Electrical Properties and Chemical Composition. 43 



conclusion that a glass, which approaches in composition to a definite 

 chemical compound, has a high resistance. 



Our knowledge of the chemistry of glasses is still very imperfect, 

 and we cannot say in what way the silica is distributed amongst the 

 bases. We give, therefore, merely the relative number of formula 

 weights of each oxide present, calculated from the analytical num- 

 bers, after allowing for the elimination of traces of foreign matter. 



Specimen XXL 



After allowing for traces of iron and alumina, the composition of 

 this glass may be expressed by the formula 



22Si0 2 ,5PbO,2K 2 0, 

 or 



5(PbO,2Si0 2 ) + 2(K 2 0,6Si0 2 ). 



Found. Calculated. 



Si0 2 50-72 50-53 



PbO 42-32 42-33 



K 2 6-96 7-13 



100-00 99-99 



Specimen XXII. 



This glass is essentially a lead potash glass mixed with a very 

 small quantity of a lead soda glass. Allowing for the small quan- 

 tity of manganese oxide, arsenious oxide, and other impurities 

 present, and eliminating the soda and a corresponding quantity of 

 silica, calculated on the assumption that each formula weight of soda 

 is in combination with three of silica, we obtain as an expression for 

 the composition of the lead potash glass 



17Si0 2 ,5PbO,2K 2 0. 



Found. Calculated. 



Si0 2 44-07 44-11 



PbO 47-72 47-82 



K 2 8-19 8-06 



99-98 99-99 



Specimen XXIII. 



This is a borosilicate of barium and alumina. After allowing for 

 the small quantities of arsenious oxide and manganese oxide which 

 it contains, it has the composition 



28Si0 2 ,16BaO,3Al 2 3 9B 2 3 . 



E 2 



