Manchester Memoirs, Vol. xlv. (1900), No. % 9 



Dr. Thorpe, both in the Blue Book and in the Lecture, 

 surmises that this circumstance is due to the existence of 

 at least two different lead compounds, one of which is 

 easily soluble in dilute acid and is easily extracted in one 

 hour, and the other practically insoluble. 



If the latter surmise is correct it would appear that 

 the statement of Zulkowski,* to the effect that insolubility 

 only occurs in trisilicates, is incorrect. 



We are of opinion that Dr. Thorpe has in his surmise 

 overlooked a very important fact, namely, that on the 

 surfaces of the solid non-porous particles an insoluble 

 coating of oxides or salts is left, which at once puts a stop 

 to further action by the solvent. Hence it is at once 

 evident that until this coating is removed no further 

 solution is possible, that is, maximum solubility has been 

 attained, notwithstanding that the interior portions of the 

 particles have never been brought under the influence of 

 the solvent. The existence of this coating and its nature 

 may be deduced from figures to be found on page 32 of 

 the Blue Book. Three grams of a fritt lost 3-02 per cent. 

 of its weight in dilute acid and the remainder on further 

 treatment with acid was found to be insoluble. The table 

 on the following page shows the nature of the material 

 dissolved. 



From this it is seen that all the bases and the boracic 

 acid are dissolved in practically the same proportion, but 

 the quantity of silica dissolved is remarkably small, 

 namely, about ^V of the proportion in which the bases 

 and boracic acid are dissolved. We must, therefore, con- 

 clude that nearly all the silica separated from combina- 

 tion remains undissolved and will be found deposited, in 

 a great measure, at the place of its liberation, that is, on 

 the particles themselves ; tinlcss, indeed, the bases exist in 



• Chem. Ind., 1S99, 22,280 ; 1900, 23,108 ; Chem. Ztg. Repert, 1900, 24,98. 



