116 CLASSIFICATION AND DESCRIPTION OF BACTERIA 
Hydrogen Sulphide. Hydrogen may be formed from many of the 
sulphur-containing compounds. When formed from proteins it probably 
comes from cystine or some other sulphur linkage. It may be detected 
by suspending a strip of bibulous paper saturated with lead acetate to 
which a little glycerol has been added. This method has been found 
to yield satisfactory results by the author. 
Ammonia Production. Dilute the culture with ammonia-free water 
and add 1 ¢c.c. of Nessler’s reagent. The presence of a yellowish-green 
color indicates the formation of ammonia. 
Nitrate in Nitrate Broth. Ammonia, see above. Nutrate. The 
amount of nitrate decomposed by an organism may be determined 
quantitatively by the aluminum-reduction method which has been 
outlined in the Chapter on Water Analysis. For qualitative results 
the presence of nitrites or ammonia in a medium which was free from 
these substances before inoculation may be taken as sufficient evidence 
of nitrate reduction. 
Test for Nitrites. The same method may be used for the qualita- 
tive estimation of the presence of nitrites that 1s used in the quantitative 
determination. The Griess method has had much application and 
depends on the formation of azobenzolnaphthylamin whenever naph- 
thylamin and sulphanilic acid are present in an acid solution of nitrites. 
Mason (1912) gives the reaction as follows: 
_-~ N=N 
NHe—CeHi—HSO3 + HNOe = Cols _ + 2HeO 
| 
QO 
_- N—- H N 
_ anal H a \ a a’ Na 
ae wa =( ing" 
HSO3 ar 
This is a very delicate test and is regarded by a few bacteriologists 
as too delicate for bacteriological work. If a control tube is made 
accurate results should be secured by its use. 
To determine nitrites, dilute 2 or 3 c.c. of the culture with ammonia- 
free water and add 3 c.c. each of sulphanilic acid and naphthylamin 
hydrochloride. The presence of a red color when viewed the long way 
of the Nessler tube indicates the presence of nitrites. 
