CARRIAGE BY HOUSE-FLIES 153 



these cysts when mixed with dilute eosin show under the micro- 

 scope a red background of stained debris while the white unstained 

 cysts stand out clearly. A few of the cysts, however, may stain 

 with eosin and these are probably dead ones. If the fasces mixed 

 with a little water is heated to boiling point for a second it will be 

 found that after this treatment the eosjn will stain all the cysts 

 a deep red instantaneously. In this case there can be little doubt 

 that the cysts have been killed by heat. Drying the cysts has a ' 

 similar effect so far as making them stain with eosin is concerned. 

 With disinfecting agents, the stronger the solution employed, the 

 more quickly do the cysts acquire the property of taking up the 

 stain. Reagents such as strong sublimate solutions which would 

 be expected to kill the cysts instantaneously similarly cause them 

 to stain with eosin. It seems, therefore, clear that the eosin-staining 

 cysts are dead, though it may be argued that others which do not 

 stain may be dead also or, at any rate, non-infective. Still, if we 

 accept the eosin test as a criterion and regard all unstained cysts 

 as living, the error in judgment will be on the safe side. The 

 following experiments have been made : 



(1) Drying. The cysts of E. histolytica do not appear to with- 

 stand drying, for they stain with eosin at once after this. Kuenen 

 and Swellengrebel likewise found that drying killed the cysts. The 

 test can be applied by simply allowing faeces to dry at laboratory 

 temperature. The dried faeces is emulsified with saline solution 

 or water, when it will be found that the cysts will stain at once 

 if eosin is added. 



(2) Moisture. The cysts will survive for over thirty days in 

 water, an observation which confirms the results of Kuenen 

 and Swellengrebel. Apparently the cysts survive best if there is 

 considerable dilution of faeces with water, so that intense bacterial 

 or fungoid overgrowth does not take place. 



(3) Chemical Agents. The experiments were conducted by 

 mixing solutions of chemicals of certain strength with equal 

 volumes of emulsion of faeces in water. Small quantities of the 

 mixture were taken out from time to time and tested by the 

 addition of a drop of eosin solution. The staining of the cysts by 

 the eosin occurred practically instantaneously when it took place 

 at all. When some time was required to kill all the cysts it was 

 found that during this period the percentage of stainable cysts 

 gradually increased. 



