198 Profs. A. Gray and J. J. Dobbie. 



specimens are referred to in the paper by the numbers XXI 

 XXIV. 



We have now to communicate the results of some further experi- 

 ments, the object of which was to throw light on various points which 

 had arisen in connection with the previous experiments, and to afford 

 information as to whether the resistance or capacity of the glass was 

 affected by the process of annealing, or varied with time. The speci- 

 mens here referred to are numbered XXV XXXII, and were all 

 made, as nearly as possible, according to a previously prescribed com- 

 position, and in the form of flasks, with long thick-walled necks, 

 adapted for experiments by the direct-deflection method formerly 

 employed. Full particulars of the different specimens are given in 

 Table I below. 



The method of experimenting and the method employed were the 

 same as those described in our former paper. The chief sources of 

 error which had to be guarded against were, as before, surface conduc- 

 tion, due to moisture on the surface of the glass, and leakage at other 

 parts of the apparatus, due to the want of perfect insulation. The 

 most careful watch was kept throughout the experiments against the 

 possibility of inaccuracy from these causes, and tests were made in 

 connection with each determination to make sure that everything was 

 working correctly. 



The Resistance Experiments. 



Only a few days before the meeting of the Royal Society at which 

 the paper referred to above was read, a rough test was made of the 

 resistance of a flask (XXVII below) made for us by Messrs. Powell 

 and Sons, which had approximately the same composition as Specimen 

 XXI, the potash, however, being replaced by soda. It will be seen by 

 a reference to our former table of results, that XXI was a lead-potash 

 glass of very high specific resistance, certainly above 18000 x 10 10 at 

 1 30 C. It was anticipated from our experiments that the substitution 

 of soda for the potash in this glass would very greatly diminish the 

 specific resistance, and it was stated when the paper was read that 

 this conclusion had been verified. More accurate determinations made 

 since that time have confirmed this result, as will be seen by a com- 

 parison of Table I with the table given in the former paper. While 

 XXI had the resistance at about 130 C. quoted above, the specific 

 resistance of XXVII at about the same temperature was only 136 x 

 10 10 ; so that the substitution of soda for potash in the composition of 

 the glass diminishes the resistance of the glass to y^ of its former 

 amount. 



The influence of the substitution of soda for potash is still more 

 clearly brought out by a comparison of XXIX with XXXI, and XXX 

 with XXXII. Specimens XXIX and XXX are lead-potash glasses, 



