244 Development of a Standardised Agar Medium 



brands of agar and three different brands of gelatine. The agar media 

 were sterihsed in the autoclave at 15 lbs. for 15 minutes and the gelatine 

 for 20 minutes at 100° C. on three consecutive days. The media A and B 

 were made up with the same agar, all the constituents being separately 

 weighed out in each case, in order to test the error involved in preparing 

 the media. This error appears to be negligible. The media were tested 

 with regard to their acidity and their capacity for colony development. 



The H-ion concentration was measured by the indicator method of 

 Clark and Lubs, both before and after sterilisation. 



The greater constancy of agar is noticeable both in the original re- 

 action and in the smaller change which occurs on sterilisation. To test 

 the colony development, six parallel platings were poured with each 

 medium, from a single diluted suspension of Rothamsted soil. The results 

 again show the advantage of agar in that it is less variable in its effects, 

 a feature evidently connected with the greater constancy in reaction 

 between samples. 



Table I. 



Variability between Samples of Agar and Gelatine. 



Sample E in the above experiment was an obviously impure agar 

 powder such as would not have been used in routine work. The somewhat 

 abnormal results obtained with it, however, show the advisability of 

 employing some method of washing the agar before use. Fellers (7) 

 found that agar contained compounds of Ca, Mg, S and N which were 

 soluble in 0-5 per cent. HCl. He also observed that agar could support 

 a slight growth of bacteria which produced ammonia therefrom. Several 

 methods of washing and purifying agar have been tried by various authors. 

 Thus Fellers (7) made a sol of 5 per cent, agar in distilled water and 

 precipitated this in alcohol. Cunningham (8) washed agar in dilute acid, 

 filtered it through cotton-wool in an autoclave, and dried the filtered 

 product in an oven. It is claimed that this product is purified and that 



