76 



LABORATORY EXERCISES IN BACTERIOLOGY. 



dish may profitably be left closed and uninoculated for the purpose of 

 observation. It is very common that in spite of all care exercised the 

 potatoes soon show the growth of numerous colonies, infection having 

 taken place from the air which was inclosed in the dish (unsterilized air) at 

 some time when the cover was removed, or entered the closed dish through 

 unobserved imperfections in the line of application of the cover to the 

 edge of the dish (should slight currents be in some way induced through 

 such small apertures). This last possible method of convection of infec- 

 tion may be prevented by applying a thick layer of vaseline to the margin 

 of the dish before applying the cover. 



(6) Potato Tubes. 



Exercise 24. One dozen potato tubes have been previously prepared 

 and once heated in the oven (Ex. 22). The same care 

 as to the selection of suitable potatoes is to be observed 

 as in the preceding exercise. They are then carefully 

 washed and rinsed in running water. The ends of each 

 potato are cut squarely off, and with a cork-borer of 

 a diameter slightly less than that of the tubes (or with 

 a knife if the cork-borer be not at hand), cylinders of 

 the potato substance are cut from each and placed 

 in water. After half a dozen such cylinders have been 

 obtained, each is to be cut in an oblique fashion so 

 that each resulting piece presents a round, flat end 

 and a large oval beveled surface for exposure in the 

 tube (Fig. 23). These pieces are now left in running 

 water for several hours (or over night if convenient), 

 the washing preventing their discoloration in the sub- 

 sequent sterilization. Thereafter each bit is intro- 

 duced into a tube, beveled surface uppermost, the flat 

 end resting on the bit of glass rod (or wad of cotton or 

 other rest) placed in the bottom of the tube, and the 

 cotton stopper is readjusted. When all the tubes are thus filled, they 

 are placed in the steam-bath (at 100 C. for thirty minutes, repeated 

 for fifteen minutes on the second and third days) or in the autoclave 

 (120 C. for thirty to forty minutes) for sterilization. 



(c) Occasionally, after the potatoes are washed and pared they are cut into broad, 

 thin slices, which are placed in Petri dishes and sterilized there just as if in tubes. In 

 this way extensive surface exposure may be obtained, of material advantage in the 

 separation of mixed cultures. 



Further, potatoes, after being cleansed and boiled, may be mashed and the result - 



FIG. 23. CULTURE 

 TUBE CONTAIN- 

 ING CYLINDER OF 

 POTATO RESTING 

 ON SMALL GLASS 

 ROD. 



