Methods of Determination of Pathogenic 3iicrohes. 123 



for bouillon. The product should be of a citron color 

 and perfectly transparent after filtration. It is then 

 introduced into test tubes plugged with wadding, and 

 sterilized. These tubes, filled to one-third of their 

 height, are then placed in a wire cage and kept in 

 the autoclave at 105°, during ten minutes. The 

 gelatin solidifies on cooling and the tubes are then 

 ready for use. 



Gelose or agar-agar. — This is a gelatinous substance 

 coming from certain algse of the Indian Archipelago. 



It is added to the bouillon whilst in preparation, in 

 the proportion of one to two per cent, and the further 

 process conducted as has already been described for 

 gelatin. The addition of the agar often renders the 

 liquid turbid, and this tarbidi'ty persists in spite of 

 filtration; it is with the view of obviating this in- 

 convenience that it has been recommended to inti- 

 mately mix with the mass, after cooling to 50°, the 

 white of an egg beaten up, and then bring the whole 

 to ebullition again. In coagulating, the albumen 

 carries with it all the substances in suspension and 

 the product becomes clearer. However it always re- 

 mains slightly opalescent. 



The filtration of this liquid, like that of gelatinized 

 liquids in general, ought to be performed while hot, 

 and it requires some time. A hot filtering apparatus 

 is employed, consisting of a glass funnel contained in 

 a larger one of copper and separated from this last 

 by a space full of water kept at the temperature at 

 which the gelatinous mixture becomes fluid. The 

 filtration thus obtained is slow, for the gelatin dries 

 and hardens upon the filter where it contacts the 

 walls of the glass funnel. 



