Manchester Memoirs, Vol. xlv. (1901), No. 15. 5 



^^ Protective Layer" Hypothesis. — Messrs. Jackson and 

 Rich endeavour to explain this behaviour by supposing 

 that an insoluble coating of silica is deposited upon the 

 particles, and that this protects them from further action 

 of the solvent {Joe. cit., pp. 9-15). 



Silica does 710 1 form a "-protective layer" — Now, it is 

 demonstrable that in many cases no such " protective " 

 layer is formed. In these cases the lead is not " protected " 

 by the silica or any other compound : it is wholly, or 

 almost wholly, removed on treatment with dilute acid. 

 The following experiments prove this point : 



It is beyond question that in such cases as the above 

 the silica does not form an effective protecting layer. It 

 is difficult to see what reason can be adduced to show why 

 silica should act differently in other cases. 



If it be said that the quantity of silica in the above 

 fritts is too small to act as a protective layer, the reply is 

 that some of them contain more silica than other fritts 

 which have much smaller solubility. For instance : — 



