62 BACTERIA IN RELATION TO PLANT DISEASES. 
with just enough HCl added to counteract the alkali in the peptone, and in neutral 
or slightly acid peptone-water or sugar-free bouillon containing acid fuchsin. On 
titration of acids and alkalies see Sutton (Bibliography of General Literature, IV). 
REDUCING POWERS. 
Determine rapidity of reduction of litmus, methylene blue, and indigo carmine 
in various fluids and solids (with and without grape-sugar). Probably all bacteria 
can reduce litmus, etc., but as the rapidity of reduction varies greatly in different 
species and in different media, it is desirable to make comparative tests. Consult a 
recent paper by Albert Maassen (‘Ueber das Reduktionsvermégen der Bakterien, 
und ueber reduzierende Stoffe in pflanzlichen und tierischen Zellen,” Arb. a. d. Kais. 
Gesundheitsamte, Bd. XXI, 3 Heft, 1904, pp. 377-384). 
HYDROGEN SULPHIDE. 
This gas is the product of a reduction. From what media and under what 
conditions is hydrogen sulphide given off with browning of lead acetate paper? 
This paper is readily prepared by dipping strips of white filter paper into a strong 
solution of lead acetate in distilled water. It should be kept in a tight tin box 
or a glass-stoppered bottle. Probably most, if not all, bacteria are able to produce 
hydrogen sulphide in nutrient media containing readily decomposable sulphur com- 
pounds. Is an enzyme necessary? When an organism grays potato cylinders in 
test-tubes, why is no hydrogen sulphide given off? ‘The student should read papers 
by Petri and Maassen (Bibliog., XXVIII). 
MERCAPTAN AND OTHER ODORS. 
We need an odor chart to go along with our color charts. If we could have a 
set of standard substances with peculiar smells for comparison with the many 
odors evolved from bacterial cultures it would certainly be a great convenience. 
The difficulty at present is that the judgment of people varies greatly, in many 
instances, as to what the smell should be likened. As it is, the bacteriologist must 
do the best he can to define these penetrating smells, which are sometimes very 
characteristic of particular organisms. Some of the fishy odors are due to amins. 
Mercaptan is a very vile-smelling sulphur compound. 
INDOL, PHENOL, LEUCIN, TYROSIN, ETC. 
The production of indol is best studied in peptonized beef-broth naturally free 
from sugar or which has been deprived of its muscle sugar by growing in it (for a few 
hours only) Bacillus coli (Theobald Smith), after which it should be filtered clear. 
If B. coli or B. cloacae will not produce gas in beef-broth in the closed end of 
fermentation-tubes, it is free from sugar and suitable for this use. Many organisms 
give the indol reaction in Uschinsky’s solution to which peptone has been added. 
The writer has never been able to obtain the indol reaction in any culture medium 
which did not contain peptone (using this word in the commercial sense.) Cultures 
which do not show the red reaction with sodium nitrite (0.02 per cent solution) and 
sulphuric acid at room temperature will frequently do so when put into hot water 
