EFFECT OF DRYING CYSTS 91 



margin is too small to render this method of any 

 service in isolating cysts from a mixture of bacteria. 

 (4) Drying. As a preliminary test cysts in an 

 impure culture were smeared over the inside of test- 

 tubes which were plugged with wool and placed in 

 a closed vessel over concentrated sulphuric acid. 

 After three weeks cultures were made, and ex- 

 cystation readily occurred with two varieties of 

 sporing bacteria which had withstood the action 

 of the drying ; all non-sporing forms being appar- 

 ently killed. A series of similar tubes of cysts 

 with B. f. liq. and B. brunneum have since this been 

 desiccated over phosphoric pentoxide for six weeks, 

 and on examination all the bacteria have been 

 found to be killed, and, although a few of the cysts 

 look degenerate, the majority appear quite normal 

 and a pure mixed culture was made from them 

 with B. subtilis. If cysts are dried with pure 

 strains of bacteria, which are known to be destroyed 

 in this way, one can obtain a stock of cysts free from 

 all living bacteria, which are not open to the objec- 

 tion that the use of a germicide might injure the 

 cysts and so interfere with experiments carried out 

 with them. But, as remarked above, the HC1 

 method is more practicable. 



B. CHEMICAL METHODS : THE USE OF 

 GERMICIDES 



(1) Formalin. The employment of this sub- 

 stance has given us considerable trouble and we are 

 not yet sure of its action on the amoeba cyst itself. 

 Cultures of old cysts have been exposed to its 

 action both in solutions and on jelly media in 1-per- 



