86 METHODS OF CULTIVATION OF BACTERIA 



of the reaction a quantitative estimate of the amount of indol 

 formation can be obtained. To do this a large culture, say 

 100 c.c., is distilled, and the colour obtained by applying the 

 test to the distillate in a Nessler's tube is matched against that 

 obtained with different amounts of a standard solution of indol 

 (prepared by dissolving 1 gr. indol in 5 c.c. absolute alcohol, and 

 making up to 500 c.c. with distilled water). 



There is no doubt that the Ehrlich test is from five to ten 

 times more delicate than the ordinary nitroso-indol reaction, and 

 it is of especial value in dealing with organisms of the coli- 

 typhoid group. With strains of b. coli it can often be obtained 

 in from twenty-four to forty-eight hours, but in the case of a nega- 

 tive result a culture of from six to seven days ought to be used. 

 The reaction is also obtainable with the cholera vibrio, but further 

 investigation is here necessary, as Marshall states that under 

 certain circumstances the nitrites formed by this bacterium may 

 have an inhibitory effect on the production of the rose colour. 



The Drying of Substances in uacuo. As many substances, 

 for example toxins and antitoxins, with which bacteriology is 

 concerned would be destroyed by drying with heat as is done in 

 ordinary chemical work, it is necessary to remove the water at 

 the ordinary room temperature. This is most quickly effected 

 by drying in vacua in the presence of some substance such as 

 strong 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 must always be water vapour present. An 

 air-pump is therefore to be employed. Here we have found the 

 Geryk pump most efficient, and it has this further advantage, 

 that its internal parts are lubricated with an oil of very low 

 vapour density, so that almost a perfect vacuum is obtainable. 

 The apparatus is shown in Fig. 37. The vacuum chamber 

 consists of a bell-jar set on a brass plate. A perforation in the 

 centre of the latter leads into the pipe b, which can be connected 

 by strong- walled rubber- tubing with the air-pump, and which 

 can be cut off from the latter by a stop-cock a. In using the 

 apparatus the substance to be dried is poured out in flat dishes 

 (one-half of a Petri capsule does very well), and these are stacked 

 alternately with similar dishes of strong sulphuric acid on a 

 stand which rests on the brass plate. The edge of the bell-jar 

 is well luted with unguentum resinse and placed in position and 

 the chamber exhausted. In a few hours, if, as is always advis- 

 able, each dish have contained only a thin layer of fluid, the 

 drying will be complete. The vacuum is then broken by 

 admitting air very slowly through a bye-pass c, and the bell-jar 



