CULTURE MEDIA. 47 



Winogradsky uses in the preparation of his silicate jelly the 

 following salts : 



Ammonium sulphate, .... 0.4 gramme. 



Magnesium sulphate, . . . . 0.05 



Potassium phosphate, . . . ~ 0.1 



Calcium chloride, .... a trace. 



Sodium carbonate, . . . 0.6 to 0.9 gramme. 



Distilled water, . . . . 100 grammes. 



To this he adds a solution of silicic acid. According to Kiihne, a 

 solution containing 3.4 per cent of silicic acid and having a specific 

 gravity of 1.02 may be preserved in a liquid condition. To this the 

 salts are added in greater or less amount, according to the consis- 

 tence desired. 



Sleskin states that a suitable jelly is formed by the addition of 

 1.15 to 1.45 per cent of the salts, and recommends that concentrated, 

 sterilized solutions be added to the acid. He dissolves separately, in 

 as little water as possible, the sulphates, the potassium phosphate 

 and sodium carbonate, and the calcium chloride. 



The use of a culture medium containing an extract from the 

 jequirity seeds has recently been recommended by Kaufmann (1891), 

 who has found, by experimenting upon various bacteria, that such a 

 medium is useful in differentiating species. 



'The jequirity solution, which may be used as a liquid medium 

 "or may be employed in the preparation of nutrient gelatin or agar, is 

 prepared as follows : Ten grammes of jequirity seeds are bruised in 

 a mortar and the shells removed ; they are then placed in one hun- 

 dred cubic centimetres of water and cooked for two hours in the steam 

 sterilizer ; after allowing the infusion to cool it is filtered. The fil- 

 tered liquid has a pale-yellow color and a neutral or slightly alkaline 

 reaction. Certain bacteria grow in this solution without producing 

 any change in its color ; others, which produce an acid reaction, 

 cause it to be decolorized ; others, which produce an alkaline reac- 

 tion of the medium, change the color to green. 



Cooked Potato. Schroter first used cooked potato as a culture 

 medium for certain chromogenic bacteria (1872), and Koch subse- 

 quently called attention to the great value of potato cultures for 

 differentiating species. His plan of preparing potatoes is as follows: 

 Sound potatoes are chosen in which the epidermis is intact. These 

 are thoroughly washed and scrubbed with a brush to remove all 

 dirt. The " eyes" and any bruised or discolored spots are removed 

 with a sharp-pointed knife. They are again thoroughly washed in 

 water, and are then placed for an hour in a bath containing 

 mercuric chloride in the proportion of 1 : 500, to thoroughly disinfect 

 the surface. They are then placed in a steam sterilizer for about 

 three-quarters of an hour, and after an interval of twenty-four hours 



