BACTERIOLOGICAL TECHNIQUE 25 



Starch gelatine. For the purpose of detection of the 

 enzyme amylase among the products of bacteria or other 

 growing cells, 2 per cent, of soluble starch is thoroughly 

 mixed with the melted gelatine medium. The starch in this 

 case should first be boiled with water to a clear paste in order 

 to obtain a homogeneous mixture with the nutrient gelatine. 



Silica jelly. Certain organisms will not grow on ordinary 

 nutrient gelatine, and a method was devised by Kiihne and 

 by Percy Frankland in which gelatinous silica is used instead of 

 gelatine, the medium being entirely free from organic matter. 



The method of preparation is as follows : 



Two solutions of the following composition are prepared : 



(a) Ammonium sulphate . . . . 0'4 gramme 



Magnesium sulphate . . . . 0'05 



Calcium chloride . . . . trace 



Distilled water . . . . x 50'0 c.c. 



(6) Potassium phosphate . . . . O'l gramme 



Sodium carbonate . . . . 0'75 



Distilled water . . . . 50'0 c.c. 



These two solutions are rendered sterile, and then mixed. 



A sterile solution of dialysed silicic acid is now prepared 

 as follows : A solution of potassium or sodium silicate (1.05- 

 1.06 sp. gr.) is poured into an equal volume of dilute hydro- 

 chloric acid (1.1 sp. gr.) ; 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 concentrated 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 its volume of the above mixed 



