31 



with the dilutions in the tubes, thus, plate i 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. 



§ 38. Making gelatin plates. These are prepared pre- 

 cisely as the agar, plates with these exceptions. ( i ) The 

 gelatin is liquefied at a temperature of 45° 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 some- 

 times necessary to put a piece of ice in the reservoir under 

 the glass plate on the leveling tripod to congeal the gelatin. 



The directions given above for luaking the dilutions are 

 applicable only when the original culture is moderately 

 clouded. If there are comparatively very few bacteria in 

 the liquid a larger quantity of the culture will be necessary. 

 If there are many more, as in turbid bouillon or slant agar 

 culture, it will be necessary to take a much smaller quantity 

 for the first dilution. It is often desirable to make the first 

 dilution in a tube of sterile water or bouillon instead of gela- 

 tin or agar, and to make two rather than three plates. 



§39. 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 de- 

 sired number of large tubes of gelatin, liquefy, inoculate, 

 label and number the dilutions as in making gelatin 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 hot media or water. The inocu- 

 lated tubes are tipped and rolled so that the liquid gelatin 

 moistens the inside of the tube to within about a centimeter 

 of the plug. Then roll the tube rapidly in the groove on 

 the ice until the medium becomes solid. The gelatin should 

 not come in contact with the plug. In rolling the tube the 

 plugged end should always project beyond the ice. See 

 illustration in text books. 



