44 BACTERIOLOGY 



steam steriliser, by which means the turbidities sink to the 

 bottom, so that the solidfied mass of agar is perfectly clear 

 in the upper part. This mass is removed from the glass 

 by slightly warming it, the clear part is separated from the 

 turbid by cutting it with a knife, and is then chopped up 

 and re-melted. 



To simplify the preparation of nutrient agar it is 

 advisable to dissolve the finely-cut pieces of agar in boiling 

 water over the open fire, a sheet of asbestos being inter- 

 posed between the flame and the vessel ; this takes from 

 half to three-quarters of an hour, and the agar should not 

 be mixed with the broth until solution is complete. In this 

 case also it is better not to heat the liquid too long, as the 

 medium becomes dark if allowed to boil away, but from the 

 first to add as much more water from time to time as might 

 be lost through evaporation. In order to secure a clear 

 solution, the pieces of agar may be first of all laid in 

 2 per cent, hydrochloric or 5 per cent, acetic acid, which 

 is afterwards washed away with water. By Kichter's 

 method the agar is dissolved in wine by two hours' mace- 

 ration and subsequent boiling, and the solution is added to 

 bouillon. 



Tischutkin allows the required quantity of agar to swell 

 for 15 minutes in a very dilute solution of acetic acid, washes 

 it in pure water, and thereupon adds it to the broth, in 

 which it dissolves after only 3 to 5 minutes' boiling. When 

 it has been neutralised and cooled, the whites of two hen's 

 eggs are poured in, and the mixture kept in the steam 

 apparatus for half to three-quarters of an hour. The sub- 

 sequent filtration occupies only a short time, even without 

 the hot-water funnel. 



The agar when ready is filled into sterilised test-tubes 

 closed with cotton-wool, in such a way that about a third 

 of the test-tube is occupied by the fluid. Care must next 



