Oil the Electrical Properties of Different Kinds of Glass. 207 



small quantities of ferric oxide, alumina, and sodium oxide, its compo- 

 sition may be represented by the formula 



20SiO L >, 4BaO, 5K 2 0. 



Found. Calculated. 



Si0 2 53-29 52-54 



Bad 26-59 26-87 



K,0 20-12 20-58 



100 99-99 



XXIX and XXX. Both of these glasses are from the same pot, 

 XXIX being annealed, and XXX unannealed. Although they yielded 

 slightly different numbers on analysis, they are essentially the same, 

 and may be represented by one formula, viz., 



23SiOo, 5PbO, 3K-.0. 



Found. 



XXIX. XXX. Calculated. 



SiO, 48-39 50-17 49-69 



PbO 40-92 39-78 40-15 



K 2 10-68 10-04 10-15 



99-99 99-99 99-99 



XXXI and XXXII. These two glasses correspond, closely to XXIX 

 and XXX, soda, however, being substituted for potash. It was hoped, 

 in preparing them, to obtain a glass closely corresponding to XXIX 

 and XXX, soda being substituted for potash in exactly equivalent 

 quantity ; but Messrs. Powell found that in order to make the glass 

 Avorkable, it was necessary to add a slight excess of soda. XXXI was 

 annealed ; XXXII remained unannealed. The composition of both 

 may be expressed by the formula 



24Si0. 2 , 5PbO, 4Na 2 0. 



Found. 



99-99 99-99 99-98 



We desire, in conclusion, to express our thanks to Messrs. Powel. 

 and Sons for their kindness in preparing glasses for us, and to Messrs. 

 0. W. Griffith and Robert Abell for their help in the electrical and 

 chemical experiments respectively. 



VOL. LXVII. 



