An Analysis of Plate-Glass. 317 



specimens, whilst digesting in water, gave any reaction with 

 the most delicate test-papers. 



To determine the extent of their solubility in water, from 

 four to live grammes were digested in that menstruum for 

 about forty-eight hours, the clear solution in each ease yielded 

 on evaporation but a slight residue, too small for determina- 

 tion. 



The specific gravity of these specimens of glass is as fol- 

 lows : — 



British Plate-Glass, . . . 2-319 



London Thames Plate-Glass, . . 2'242 



London and Manchester Plate-Glass, . 2-4:08 



A qualitative examination shewed the presence of silicic 

 acid, potash, soda, sesquioxide of iron, alumina, lime, and, in 

 one case, traces of manganese. 



The silicic acid was determined in the usual manner by 

 fusion with pure carbonate of potash. The sesquioxide of 

 iron, the alumina, and the lime, were afterwards precipitated 

 from the hydrochloric filtrate. 



To detei'mine the alkalies the glasses were decomposed by 

 means of hydrofluoric acid, in an apparatus recommended by 

 Brunner,* which consists of a leaden capsula with a flat 

 bottom, about 6 inches in diameter, and 4 inches high, in the 

 centre of which is placed a small leaden ring about an inch 

 and a half high, which serves as a support for a platinum 

 dish. The leaden capsula has a cover fitting perfectly tight. 



To set the apparatus in action it is necessary to cover the 

 bottom of the capsula with a layer of pulverised fluor-spar 

 about half an inch in thickness, and to pour upon it some 

 sulphuric acid, sufiicient to form a thick paste. A weighed 

 portion of the finely-powdered glass, after being put in the 

 platinum dish, is covered with water, and placed on the 

 leaden ring. The whole is then kept at a gentle heat either 

 on a sand-bath, or by means of a spirit-lamp. 



By a few preliminary experiments we found the action on 



* Poggendorff's Annaleo, xliv., p. 134. 

 VOL. XliVII. NO. XCIV.— OCTOBKIl 1849. 



