474 Jacques Loeb 



in the precipitating action of different salts should be ascribed to a diffe- 

 rence in the depressing effect which salts have on the P. D. between the 

 colloidal partióles and the aqueous solution. Northrop and De Kruif ^ 

 have shown that certain bacteria agglutinate when their P. D. is depres- 

 sed by electrolytes below the critica! valué of 15 millivolts. 



The writer has recently investigated the connection between the in- 

 fluence of different salts on the P. D. of fine collodion partióles and the 

 precipitation of the suspensión by the same salts. In these experiments 

 there existed a sharp limit for the precipitating effect of different salts. 

 The suspensión was made by dissolving collodion (after the evaporation 

 of the ether and alcohol) in a mixture of much acetone with little water. 

 When the acetone was removed by evaporation, a milky suspensión of 

 the collodion partióles remained. The suspensión was centrifuged and 

 in this way a layer of partióles not varying too much in size was obtained. 



One drop of this suspensión was added to lO ce. of a salt solution 

 and the time of settling observed. The partióles settle very slowly by 

 gravity, the larger partióles settling, of oourse, more rapidly than the 

 smaller ones. The suspensión remains milky for days and slightly opa- 

 que tor weeks. In a salt solution of the proper conoentration the sus- 

 pensión beoomes perfeotly olear in less than 12 hours (at about 1 8° C). 

 In this case the small partióles ooalesoe into larger ones which settle very 

 rapidly. It was thus possible to ascertain with great acouracy the oriti- 

 oal precipitating or ooagulating conoentration for each salt. 



The potential difference between the partióles and the surrounding 

 aqueous solution was ascertained by experiments on cataphoresis. The 

 rate of migration of the partióles in an electrical field was measured with 

 Northrop's apparatus - with non-polarizable eleotrodes, and from the rate 

 of migration the P. D. between the partióles and aqueous solution oould 

 be oalculated wiLh the aid of the equation used by Ellis, Powis, Burton, 

 Northrop, and others '. The details of these measurements will be given 

 in a fuUer paper to appear in the Journal of General PJiysiology, and the 

 writer will confine himself to the publication of a few figures which corre- 

 late the precipitating effect of salts on the suspensión with the effect of 

 the same salts on the P. D. between partióles and aqueous solutions. 



' Northrop, J. II., and De Kruif, PH. J. Gen. PhvsioL, 1921-22, iv, 639. 



2 Northrop,]. H.: J. Gen. P/iysioL, 1921-22, iv, 629. 



* This ec^uation can be found in Burton'sbook (p. 137) and in Powis' paper. 



