CULTURE-MEDIA. 77 



of one or two eggs and boil thoroughly. Avoid breaking 

 the eoagulum of egg which is designed to entangle the solid 

 particles that make the medium cloudy; stir at the bottom, 

 however, to prevent burning. Filter while hot, using filter- 

 paper or absorbent cotton covered with cheese-cloth. The 

 hot water funnel originally devised for the filtration of agar 

 is not necessary. If filtration is slow, the funnel and flask 

 may be placed inside of the steam sterilizer and kept heated 

 during filtration. The medium is collected in suitable flasks 

 or tubes plugged with cotton, and sterilized once in the auto- 

 clave or in the ordinary steam sterilizer for fifteen minutes 

 on each of three consecutive days. As agar is frequently 

 used for smear-cultures where a slanted medium is desired, 

 some of the tubes may be allowed to cool in a slanting posi- 

 tion. It is not well to keep on hand many tubes which 

 have been slanted, as the medium dries more rapidly. Agar 

 is not liquefied by bacteria as is gelatin. Its solidifying 

 qualities are impaired somewhat if the reaction be acid. 



The remarks on pages 72 to 74 with regard to the use of 

 fresh beef and the titration method for the preparation of 

 bouillon apply equally to agar-agar. 



Glycerin-agar is used extensively. It is agar, made as 

 above directed, to which 6 per cent, of glycerin is added 

 before sterilization. It is very useful in cultivating the 

 bacilli of tuberculosis and diphtheria. 



Sugar-cigar. Before sterilizing, i per cent, of either dex- 

 trose, lactose, saccharose, or other sugars may be added to 

 agar. With media containing sugar, litmus forms a useful 

 indicator of the production of acid. Enough tincture of 

 litmus is used to give the medium a blue color before ster- 

 ilization; the litmus is somewhat unstable and prone to 

 change its color during sterilization. Neutral red may also 

 be added in the same manner ; its color is said to be changed 

 by certain bacteria and not by others (see bacillus of typhoid 

 fever, and bacillus coli communis, Part IV.). 



