PROTOZOAN differences: resistance to heat 143 



and Blepharisma lateritia were transferred to an n/900 H2SO4 

 solution and then subjected to heat in the usual way, the fatal 

 temperature was not different from that of the animals in the 

 ordinary medium. Fine ('12) states that the acid content of an 

 infusion could be increased seven or eight times before interfering 

 with the development of animals present. There is reason to 

 believe that the acidity of the medium can be increased con- 

 siderably before any effect is produced on the thermal resistance. 



As to the effect of alkalies in modifying the death temperature 

 curve no decided- answer can be given as yet. The OH con- 

 centration increases during the life of a hay infusion as indicated 

 by the change of color (Peters '07). From statements made 

 above it is plain that the ordinary increase is not enough to 

 affect perceptibly the thermal resistance. 



The effect of salts in fairly high concentrations may be quite 

 marked. Paramecium caudatum transferred, with as little of 

 the medium as possible, to n/50 NaNOs and then subjected to 

 heat in the usual way was killed at a temperature two degrees 

 higher than was the case in the unchanged medium. Parame- 

 cium caudatum from another culture was transferred to n/50 

 NaCl and also to n/50 KNO3. In both these cases the whole 

 death temperature curve was shifted two degrees higher on the 

 scale. These salts then have the effect of increasing the resist- 

 ance to heat. To determine to what extent osmotic and to 

 what extent chemical factors are involved in the above results, 

 a more extensive series of experiments has been undertaken, 

 which will be discussed elsewhere. 



SUMxMARY 



1. Of the common species of Protozoa examined each one has 

 a resistance peculiarly its own. Under given conditions its resist- 

 ant is quite constant and differs more or less from that of other 

 species subjected to the same conditions. 



2. The amount of variation within the species may be con- 

 siderable. Under the conditions employed in these experiments 

 the fatal temperature zone may spread out over as much as six 

 degrees (Paramecium caudatum). 



