66 APPLIED BACTERIOLOGY 



grow on an organic medium. In this preparation the 

 gelatinous consistency is obtained by means of dialysed 

 silicic acid. The method of preparation is as follows : Two 

 solutions of the following composition are prepared : 



(a) Ammonium sulphate, 0*4 (b) Potassium phosphate, 0' 1 



gramme. gramme. 



Magnesium sulphate, 0-05 Sodium carbonate, 0*75 



gramme. gramme. 



Calcium chloride, trace. Distilled water, SO'O c.c. 



Distilled water, 50"0 c.c. 



These two solutions are rendered sterile by the usual 

 method, after which they are mixed. 



A sterile solution of dialysed silicic acid is now prepared 

 as follows : A solution of potassium or sodium silicate is 

 poured into dilute hydrochloric acid ; the mixture is then 

 placed in a dialyser, the outside of which is kept surrounded 

 with running water during the first day, and subsequently 

 with distilled water, which is frequently changed until it 

 yields no trace of turbidity with silver nitrate, thus showing 

 the whole of the chlorides to have been extracted. The 

 contents of the dialyser, if the solution of alkaline silicate 

 originally employed was not too strong, will be quite 

 clear. This liquid is then poured into a flask and concen- 

 trated by boiling until it is of such a strength that it 

 is found that, on cooling a little of the solution and 

 mixing it with one-third of the above mixed alkaline solu- 

 tions, it readily gelatinises on standing. When the solution 

 of silicic acid is found to give this result, it is cooled, and 

 one-third to one-half of its volume of the mixed alkaline 

 solutions {a and b) are added, the solutions well mixed, and 

 at once poured into Petri dishes or flat-bottomed flasks. 

 The medium should gelatinise in from five to fifteen 

 minutes. The material containing the organisms for ex- 



