250 



REPORT 1879. 



5. On Secular Changes in the Specific Inductive Capacity of Glass. By 

 J. E. H. Gordon, B.A., Assistant Secretary of the British Association. 



At Christmas 1877 I made some determinations of the specific inductive capacity 

 of optical glass by a method which has already been fully described both before 

 this section and elsewhere. 1 



At the end of July 1879 I commenced a repetition of the experiments, using 

 the same slabs of glass, and was surprised to find a large increase in the specific 

 inductive capacity in every case. In some cases the increase was as much as 

 twenty per cent. 



The following is a table of the results : — 



Specific Inductive Capacity of Optical Glass. 



The arrangement of the apparatus, including the coil and rapid break, was 

 precisely the same as in my earlier experiments. The electromotive force was as 

 nearly as possible the same, and experiment has shown that moderate variations 

 in it do not affect the results. 



The differences observed might have been caused by any one of three things : — 



(1) By error in the 1879 experiments ; 



(2) By error in the 1877 experiments ; 



(3) By a change in the specific inductive capacity of the glass between 

 Christmas 1877 and July 1879. 



Careful repetition of the 1879 experiments has convinced me that there is no 

 error in them. 



If the difference is caused by error in the 1877 experiments, then in 1877 I 

 must have obtained too low a result. With my induction balance the effect of 

 covering the dielectric with a well-conducting film is to prevent observation ; the 

 effect of covering it with a badly-conducting film is to give too low a result. 



Before rejecting the second explanation of the difference, based on the hypothesis 

 of error in the 1877 experiments, it is therefore necessary to prove that in 1877 

 there was no film on the surface of the glass of sufficient conducting power to 

 cause a large error in the results. 



In 1877 the glasses were not washed by immersion in water, but were 

 thoroughly cleaned with a glass-cloth and wash-leather. To the best of my recol- 

 lection they were first rubbed with a damp cloth, then with a dry one, and then 

 polished with the leather, being frequently breathed on during the process, and 

 then usually warmed at the fire. This process was so far efficacious in removing 

 any conducting film of moisture from the glasses, that at the end of it they were 

 usually found to be electrified by the friction of the leather. When this occurred 

 they were passed rapidly a few times over the flame of a spirit-lamp to discharge 

 them. They were always so warm that any visible moisture deposited by the 

 spirit-lamp disappeared instantly. 



In the 1879 experiments, which are quoted in the preceding table, the glasses 



1 Report Brit. Assoc, 1878; Proc. Roy. Soc, 191, 1878; Phil. Trans., 1879; 

 Four Lectures on Electric Induction, delivered at the Royal Institution (Sampson 

 Low & Co.), 1879. 



