151 



In making the solntions a quaJitity of zinc eliloride was weiglied out and 

 dissolved in sucli an amount of water as to malve a solution somewhat 

 stronger than 3%. Some precipitate was formed and this was filtered out, 

 the remaining clear solution being then standardized and diluted to the 

 required strength. Colloidal membranes were iirepaicd liy coating vege- 

 table parchment, of as uniform thickness as possible, with collodion. These 

 were fastened on the ends of tulles having a diameter of about five centi- 

 meters, the tubes l)eiug of equal cross-section. Of tlie respective solutions 

 ."lO cc. were placed in separate tubes and the latter were immersed in sepa- 

 rate beakers of distilled water, adjustment being made so that the water 

 outside was at the same level as the solutions inside. No attempt was 

 made to keep the temperature constant l)ut changes in temperature affected 

 the tubes alike. After certain periods of time, as stated in the tables 

 below, the l>eakers containing the water and the diffused zinc salt were 

 removed and substitrrted by beakers containing fresh water. The zinc 

 which had diffused through the membranes was determined by titration 

 witli standard solution of potassium ferrocyanide, uranium nitrate being 

 used as indicator. Tlie amount of water placed outside the membranes was. 

 in every case, 100 cc. The standard solution of potassium ferrocyanide was 

 of .such concentration that 1 cc. was equivalent to 0.004.55 gram of zinc. 



In order to correct for arry possible difference in thickness or perme- 

 ability of the two membranes the solutions were interchanged after 284 

 hours, each .solution thereafter diffusing through the membrane that had 

 previously held the other. 



Iir the tables the symbols have the following significance : 



B. 'SI. = solution of zinc chloride 37^ mid aluminum sulphate '[.'>%, 



B. I'. =^ solution of zinc chloride 3%, 



t = hours for each period of diffusion, 



T = total hours before reversal of membranes, 



Tr = total hours after reversal of membranes, 



m = milligrams zinc chloride diffused each period, B. M.. 



M = total milligrams ziirc chloride dift'used, B. M., 



Mr = total milligrams diffused after reversal, B. M.. 



p = milligrams zinc chloride, diffused each period, B. P., 



P = total milligrams dift'used, B. P., 



Pr = total milligrams diffused after reversal, B. P., 



Cm = milligrams zinc chloride left in B. M. = 1,500 — M, 



Cp = milligrams zinc chloride left in B. P. = 1500 — P. 



