ROSA, VINAL, AND McDANIEL! SILVER VOLTAMETER 55 



electroh^te appreciabh' acid the decrease might be due to a de- 

 positing out of the hydrogen ions before the silver begins to de- 

 posit, since there is probably at the instant the current begins to 

 flow a greater difference of potential at the cathode than obtains 

 later, as the discontinuous character of the deposit indicates that 

 the potential difference must break thru a sort of surface film 

 and establish outlets for the current to the platinum cathode. 

 If this were possible, by making deposits of about 1 mg. each 

 from solutions containing acid and without acid, the difference 

 ought to be apparent. This we have done for varying lengths of 

 deposit, measuring the silver by titration with ammonium sul- 

 phocyanate. The results showed always an agreement of both 

 deposits between themselves and with the computed value for the 

 silver within the experimental error. But when we electrolj'zed 

 oxygen into the platinum cathode for four hours previous to 

 making a deposit a loss of about 0.2 mg. was observed, this was 

 not again found on subsequently using the bowl unless it had been 

 oxygenated as before. 



We have made a careful study of the effect of various septa 

 other than filter paper on the voltameter. First raw silk was in- 

 vestigated. Extracting raw silk in neutral double distilled water 

 we find the extract to be distinctly basic toward both methyl 

 and iodeosine and the concentrated extracts will restore the 

 color to magenta solution previously decolorized by sulphurous 

 acid, thus indicating the presence of an aldehyde. The colloidal 

 character of the solutions was ascertained by the ultra-microscope. 

 After prolonged washing or use in the voltameter the so-called 

 ''silk glue" which probably gives rise to the above effects is elimi- 

 nated and a hydrolytic process into amino acids appears causing 

 a reverse action in the voltameters. We have never claimed that 

 these two actions take place at the same time, as implied recently 

 by von Steinwehr. We think that silk like filter paper should be 

 excluded from use in the silver voltameter. 



Our experimental evidence indicates that porous pots if prop- 

 erly prepared are without influence on silver nitrate. A new 

 pot or one that has been baked may contain free alkali which must 

 be washed out with nitric acid and water until perfectly neutral 



