fact remains, however 
it may be explained, that if the same acidity to phenolphthalein be attained with hydro- 
chloric acid before inoculation sulphide formation does not proceed. 
C. Temperature 
Cold exerts a retarding influence on the formation of sulphide. A temperature 
of 37" C. proves very favourable, 25° C. less so, and 12° C. very unfavourable. 
D. Oxygen 
The strong sulphide formers act quite as well anaerobically as aerobically. Some 
of them seem to prefer a limited amount of oxygen as is evidenced by the formation 
of sulphide in the depths of stab cultures and plate colonies, rather than at the surface. 
Indeed, when surface colonies begin to darken they not infrequently do so first in 
their depth, the surface remaining of its original colour. It is noteworthy that a 
number of these very powerful hydrogen sulphide forming bacteria, while giving rise 
to a deep black throughout the ten per cent, gelatine and agar tubes, fail entirely to 
form black colonies in identical media, in poured plates, under aerobic conditions. 
Attempts made to induce black formation anaerobically, according to Buchner's 
method, failed also. 
