498 



On the Electrical Qualities of Glass. [Apr. 24, 



intervals of time which the method renders necessary, we decided, 

 after all onr apparatus and arrangements had been planned, to adopt 

 some other method, and we are indebted to Sir "William Thomson for 

 the suggestion of a method which promises to be at once easy and 

 likely to give results nearly or altogether independent of " electrifica- 

 tion," and we have made arrangements for using it in our continued 

 experiments. One plate of an air-condenser, the capacity of which is 

 known, is joined to the upper end of a fine wire which forms the 

 thread of a pendulum, of which a small metal ball is the bob. 

 Another exactly similar pendulum has, connected to the upper end 

 of its wire, one plate of the experimental condenser, while the 

 other two plates of the condensers are connected to the case of an 

 electrometer used to measure the potential to which the air-condenser 

 is charged. Supposing the air-condenser to be charged, and its 

 potential measured by means of the electrometer, and the experi- 

 mental condenser to be uncharged, the ball connected with either 

 condenser is drawn aside, and let go so as to fall against the other 

 ball. The balls being of an equal size and of the same material, the 

 first ball will be brought to rest, and the other ball will immediately 

 be set into motion, the time of contact being very short and capable of 

 being approximately estimated. The second ball is caught, and pre- 

 vented from returning. The charge of the air-condenser has been 

 shared by the contact with the experimental condenser, and the dimi- 

 nution of potential is read off by the electrometer, and from the result 

 the specific inductive capacity of the material forming the experimental 

 condenser can be found ; and, as the time of contact is very short, 

 the result will be but very slightly, if at all, influenced by " electrifi- 

 cation." We may easily arrange, in ways which we need not here 

 point to, this method for use as a null method. 



We propose in a continuation of our experiments to eliminate the 

 uncertainty which still exists as to the effect of impurities in the 

 glass, and to test the effect of varying the proportions of the ingre- 

 dients, and of adding impurities by making up glasses from pure 

 chemicals. The results of experiments, such as those described above, 

 are so complicated by the presence of small quantities of what may 

 be called "foreign" ingredients, that it seems unlikely that more 

 definite results than those already obtained can be expected as to the 

 exact relation between composition and resistance unless such experi- 

 ments as we propose be made. 



We are also extending our experiments to crystals of quartz, fel- 

 spar, and other minerals, and to natural glasses, and further to glasses 

 produced by melting natural crystals which are capable of assuming 

 the vitreous condition. We hope to obtain results as to the relation 

 s>f electrical quality to the crystallographic axes of the crystal. 



