29 



ANIWNE METHYWIOI.ET (EHK.I,ICH-WEIGERT). 



Saturated alcoholic solution of methyl-violet ii c.c. 



Absolute alcohol lo c.c. 



Aniline water lOO c.c. 



§ 34. Making aniline water. Aniline water is a saturated 

 aqueous solution of aniline oil. It is prepared by adding i 

 c.c. of aniline oil to 20 c.c. of distilled water and shaking 

 frequently for from 15 to 30 minutes. It is convenient to 

 use a stoppered vial or large test tube for mixing it. Filter 

 through a moistened filter paper. The filtrate should be 

 perfectly clear. If it is cloudy it should be refiltered before 

 using. 



EXERCISE VIII. 

 MAKING PLATE AND ESMARCH ROLL CULTURES. 



§ 35. Explanatory note. The general principle under- 

 lying the separation of bacteria by means of plate and roll 

 cultures is to dilute the substance containing the bacteria so 

 that the individual organisms will be separated from each 

 other by an appreciable distance and then fixed in a solid 

 medium where each organism can multiply into a growth 

 or colony without coming in contact with any other 

 organism or colony. For this purpose agar and gelatin are 

 used. Originally, Koch employed a rectangular piece of 

 glass for holding the layer of medium and protected it from 

 contamination by putting it under a bell jar. Later 

 Esmarch introduced the ' ' roll culture ' ' method which was 

 extensively followed, until the Petri dishes were introduced. 

 Since that time they have been largely used in place of the 

 Koch plate and Esmarch tube. On this account the plate 

 cultures of to-day are usually made in Petri dishes. The 

 roll culture, however, is occasionally made. 



