OBSERVATION OF INDOL FORMATION. 8l 



(see p. 43). Indol production is recognised by the fact that 

 when acted on by nitric acid in the presence of nitrites, a nitroso- 

 indol compound is produced, which has a rosy red colour. Some 

 bacteria (e.g. the cholera vibrio) produce nitrites as well as indol, 

 but often, in making the test (e.g. in the case of B. coli), the 

 nitrites must be added. This may be effected by using yellow 

 nitric acid (which of course contains nitrous acid) for the test, 

 or by adding to an ordinary tube of medium i c.c. of a .02 per 

 cent solution of potassium nitrite, and testing with pure nitric 

 or sulphuric acid. In any case only a drop of the acid need be 

 added to say 10 c.c. of medium. If no result be obtained at 

 once, it is well to allow the tube to stand for an hour, as some- 

 times the reaction is very slowly produced. The amount of indol 

 produced by a bacterium seems to vary very much with certain 

 unknown qualities of the peptone, and differences in ability to 

 form indol from a given sample of peptone are noticeable, too, 

 in races of bacteria of the same species. It is well therefore 

 to test a series of peptones with an organism (such as the B. 

 coli) known to produce indol, and noting the sample with which 

 the best reaction is obtained, to reserve it for making media to 

 be used for the detection of this product. 



The Drying of Substances in vacuo. As many substances, 

 for example toxins and 

 antitoxins, with which 

 bacteriology is con- 

 cerned would be de- 

 stroyed by drying with 

 heat as is done in or- 

 dinary chemical work, 

 it is necessary to re- 

 move the water at the 

 ordinary room tem- 

 perature. This is most 

 quickly effected by 

 drying in vacuo in the 

 presence of some sub- 

 stance such as strong 



. , . , FIG. 46. Gervk's air-pump for drying m vacuo. 



sulphuric acid which 



readily takes up water vapour. The vacuum produced by a 



water-pump is here not available, as in such a vacuum there 



