BOUILLON, AGAR AND MILK. 235 



heated f or ^ an hour at 110°, or, what is even more prefer- 

 able, it is concentrated over a flame to less than i its 

 original volume. The vessel is finally weighed, and the 

 distilled water necessary to bring back the bouillon to the 

 original weight is added. The bouillon is again filtered 

 and the filtrate is then filled into tubes. The small test- 

 tubes (12 X 150 mm.) are, as a rule, desirable for this 

 purpose. These are plugged and sterilized in the usual 

 manner. 



The bouillon is then sterilized according to the direc- 

 tions given under gelatin (p. 162); that is to say, it is 

 exposed to steam-heat for 15 minutes on each of three 

 consecutive days. 



The bouillon prepared as above should be (1) perfectly 

 clear; (2) should be slightly alkaline in reaction; and, (3) 

 should not cloud on heating. 



Agar-Agar. 



One liter of bouillon is prepared as above, except that 

 the second heating is omitted. 20 g. of agar (2 per cent.) 

 are cut up into as small pieces as possible, and these are 

 then added to the bouillon which has been returned to the 

 clean jar. The jar and contents are weighedj, and then 

 placed over a free flame to boil. The heat shojild.be just 

 sufficient to gently boil the liquid, otherwise the liquid is 

 liable to froth and run over. The liquid is boiled for ^ - f of 

 an hour, or until the agar has completely dissolved. This 

 point can be readily ascertained by drawing up the liquid 

 into a clean glass tube or pipette. No solid, translucent 

 lumps should be visible. The jar and contents are now 

 weighed, and distilled water is added to replace that lost 

 by evaporation. 



The jar is then placed in the steam sterilizer; or, better, 

 immersed in a water-bath at a constant temperature of 



