198 Cultivation of Micro-organisms 



be used, the cylinders being cut transversely, so that a number, 

 each about an inch and a half in length, can be cut from one potato. 

 The skin is removed from the cylinders by cutting off the ends, after 

 which each cylinder is cut in two by an oblique incision, so as to leave 

 a broad, flat surface. The half-cylinders are placed each in a test- 

 tube previously sterilized, and are exposed three times, for half an 

 hour each, to the streaming steam of the sterilizer. This steaming 

 cooks the potato and also sterilizes it. Such potato cylinders are 

 apt to deteriorate rapidly, first by turning very dark, second by 

 drying so as to be useless. Abbott has shown that if the cut cylinders 

 be allowed to stand for twelve hours in running water before being 

 dispensed in the tubes, they are not so apt to turn dark. Drying 

 may also be prevented by adding a few drops of clean water to each 

 tube before sterilizing. Some workers insert a bit of glass or a 

 pledget of glass wool into the bottom of the tube so as to support 

 the potato and keep it up out of the water. It is not necessary to 

 have a special small chamber blown in the tube to contain this water, 



only a small quantity of which need be added. 



The special reservoir increases the trouble of 



cleaning the tubes. 

 If the work to be done with potatoes is to be 



accurate, it is necessary to correct their variable 



reaction, especially if the acids have not been 



suflSciently removed by the washing in running 



water already described. 



To do this the cut cylinders are placed in a 



measured quantity of distilled water and 

 Fig. 46. — Ravenel's steamed for about an hour. The reaction of 

 potato cutter. the water is then determined by titration and 



the desired amount of sodium hydroxid added 

 to correct the reaction, after which the potatoes are steamed in the 

 corrected solution for about thirty minutes before being placed in 

 the tubes. 



A potato-juice has also been suggested, and is of some value. 

 It is made thus: To 300 cc. of water 100 grams of grated potato are 

 added, and allowed to stand on ice over night. Of the pulp, 300 

 cc. are expressed through a cloth and cooked for an hour on a water- 

 bath. After cooking, the liquid is filtered, titrated if desired, and 

 receives an addition of 4 per cent, of glycerin. Upon this medium 

 the tubercle bacillus grows well, especially when the reaction of the 

 medium be acid. 



Milk. — Milk is a useful culture-medium. As the cream which 

 rises to the top is a source of inconvenience, it is best to secure fresh 

 milk from which the cream has been removed by a centrifugal ma- 

 chine. It is given the desired degree of alkaUnity by titration, dis- 

 pensed in sterile tubes, and sterilized by steam by the intermittent 

 method or in the autoclave. The opaque nature of this culture-me- 



