so , PRACTICAL BACTERIOLOGY. 



air steriliser at a temperature of about 160-1 80° C, in 

 the same way as t^e test-tubes ii^ed for receiving the 

 blood serum. The slices of potato on their dishes must 

 be kept in the steam steriliser for at least an hour, not 

 only to ensure their being thoroughly cooked, — bao- 

 teria, as a rule, will not grow on raw potatoes, — but 

 also to completely rid them of all germs that might be 

 clinging to them. 



The reason why potatoes are so diflScult to sterilise 



Fig. 5. — Cultivation Dish. 



is this : there are some kinds of bacteria, with great 

 powers of resistance (frequently classed together under 

 the common name of potato bacilli), which occur in 

 potatoes, especially so in the buds or " eyes," where 

 they are so well protected that it is very difficult to 

 kill them by disinfectant agents. It is therefore 

 necessary, in peeling the potatoes, to take the greatest 

 care to cut out all the eyes completely, and also to get 

 rid of all black or dirty places. If there are a great 

 many such black places in a potato, it shows that it is 

 unsuitable for purposes of cultivation. In the spring 

 especially it is ofcen difficult to obtain good potatoes 

 for this purpose. 



The potatoes may be cooked without havincr been 



