52 BACTERIOLOGY 



a medium upon which to cultivate {EsmarcWs potato discs) ; 

 but they must be separately sterilised on from three to 

 five successive days in the steam steriliser. They may 

 also be kept stored in several glass dishes standing one 

 above the other, and which mutually cover and close each 

 other. 



In order to use potatoes in a transparent form, thin 

 slices can, after Wood's method, be cut from very white 

 potatoes, and firmly pressed upon sterilised slips of glass, 

 which are introduced into test-glasses and then sterilised. 



Instead of discs, cylinders may be punched with the 

 aid of a cork-borer out of cleaned and peeled potatoes. 

 Each cylinder can be split into two halves in its long axis, 

 and each half placed in a test-tube closed with cotton-wool. 

 After these test-tubes have been sterilised in the steam 

 apparatus for three days m succession, the surface of the 

 piece of potato is inoculated. It is advisable to support 

 the potato cylinders on cotton-wool or small glass tubes, 

 which can absorb the water of condensation. 



Potatoes are often pounded up after being peeled and 

 boiled, and are then pressed into little Erlenmeyer's flasks 

 {potato pap) ; and in this way a useful culture medium is 

 obtained after proper sterilisation. Eisenberghas modified 

 the process by using, instead of flasks, small boxes capable 

 of being closed with a glass lid, and by sealing these with 

 parafiin for permanent cultures. 



Nutrient materials made from potatoes possess a 

 slightly acid reaction, so that the surface of the potato 

 must be rendered alkaline with sodium bicarbonate solu- 

 tion for certain micro-organisms, whose growth demands 

 alkalinity. 



Eice, bread, and wafers. — Milk rice is prepared by 

 mixing skim-milk with two and a half times its quantity of 

 powdered rice. This mixture is boiled until a thick pap is 



