24 MICROORGANISMS IN WATER 



2 per cent, solution of sodium bicarbonate, or by pour- 

 ing some cubic centimetres of the solution into the 

 culture dish, so that the bottom is only just covered. 



In a paper by Kresling 1 the careful selection of the 

 particular potato employed is insisted upon in connec- 

 tion with the growth of the glanders bacillus (B. mallei) 

 on this medium. This organism produces acid in solu- 

 tions containing grape or milk sugar, but not in their 

 absence. This author states that if potatoes which have 

 been frost-bitten or which have begun to bud are used 

 for its cultivation the glanders bacillus produces so 

 much acid that its growth is not only greatly retarded 

 but entirely checked, for the above descriptions of 

 potatoes contain sugar. 



LIQUID CULTUEE MEDIA 



Liquid media have been long used by bacterio- 

 logists for cultivation purposes, in fact all the earlier 

 work on micro-organisms was carried out exclusively 

 by their means. Already, in 1857, Pasteur published 

 a memoir on the lactic ferment, in which he employed 

 a transparent culture liquid, and for many purposes 

 these liquid culture media are still preferable, and in 

 some cases quite indispensable. Indeed it may be said 

 generally that whilst the solid media afford great 

 advantages for the preparation of pure cultures and 

 the maintenance of pure growths, the liquid media are 

 of more especial value in the study of the chemical 

 products to which the micro-organisms give rise. 



The simplest, although but rarely available, method 

 of obtaining pure cultivations consists in starting a 

 growth of organisms in a liquid medium, which experi- 



1 ' Sur la Preparation et la Composition de la Malleine,' Archives des 

 Sciences Biologiques publiees par I'Institut Imperial de Medecine 

 Experimentale a St. Petersbourg, vol. i. No. 5, 1892, p. 723. 



