PRACTICAL BACTERIOLOGY. 



one litre of meat juice, twenty grammes of agar-agar, 

 ten grammes of peptone, five grammes of common salt. 

 The mixture is boiled in the steam steriliser until the 

 agar-agar commences to dissolve ; this takes about 

 four hours. Since the agar-agar only becomes com- 

 pletely dissolved at a higher temperature than the 

 boiling point of water, and further, since the boiling 

 point of the solution rises above 100° C, as it becomes 

 stronger, the operation is now best carried on in a 

 bath of concentrated salt solution. The agar-agar 

 solution is in this manner kept boiling from two to 

 three hours, until it becomes quite liquid and homo- 

 geneous. It is now filtered, being kept at boiling 

 point in the steam steriliser. This filtering is a some- 

 what wearisome process, which can only be shortened 

 by the thorough boiling of the solution in the salt- 

 water bath. Even then it takes many hours, or even 

 days, before all has passed through the filter. When 

 agar-agar is liquid, it is quite transparent, and of a 

 somewhat darker yellow than gelatine ; when, how- 

 ever, it solidifies, it is only transparent in thin plates ; 

 somewhat thicker plates are only translucent, and in a 

 mass it is nearly opaque. Should the liquid be not 

 quite clear after filtration, it must, like the other solu- 

 tions, he cleared with the white of an &^^, in order to 

 procure a nutrient medium suitable for all purposes. 

 In order to avoid this wearisome business of filtering, 

 many bacteriologists pour the solution into a tall 



