232 Cultivation of Micro-organisms 



(pharmaceutic filter), wet with boiling water, and if the 

 gelatin be properly dissolved, the whole quantity should 

 pass through before cooling too much. Should only half 

 go through before cooling, the remainder must be returned 

 to the pot, heated to boiling once more, and then passed 

 through a new filter paper. As a matter of fact, gelatin 

 usually filters readily. A wise precaution is to catch the 

 first few centimeters in a test-tube and boil them, so that 

 if cloudiness show the presence of uncoagulated albumin, 

 the whole mass can be boiled again. The finished gelatin, 

 which is perfectly transparent and of an amber color, is at 

 once distributed into sterilized tubes and sterilized like the 

 bouillon by the intermittent method. The sterilization can 

 also be satisfactorily performed by the use of the autoclave 

 at i io-i 15 C. for fifteen minutes, but this method is prob- 

 ably less well adapted to the sterilization of gelatin than of 

 the other media, as the high degree of heat injures its sub- 

 sequent solidifying power. 



Sterilized gelatin or other culture medium can be kept 

 en masse indefinitely, but should a contaminating micro- 

 organism accidentally enter, the whole quantity will be 

 spoiled ; if, on the other hand, it be dispensed and kept in 

 tubes, several of them may be contaminated without serious 

 loss. When properly sterilized and protected, it should keep 

 indefinitely. 



AGAR-AGAR. 



Agar-agar is the commercial name of a preparation 

 made from a Ceylonese sea-weed. It reaches the market 

 in the form of long shreds of semi-transparent, isinglass- 

 like material, less commonly in long bars of compressed 

 flakes, rarely in the form of powder. It dissolves slowly 

 in boiling water with a resulting thick jelly when cold. 

 The jelly, which solidifies between 40 and 50 C., cannot 

 again be melted except by the elevation of its temperature 

 to the boiling-point. The culture medium made from agar- 

 agar is nearly transparent, and is almost as useful as gelatin, 

 as in addition to its ability to liquefy and solidify, it has 

 the decided advantage of remaining solid at comparatively 

 high temperatures so as to permit keeping the cultures grown 

 upon it at the incubation temperature, i. e., 37 C., at 

 which temperature gelatin is always liquid. 



The preparation of agar-agar is commonly described in 



