NUTRIENT AGAR- AGAR 61 



soluble. The agar is then thoroughly washed in water to 

 free it from all traces of acid, after which it is well boiled 

 with a litre of nutrient broth, prepared by the method 

 already described, for about thirty-five to forty-five minutes, 

 until all the agar has become quite dissolved ; the water 

 lost by evaporation is replaced from time to time. Care is 

 then taken to see that the medium is faintly alkaline, after 

 which it is cleared with egg-albumin, as described under 

 the preparation of gelatine. The agar is then filtered 

 through a damp filter, as directed for the making of 

 nutrient gelatine. A very quick and good method of 

 filtering agar is to use a small jelly-bag, which is suspended 

 in the steam steriliser. Some workers prefer that the hot 

 agar should be allowed to stand in the steam steriliser in a 

 tall, cylindrical vessel till the flaky particles which cause 

 the turbidity sink to the bottom, when the clear agar can 

 be drawn off. 



The agar, when filtered, is run into test-tubes, as already 

 directed for gelatine. During the solidification of agar- 

 tubes, a few drops of water, the ' water of condensation,' 

 separates out, and prevents the firm adherence of the 

 medium to the tubes. Esmarch recommends the addition 

 of gum arabic to the medium, to prevent the slipping away 

 from the surface of the glass. The water of condensation 

 can also be got rid of by removing the india-rubber caps, 

 and allowing the tubes to remain for a few days in the 

 incubator at blood-heat. 



Agar jelly has the distinctive property of remaining solid 

 at 40° C, and only melting completely at 90° C. ; hence this 

 medium is well adapted for use as a culture medium for 

 those micro-organisms which must be grown at the higher 

 incubating temperatures. Nutrient agar is often quite clear 

 when hot, but is almost always cloudy and opaque on 

 cooling. 



