240 THE AMERICAN MONTHLY [Aug 



doing this remove the plug, flame the mouth of the tube, 

 and after quickly cooling, raise with the left hand the 

 edge of the cover on one side of the dish suflBciently to 

 allow of inserting the mouth of the tube, and hold it un- 

 til the agar is poured and replace it immediately. Label, 

 and number the Petri dishes to correspond with the dilu- 

 tions in the tubes, thus, plate 1 is from tube one, plate 2 

 is from tube 2, and plate 3 is from tube 3. In making the 

 dilutions it is important that the wire loop should be 

 flamed after making each transfer. 



Making Gelatin Plates. — These are prepared pre- 

 cisely as the agar plates with tliese exceptions. (1) The 

 gelatin is liquefied at a temperature of 45 deg. C. (2) 

 The plates when made are to be kept in the locker the 

 same as the gelatin stab cultures. (3) In hot weather it 

 is sometimes necessary to put a piece of ice in the reser- 

 voir under the glass plate on the leveling tripod to con- 

 geal the gelatin. 



The directions given above for making the dilutions are 

 applicable only when the original culture is moderately 

 clouded. If there are comparatively very few bacteria in 

 the liquid a larger quantit'y of the culture will be neces- 

 sary. If there are many more, as in turbid boullion or 

 slant agar culture, it will be necessary to take a much 

 smaller quantity for the first dilution. It is often desir- 

 able to make the first dilution in a tube of sterile water or 

 bouillon instead of gelatin or agar, and to make two rather 

 than three plates. 



Making Esmarch Roll Cultures. — For this purpose 

 gelatin is ordinarily used as agar does not adhere readily 

 to the sides of the tubes. It is sometimes used. Take 

 the desired number of large tubes of gelatin, liquefy, in- 

 oculate, label and number the dilutions as in making gela- 

 tin plate cultures. Place a block of ice about 6 inches 

 long in an agate iron tray. Melt a slight horizontal groove 

 in the ice with a test tube containing hotmediaor water. 



