On the Electrical Properties of Different Kinds of Glass. 203 



It was found that for the same fibre the same twisting couple pro- 

 duced the same residual twist, provided the two determinations were 

 not made in immediate succession. The rate at which the residual 

 twist came out was very great immediately after the twisting couple 

 had been taken off; and this rate diminished rapidly as the spot of 

 light approached its zero position on the scale. 



Chemical Composition of the Specimens of Glass. 



The results of the chemical analyses made for the different speci- 

 mens are stated briefly in Table I, which affords a conspectus of all the 

 results of the experiments now described. 



The following notes regarding the different specimens, containing 

 approximate empirical formulae for their composition, are, however, set 

 down here. 



XXV. This is an ordinary lime glass. The alkalies were not esti- 

 mated separately, and no formula can therefore be given for it. 



XXVI. This is an ordinary lead-alkali glass, containing consider- 

 able quantities both of potash and soda. After deducting ferric oxide, 

 alumina, and manganese, its composition may be represented by the 

 empirical formula 



43Si0 2 , 5PbO, 5Na 2 0, 3K 2 0. 



Found. Calculated. 



Si0 2 59-60 60-18 



PbO 26-44 26-00 



Na 2 7-44 7-23 



KoO 6-50 6-57 



99-98 99-98 



XXVII. This glass is composed of silica, lead oxide, and sodium 

 oxide, and is free from potassium. Allowing for the small amount of 

 ferric oxide and alumina which it contains, its composition may be 

 expressed by the empirical formula 



10Si0 2> 3PbO, 3Na 2 0. 



Found. Calculated. 



Si0 2 40-87 41-24 



PbO 45-33 45-98 



Na 2 13-80 12-78 



100 100 



XXVIII. This is a barium-potash glass, free from lead, and con- 

 taining only a very small amount of soda. After allowing for the 



