BY R. GREIG SMITH. 109 



methods or media certain hypotheses regarding the behaviour of 

 the bacteria must first be formulated. Stutzer and Hartleb would 

 probably have first formed the hypothesis regarding the action of 

 nitrif3dng organisms, and then have endeavoured to induce nitri- 

 fication in a solution of ammonium sulphate. The dilute solution 

 of ammonium sulphate employed in testing nitrification is a 

 selective medium, since it favours the growth of the nitrous 

 organisms and hinders the multiplication of others. Since nitri- 

 fication has been suggested, one of the points of this investigation 

 should be to test the idea, and see to what extent it is corrobo- 

 rated. 



Another idea that gives the cue for a selective medium is based 

 upon the disintegration of the cement through loss of lime. 

 Whether it is removed as the hydrate, the bicarbonate, or other 

 salt, e.g., nitrite, we cannot tell. It is probable, however, 

 that the surfaces of the disintegrating cement particles are more 

 or less alkaline, and if such is the case the bacteria that can 

 withstand or grow only in the presence of alkali are more likely 

 to be responsible for the disintegration than those which show an 

 antipathy to alkaline media. The majority of bacteria prefer 

 small amounts of alkali, say a medium containing 0'05 % sodium 

 carbonate, but there are not so many that can grow in a medium 

 as alkaline as lime water, which contains 0*28 % calcium oxide. 

 It may be said in parenthesis that the bacteria which normally 

 inhabit the intestinal tract of mammals can grow in media con- 

 taining at least 0*75 % sodium carbonate, but as I have indicated 

 in a former paper, the water which supplies Sydney is pure, and 

 therefore the possible presence of these organisms may be dis- 

 missed. There is, however, the question of what bacteria in the 

 disintegrated cement can be separated by means of increasin«- 

 amounts of alkali. Sodium carbonate is the most convenient 

 alkali to use, since it is soluble and does not alter during the 

 processes of sterilising the media and growing the organisms. 

 Lime would be the ideal alkali to employ in this case, but its 

 relative insolubility and its tendency to form the insoluble car- 

 bonate precludes its use. 



