lo Thorpe and Simmonds, Lead Fritts. 



in a cylinder. The quantity of lead oxide dissolved was 2"28 

 per cent, of the weight of the glaze after a quarter of an 

 hour's action, I ri6 after twelve hours, and \y^2 at the end 

 of the twenty-two hours. This is considered to support the 

 hypothesis that silica forms a protective layer ; since the 

 friction with the pebbles, it is argued, would continuously 

 remove this layer and allow the acid to act without 

 cessation upon the particles of fritt. 



The experiment as described is, however, of very doubt- 

 ful value. No information is given as to the behaviour of the 

 glaze under the same conditions but without the pebbles. 

 Nor are details furnished of the composition of the glaze. 

 Only two-fifths of the usual volume of acid was employed, 

 and the strength of this small volume would be very con- 

 siderably reduced by the acid-neutralising constituents of 

 the glaze. It is quite possible that the acid thus weakened 

 would act only slowly on certain kinds of fritts, altogether 

 irrespective of any supposed layer of silica and its removal 

 by pebbles. Moreover, if, as is commonly the case, the 

 glaze contained calcium carbonate, the effect of this when 

 the acid was very weak would be to render the action 

 still slower by the tendency to form insoluble lead 

 carbonate. Thus the gradual action of the acid described 

 by the authors is explicable on quite other grounds than 

 those alleged by them ; and in the absence of further data 

 the experiment must be looked upon as inconclusive. 

 Very similar objections may be urged also against the 

 second experiment, with " commercial di-silicate," des- 

 cribed on p. 14. 



Unsatisfactory fritts and glazes used in fackson and 

 Rich's experiments. — Finally, it is to be especially noted 

 that the glaze experimented with was one which, from 

 the figures given, would evidently if tested by the 

 standard method have furnished a solubility-figure of at 



