CALCULATION OF PRELIMINARY RESULTS WITH ZINC. 2)7 



cumulative error is involved, but the shape of the curve in its different por- 

 tions the most important question can not be seriously altered, and the 

 percentage error can never be increased. 



It was now thought desirable to find a point on the curve between (5) 

 and (8). Therefore, a new amalgam, No. 9, was made. 



Weight amalgam No. 2 = 18.775 



Weight mercury = 77.90 



Factor for Zn dissolved = 1.002 



Approximate per cent of Zn in No. 9 = 0.029 



^ = 96,675 LV533 x 1>002 + l0 h = 0.71241 

 '9 18.775 13.542 ^ K c 9 



log 4^ = 0.77546 



c* 



1R 0.71241 -(-0.77546= 1.48787. 

 ^9 

 k (2-9) observed = 0.02088; corrected for errors in potentiometer, 



0.02090 ; reduced to international volts 0.02074. 



7T (2-9) calculated = 0.020895 ; Dtt for (2-9) = 0.000155. 



Probable value for Dtt (3-9) is the sum Dir (2-9) + Dtt (1-2) and 



D* (3-i). 



D* (3-9) - 00C,I 55 + 0.00141 -f- 0.00001 = 0.001575. 



Point (9) fits the curve quite closely. 



The curve as it now stands predicts that the potential between two amal- 

 gams containing less than four-hundredths of one per cent of zinc will be 

 greater than that required by the osmotic formula ; also that this effect will 

 increase as infinite dilution is approached. The latter consideration points 

 to experimental error rather than faulty assumptions in the formula, since 

 infinite dilution usually minimizes disturbing secondary effects. Oxidation, 

 the worst foe of workers in this field, was suspected ; calculation showed that 

 the absorption of 0.00003 gram of oxygen by each of the dilute amalgams, 

 No. 4, No. 9, No. 6, and No. 8, would account for the falling off of the curve. 

 It is quite possible that this quantity of the gas may have been absorbed on 

 the glass surfaces used, or contained in the hydrogen bubbled through the 

 amalgams, all our precautions notwithstanding. An amalgam so concen- 

 trated as No. 3 can not have been injured in this way ; therefore, if weighed 

 portions of it were diluted inside the cell with mercury free from air, the 

 values of D-n- thus determined would be free from error. Now, the risk of 

 losing minute drops of amalgam from the pipette tips would make it inex- 

 pedient to weigh out less than 10 grams in each determination. It is incon- 

 venient and wasteful to dilute such portions one hundred, fifty, or even ten 

 times ; there are also serious obstacles to the thorough mixing of such a com- 



