H. G. Thornton 243 



B. There must be as little interference between the developing 

 colonies as possible. For example, the rapid growth of spreading colonies 

 must be checked. 



C. The medium must not encourage rapid growth of fungi. 



D. Its reaction must vary but slightly. 



If the composition of a medium is to be sufficiently constant, it must 

 not contain food constituents whose composition varies. It is in this 

 respect that most of the older media failed, for the earlier work was 

 carried out upon media containing peptone, meat extract, "NahrstofE 

 Hey den" or some such food supply of uncertain composition. 



The first important development from this stage consisted in sim- 

 plifying the medium and greatly reducing the conteiit of organic matter. 

 Thus Fischer (4) tried a medium containing only soil extract and phos- 

 phate as food substances, and Temple (5) used 0-1 per cent, peptone as 

 the sole source of organic matter. It \vas found that this reduction in 

 organic matter lessened the growth of spreading colonies to some extent 

 and allowed higher counts to be obtained. 



At about this time there arose the idea of the "synthetic medium" 

 in which only pure chemical compounds were used as food constituents. 

 Fischer (4) describes such a medium, and Lipman and Brown (6) tried 

 agar media containing dextrose as the source of energy material and 

 KNO3 of (NH4)3S04 as the nitrogen supply. The medium which they 

 finally developed, however, contained peptone and thus was not truly a 

 "synthetic medium." Brown also tried media with casein, urea, albu- 

 men, and asparagine as sources of nitrogen. With the same idea. 

 Conn (3), in 1914, developed an agar medium to which nitrogen was 

 added as ammonium phosphate and sodium asparaginate. 



Although past work has thus shown that food substances can be 

 provided in the form of definite chemical compounds, there is great 

 difficulty in obtaining a gel-producing constituent of constant composi- 

 tion. Silicic acid is unsuitable for general use for this purpose. The 

 present author carried out some experiments with cerium hydrate gel, 

 at the suggestion of Dr Emil Hatscliek, but was not successful in using 

 it for plating. It would, therefore, appear that an organic colloid such 

 as agar or gelatine is alone suitable for this purpose. In the constancy 

 of the results obtained with it, agar is far superior to gelatine, both on 

 account of its less variable composition and because of its comparatively 

 low feeding value to bacteria. This is illustrated in the following experi- 

 ment (see Table I). Media were made up in which the food constituents 

 of Conn's sodium asparaginate medium (3), were added to four different 



16—2 



