CULTURES, AND THEIR STUDY. 119 



by turning the dish in all directions. The solution is 

 emptied out, and the dish, which is always kept closed, 

 is ready for use. 



A levelling apparatus is required (Fig. 19). This con- 

 sists of a wooden tripod with adjustable screws, and a glass 

 dish covered by a flat plate of glass upon which a low 

 bell-jar stands. The glass dish is filled with broken ice 

 and water, covered with the glass plate, and then exactly 

 levelled by adjusting the screws under the legs of the 

 tripod. When level the cover is placed upon it, and it 

 is ready for use. 



Method (Fig. 20). A sterile platinum loop is dipped 

 into the material to be examined, a small quantity se- 



FIG. 20. Method of holding tubes during inoculation. 



cured, and stirred about so as to distribute it evenly 

 through a tube of the melted gelatin. If the material 

 under examination is very rich in bacteria, one loopful 

 may contain a million individuals, which, if spread out 

 in a thin layer, would develop so many colonies that it 

 would be impossible to see any one clearly ; hence the 

 necessity for a dilution. From the first tube a loopful 

 of gelatin is carried to a second tube of melted gelatin 

 and stirred well, so as to distribute the organisms evenly 

 through it. In this tube- we may have no more than ten 

 thousand organisms, and if the same method of dilution 

 be used again, the third tube may have only a few hun- 

 dreds, and a fourth only a few dozen colonies. 



After the tubes are prepared, one of the sterile glass 

 plates is caught by its edges, removed from the iron box, 

 and placed beneath the bell-glass upon the cold plate 



