Immunity of Lower Organisms to Ethyl Alcohol 579 



In order to prevent error due to a weakening of the alcohol only 

 freshly made solutions were employed. A coverslip which could 

 be sealed in case of prolonged experimentation was used through- 

 out the series. 



Upon adding the organisms to the killing fluid considerable 

 dilution is possible. To avoid weakening of the killing fluid either 

 by the addition of more of the culture medium or less of the test 

 fluid a definite amount of culture medium — a capillary drop — was 

 added to a fixed quantity — one-fourth cubic centimeter — of the 

 killing fluid. 



Another source of error, more difficult to control, derives from 

 the condition of the organisms themselves. A study of diff"erent 

 strains of Paramecia^^ has shown that animals adjusted to diflPer- 

 ent media may react very diflTerently to a given stimulus. Only 

 animals of the same strain, therefore, should be used when a 

 comparative study of the eff'ects of diff"erent concentration is de- 

 sired. 



It is further of importance that a careful selection of typical 

 animals, even from the same strain, be made. In these experi- 

 ments the greatest care has been exercised to reject as abnormal 

 all organisms showing division, regeneration, marked variations 

 in color, considerable diff"erences in size" and the like. 



The striking diff^erences in reaction seen in animals tested at 

 difi^erent degrees of temperature make the control of variable tem- 

 peratures of the greatest importance. 



In a room rapidly heated and ranging from 20° to 22° C. I have 

 often found the culture media and reagents as low as 16° or I7°C. 

 In a condition of this kind the organisms will during the experi- 

 ments be tested at temperatures varying four or five degrees. 

 With extremes so great as these marked fluctuations in resistance 

 follow which make imperative the discarding of "room tempera- 

 ture" as manifestly too indefinite to be of service in work of this 

 kind. In the following series all materials — reagents, culture 

 media and the like — were kept at a constant temperature of 23° C. 



'^ In part i. hoc. cit. 



" It should be remarked that small cells are not necessarily weaker or less resistant than larger ones; 

 the opposite is often the case. In this selection the condition sought was uniformity of size. 



