CHARACTER OF GROWTHS 85 



24 C., and cannot therefore be placed in the blood-heat 

 incubator without becoming for practical purposes a 

 fluid medium. Agar, however and this is one of its 

 most valuable properties does not liquefy below a 

 temperature of 97-99 C., though when once liquefied 

 it does not set again until the temperature has fallen 

 to about 45 C. Gelatin is therefore usually reserved 

 for use at low temperatures, while agar, blood-serum, 

 potato, and the fluid media can be used indifferently 

 either at low or at high temperatures. Agar is often a 

 better cultivating medium than gelatin, even at low 

 temperatures, probably because it is so much moister. 

 The growths in fluid media are usually of the nature of a 

 general turbidity and are not particularly characteristic, 

 but sometimes an organism produces a film on the surface 

 which another similar organism does not, or the medium 

 remains clear, the growth forming a flocculent deposit, 

 thus affording a distinction. Not only do the characters 

 of the growths of organisms on media differ more or less, 

 but in some instances chemical changes occur in the media 

 which afford valuable information in the differentiation 

 of species. Thus many organisms exert a peptonising 

 effect on gelatin, and render it fluid sooner or later, while 

 others have no such action. Milk is coagulated by some 

 organisms, the coagulation being brought about in one 

 of two ways, either by the production of acids and pre- 

 cipitation of the caseinogen, or by the action of a rennet- 

 like ferment with the formation of a clot of casein. Most 

 organisms which liquefy gelatin coagulate milk, but 

 the converse is not the case. Agar is carbohydrate, not 

 albuminoid, in nature, and only two or three organisms 

 are known which liquefy it. In fluid media, such as 

 broth and peptone water, chemical tests can be applied 

 especially for indole, which is formed by some organisms 

 but not by others. 



