22 



MICRO-ORGANISMS IN WATER 



of the tube may collect at the bottom without interfer- 

 ing with the potato culture, whilst the latter is thus also 

 furnished with a support upon which 

 it can rest. 



The same end may be more simply 

 secured by placing a small pad of sterile 

 cotton-wool at the bottom of an ordinary 

 test-tube. 



Hoizs Potato gelatine. — This me- 

 dium [was devised by Holz ^ originally 

 for the cultivation of the typhoid bacil- 

 lus, and is prepared in the following 

 manner : Potatoes are carefully washed 

 and peeled, and then pulverised by 

 means of an ordinary kitchen grater. 

 The gratings are collected in glass dishes, 

 and subsequently pressed through a 

 clean cloth, the resulting juice being 

 collected in a flask, which is subse- 

 quently plugged with cotton-wool and 



Fig. 4.— Eoux's Me- 

 thod OF CULTIVAT- 

 ING Organisms on 



Slices oe Potato, allowcd to Stand for twcuty-four hours 



A, growth produced by „o n rrn t • ji T j. 



organism; b, suce of at lU C. ilie liQuiQ assumes a ciirty 



potato. n /> T • 



brown colour, and after thus standmg it 

 is filtered, and the filtrate heated for half an hour in the 

 steam steriliser and again filtered. To 400 grams of this 

 now quite clear potato juice are added forty grams of 

 gelatine, and the whole heated for three-quarters of an 

 hour in the steam steriliser, after which it is filtered 

 and poured into test-tubes. In the test-tubes it is again 

 sterilised for a quarter of an hour on three successive 

 days. This potato gelatine is clear, transparent, and 

 slightly brown in colour. In order to neutralise the 

 material it is necessary to add about 1*6 c.c. of deci- 

 normal caustic potash to ever)'" ten grants. 



' Zeitsclirift f. Hygiene, vol. viii. 3-890, p. 143. 



