AGAR 39 



equivalent to 0.1% HC1, the agar dissolves very readily, 

 filters quickly, the resultant filtrate being a light yellow, 

 transparent, slippery, watery solution which does not 

 solidify upon cooling. If a smaller percentage of hydro- 

 chloric acid is used, solidification occurs (below 40 C.) 

 but the jelly will not " stand up " and is therefore useless 

 for agar slant or plate cultures. A large amount of con- 

 densation water is present also. 



If agar is dissolved in a weak alkaline or neutral broth, 

 a thick, reddish-brown, viscous liquid is obtained which 

 filters slowly and solidifies quickly at 40 C., to a very solid, 

 opaque, dry jelly, having but little condensation water; 

 it retains its shape well in slants and in plates. Thus 

 the value of the agar as a solid culture medium is raised or 

 lowered according to the cjegree of alkalinity or acidity. 



It must be noted in addition, however, that when once 

 the solidifying property of agar is destroyed by the presence 

 of an excess of acid in its solution, this property can never 

 be regained by neutralization with alkali; the acid per- 

 manently destroys the reversibility of the colloid. 



The melting-point of agar (of 1.5% in neutral solution) 

 is 97 C. and although its solidifying point is at 40 C., 

 when once it has solidified it will stand up in the thermostat 

 at a temperature of 50 C. For bacteriological purposes, 

 only that form of agar can be used which remains fluid at 

 from 38 to 40 C. Agar which remains fluid only at a 

 temperature above this point would be too hot when in 

 a fluid state for use; the vitality of organisms introduced 

 would be impaired or destroyed by the high temperature. 



Difficulties are encountered in the preparation of a solid 

 culture medium from agar, due to its slow solubility, vis- 

 cosity and consequent slow filterability. Its solution 

 (digestion) is effected, as mentioned above, by a long 

 heating in a water-bath, steam sterilizer, autoclav, or over 

 a free flame. The length of time required for complete 

 digestion depends upon three things: The reaction of the 



