TOXICITY OF ACIDS TO INFUSORIA 77 



The efficiency of the alkaUne earth chlorides is as follows : 



Paramecium: Ca > Sr > Ba = Mg 

 Euplotes: Ca > Ba > Sr > Mg 



The antagonistic power of this group is on the whole greater than 

 that of NaCl as regards acetic acid. _Thus the length of life is 

 multiplied as follows: 



Ca ' Sr ' Ba_ Mg^ Na 



Paramecium : 23 X 15 X 3X 3X 4x 

 Euplotes: 10 X 4X 7x 3x2iX 



The chlorides of the heavy metals are rather less efficient an- 

 tagonists, partly because of their greater toxicity. The order of 

 antagonistic power is: 



Paramecium: Mg > Mn > Co > Cd = Ni = Zn 

 Euplotes: Mg > Mn > Co > Cd > Ni = Zn 



When used alone their relative toxicity is (table 2) 



Paramecium: Mg < Co < Mn < Cd < Ni < Zn 

 Euplotes: Mg < Mn < Co < Cd < Ni < Zn 



If the pure salts are combined with NaCl in the ratio of one to 

 ten molar, their toxicity is in most cases considerably diminished, 

 although the general relations are similar. There are a few ex- 

 ceptions: the toxicity of BaClg to Paramecium is unchanged and 

 that of SrCl2, CdCla and C0CI2 is noticeably increased by such an 

 addition. The order of toxicity of the pure salts given above 

 agrees only in part with those given for Paramecium by McClel- 

 land and Peters^" and by Woodruff. ^- 



Woodruff K < Zn < Sr < Mg < Mn < Co < Ni < Cd < H 

 McClelland and Peters Ba < Mg < Ni < Co < Zn < Mn 



In all these lists, however, Cd and Mn are among the most toxic, 

 Ni and Zn are at certain concentrations very toxic, and Co occu- 

 pies an intermediate position. The lack "of agreement is due in 

 part to differences in technique and to the concentrations studied 

 and probably also to race and species differences. 



If, instead of adding a single salt to the acid, we add both an 

 alkaline earth chloride and NaCl. in the ratio of one molecule of 



