4o6 Pcdigreed Race of Param(Bcinm [Mar. 



of the organisms tested, at about 20° C. The criterion of death 

 was the stopping of the cilia and the consequent loss of motion of 

 the organism, It was possible to distinguish this point with great 

 exactness owing to long familiarity in handHng paramsecia in 

 pedigreed culture work. To ascertain the exactness of the method, 

 the toxicity of the salt was determined on successive days, and the 

 agreement of the results proved the conditions of the experiment 

 to be highly satisfactory. In certain cases over 100 determinations 

 were made before the desired toxicity was secured. 



The salts and acids employed were: AgNOg, HgCla, CuCla, 

 FeCls, PbClg, NiCIs, HCl, H2SO4, C0CI2, CdClg, ZnS04, MnClg, 

 MgCl2, SrCls, CaClg, and KCl. In almost every case the chloride 

 of the metal was used, but the fact that a sulphate and a nitrate 

 were employed does not render the data less comparable, because 

 the ionic potential of the three anions is very nearly the same. 

 Moreover, Mathews has shown that the same concentration of the 

 nitrate, sulphate and chloride of sodium are required to inhibit the 

 development of the eggs of Fitndtdus. This is also shown by the 

 toxicity of sulphuric and hydrochloric acid as found in these ex- 

 periments on paramsecia. (For details, see original paper.) 



The data secured clearly indicate a parallelism between the 

 smallest fatal concentration of the various cations and the ease with 

 which they throw off their charge, i. e., their ionic potential. As 

 in the results of previous workers, there are certain metals which 

 do not appear in the order of toxicity which would be expected 

 from their potential. However, one must consider that the living 

 cell is composed of a large variety of materials, each having specific 

 affinities for the different ions, and it is probable that the low 

 toxicity of copper, for example, is based on differences of this sort. 

 Cadmium and ferric iron are also out of place, just as they have 

 been found to be in their action on the eggs of Fimdtdus and certain 

 seedlings. Hydrogen is somewhat more toxic than one would 

 expect on first thought, but this is probably due to the high migra- 

 tion velocity of the hydrogen ions. 



Apart from these few exceptions all of the cations tested follow 

 the Order of their ionic potentials, the slight fluctuations being within 

 the errors of the experiment and may not be considered as excep- 



