CULTURE MEDIA. 47 



1 : 2,000 to 1 : 500 of hydrochloric acid may be used for this purpose. 

 The addition of litmus to milk or other culture media is fre- 

 quently resorted to for the purpose of ascertaining whether acids or 

 alkalies are developed during the growth of bacteria under investi- 

 gation. The addition of aniline colors which are variously changed 

 by the products of growth of certain species has also been resorted 

 to in the differentiation of species. Various disinfecting agents, such 

 as carbolic acid, etc. , have also been used for the same purpose, and 

 it has been shown by experiment that some bacteria will grow in a 

 medium containing such agents in a proportion which would entirely 

 restrain the development of others. 



The soluble silicates which form a jelly-like mass have been 

 proposed as a culture medium for certain bacteria which do not grow 

 in the usual media. Kiihne (1890), Winogradsky (1891), and Sles- 

 kin (1891) have made experiments which indicate that this medium 

 has considerable value. 



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 je- 

 quirity seeds has been recommended by Kaufmann (1891), who has 

 found, by experimenting upon various bacteria, that such a medium 

 is useful in differentiating species. 



Thejequirity 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 



