130 BACTERIOLOGY 



be neutralized before the agar-agar is added. Then add 

 finely chopped or powdered agar-agar in the proportion of 

 1 to 1.5 per cent. Place the mixture in a porcelain-lined 

 iron vessel, and on the side of the vessel make a mark at 

 the height at which the level of the fluid stands; if a liter 

 of medium is being made, add about 250 to 300 c.c. more of 

 water and allow the mass to boil slowly, occasionally stirring, 

 over a free flame, from one and a half to two hours; or until 

 the excess of water i. e., the 250 or 300 c.c. that were 

 added has evaporated. Care must be taken that the 

 mixture does not boil over the sides of the vessel. From time 

 to time observe if the fluid has fallen below its original 

 level; if it has, add hot water until its volume of 1 liter is 

 restored. At the end of the time given remove the flame 

 and place the vessel containing the mixture in a large dish 

 of cold water; stir the agar-agar continuously until it has 

 cooled to about 68-70 C., and then add the white of one 

 egg which has been beaten up on about 50 c.c. of water; 

 or the ordinary dried albumin of commerce may be dissolved 

 in cold water in the proportion of about 10 per cent, and used; 

 the results are equally as good as when eggs are employed. 

 Mix this carefully throughout the agar-agar and allow the 

 mass to boil slowly for about another half-hour, observing 

 all the while the level of the fluid, which should not fall 

 below the liter mark. It is necessary to reduce the tempera- 

 ture of the mass to the point given, 68-70 C., otherwise 

 the coagulation of the albumin will occur suddenly in lumps 

 and masses as soon as it is 'added, and its clarifying action 

 will not be uniform. The process of clarification with the 

 egg is purely mechanical; the finer particles, which would 

 otherwise pass through the pores of the filter, being taken 

 up by the albumin as it coagulates and retained in the 

 coagula. 



