214 Cultures, and their Study 



So great is their historic interest, however, that it would be a 

 great omission not to describe the original method in detail. 

 Apparatus. Half a dozen glass plates, measuring about 

 6 by 4 inches, free from bubbles and scratches and ground at 

 the edges, are carefully cleaned, placed in a sheet-iron box 

 made to receive them, and sterilized in the hot-air closet. 

 The box is kept tightly closed, and in it the sterilized plates 

 can be kept indefinitely before use. 



A moist chamber, or double dish, about 10 inches in diam- 

 eter and 3 inches deep, the upper half being just enough 

 larger than the lower to allow it to close over it, is carefully 

 washed. A sheet of bibulous paper is placed in the bottom, 

 so that some moisture can be retained, and a i : 1000 bichlorid 

 of mercury solution poured in and brought in contact with 



the sides, top, and bottom 

 by turning the dish in all 

 directions. The solution is 

 emptied out, and the dish, 

 which is kept closed, is 

 ready for use. 



A leveling apparatus is 

 required (Fig. 41). It con- 

 sists of a wooden tripod with 

 adjustable screws, and a 

 glass dish covered by a flat 



Fig. 41. Complete leveling ap- 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 leveled 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. A sterile platinum loop is dipped into the mate- 

 rial to be examined, a small quantity secured, and stirred 

 about so as to distribute it evenly throughout the con- 

 tents of a tube of melted gelatin. If the material under 

 examination be very rich in bacteria, one loopful may con- 

 tain 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 further dilution 

 becomes necessary. From the first tube, therefore, a loop- 

 ful of gelatin is carried to a second and stirred well, so as 

 to distribute the organisms evenly throughout its contents. 



