Electrical Properties and Chemical Composition. 39 



difficulty in getting glasses made according to our own specifications. 

 We endeavoured to make the glasses ourselves, and several experi- 

 ments were made accordingly, both, in the laboratories here and at 

 the Ogwen Tile Works, where a large furnace had been erected for 

 the construction of tiles from slate dust. Some success was achieved, 

 but it was found impossible, without the expenditure of far more 

 time than could be spared, to obtain the glasses in a condition suit- 

 able for the experiments we wished to carry out. 



Through the kindness, however, of Messrs. Schott & Co., of Jena, 

 and of Messrs. Powell & Sons, Whitefriars, London, we have recently 

 obtained a number of specimens of glass all richer in lead than the 

 specimens formerly available, and, further, in some cases practically 

 free from soda. We have also had made to order by Messrs. Schott 

 specimens of their own glass, used, we believe, chiefly in the con- 

 struction of thermometers, as well as of a barium crown glass, 

 which have not hitherto, so far as we are aware, been experimented 

 with. 



Determination of Conductivity. The method of experimenting 

 followed was practically the same as that described in the paper 

 already referred to, but its nature may perhaps here be indicated. 



Owing to the large percentage of lead oxide in some of the glasses 

 prepared for us by Messrs. Schott, it was found impossible to blow 

 them into flasks, and they were therefore cast into plates ; the 

 arrangements therefore required some modification for their case. 



The specimens which were in the form of flasks were filled up 

 with mercury to the bottom of the stem (which in most cases was 

 about 8 or 9 inches long), and the flask thus filled was sunk in a 

 bath also containing mercury, so that the mercury was at the same 

 level inside and outside. One terminal of a circuit containing a 

 battery of about thirty secondary cells and a very sensitive galvano- 

 meter was connected to the mercury within the flask by a wire pass- 

 ing down the neck, while the other terminal was connected to the 

 mercury in the bath. 



The galvanometer was the instrument formerly used and described 

 in * Eoy. Soc. Proc.,' vol. 36, p. 287. It was carefully insulated, as 

 was also the reversing key, and all necessary precautions were taken 

 to make sure that the current passing through the galvanometer 

 was that passing through the walls of the flask between the mercury 

 coatings. Thus it was always verified that no deflection took place 

 when the wire was withdrawn from the flask and placed round the 

 outside of the neck. This test obviated the possibility of the 

 existence of any disturbing film of moisture on the surface of the 

 glass. The bath could be heated to any temperature required in 

 the experiments. 



The conductivity was calculated from deflection of the galvano- 



