Manchester JMemoirs, Vol. xlv. {\<^oo), No. %, ii 



{a) a chemical method in which the layer would be 

 removed by solution. 



(/;) a mechanical method in which it would be removed 

 by friction. 



In each case the removal of the la}'er would be 

 followed by treatment with hydrochloric acid. 



{A) CHEMICAL IMETHOD. 



Since dilute caustic soda solution readily dissolves 

 amorphous silica, we prepared a 5 per cent, solution with 

 which we treated a fritt after extraction with hydrochloric 

 acid. We found that an amount of silica passed into 

 solution and the fritt became again soluble in hydrochloric 

 acid. We found, however, that even the fresh fritt yielded 

 silica to the soda — though less in amount than after acid 

 extraction — and the solubility of the lead oxide in the 

 fritt was at the same time increased. It appeared that 

 the soda had a decomposing action on the fritt, and hence 

 a 10 per cent, solution was adopted. This we found did 

 not take up silica from the fresh fritt, though an amount 

 was extracted from the fritt after treatment with hydro- 

 chloric acid. But even in this case, with i per cent, solution, 

 there was a slight increase of the solubility of the fresh 

 fritt after treatment with the soda. 



The results are tabulated below. 



Action of 5 per cefit. NaOH Solution. 



a. I gram fritt + HCl - i -8 mg. PbO extrd. 



Ik Residue from «: + 5% NaOH soln. 103 mg. SiO., ,, 

 c. „ „ /^ + HC1 4-8 mg. PbO „ 



I gr. fresh fritt + 5% NaOH soln. 8-i mg. SiO, „ 

 .*. Layer of Silica = io'3 - 8"i - 2*2 mg. 



