PREPARATION OF NUTRIENT MEDIA. 63 



The next process requires the greatest care and 

 attention. Some micro-organisms grow best in a 

 slightly acid, others in a neutral or slightly alkaline 

 medium. For example, for the growth and cha- 

 racteristic appearances of the comma bacillus of 

 Asiatic cholera a faintly alkaline soil is absolutely 

 essential. This slightly alkaline medium will be 

 found to answer best for most micro-organisms, and 

 may be obtained as follows : 



With a clean glass rod dipped into the mixture, 

 the re-action upon litmus paper may be ascer- 

 tained, and a concentrated solution of carbonate of 

 soda must be added drop by drop, until red litmus 

 paper becomes faintly blue. If it has been made 

 too alkaline it can be neutralised by the addition 

 of lactic acid. 



Finally, the mixture is heated for an hour 

 in the water-bath and filtered while hot. For the 

 filtration the hot-water apparatus can be used 

 with advantage, furnished with a filter of Swedish 

 paper made in the following way. 



About eighteen inches square of the best and 

 stoutest filtering paper is first folded in the middle, 

 and then, as in Fig. 26, creased into sixteen folds. 

 The filter is made to fit the glass funnel by 

 gathering up the folds like a fan, and cutting off 

 the superfluous part. The creasing of each fold 

 should be made firmly to within half an inch of 

 the apex of the filter, which part is to be gently 

 inserted into the tube of the funnel. To avoid 



