110 
PROFESSOR R, THRELFALL AND MR. J. H. U. BREARLEY 
Hence 
d = -024341 ; 
A = 156-645 ; 
C = 213 X 10“®°; 
V = 3 X 10 10 . 
/X = 3-748, 
a number very like that previously obtained for the same kind of film (3"5). 
Having obtained a provisional value for the specific inductive capacity of the 
sulphur, we considered that a suitable time had come to repeat some experiments 
made a year before on the residual effect, and endeavour to obtain a comparison 
between sulphur and mica, the latter having been much recommended by M. Bouty 
(‘ C. It.,’ vol. 110, p. 1362 ; ‘ C. R.,’ vol. 112, p. 931 ; ‘ Journ. cle Phys.,’ vol. 9, p. 288, 
June, 1890; ‘Ann. de Chim. et de Phys.’ [6], vol. 24, p. 394). We propose to 
describe our experiments first, and then to discuss this very important matter 
(the literature of which is overwhelming) when we have described a number of 
cognate phenomena. 
A preliminary experiment was made to find the capacity of the quadrants of our 
electrometer and of the key and connections used in the investigation of the 
residual charge. The capacity of the quadrants was rather less than "00005 micro¬ 
farad, and of the key and connections about "000004 microfarad. The sulphur plates 
have, as has been shown, a capacity of about "002 microfarad. The smallness of the 
capacity of the quadrants and key and leads, in comparison with the capacity of the 
condenser, enables us to disregard them in estimating- the residual effect. The mica 
condenser was prepared from a specially selected bit of mica, such as is used for 
covering photographs; it was silvered on both sides, and cleaned along the edges as 
described by Bouty, the edges being lightly varnished by dipping in a dilute filtered 
solution of shellac. This condenser plate was dried at the temperature of the water 
bath for twelve hours, and was then mounted on quartz insulators in a desiccator over 
sulphuric acid. It had been in this for a year when our experiments began. Connec¬ 
tion was made to the insulated plate by means of an aluminium wire on a fused 
quartz support. The dimensions of the mica condenser were :— 
Length of rectangular silver films . . 7"2 centims. 
Breadth.3 "6 ,, 
Thickness of mica . . -Q05 centim. 
The capacity was of the order of that of the sulphur condenser. We will not 
describe any preliminary experiments on the residual effect, because there was nothing 
novel either in our method or in the difficulties we encountered. The investigation 
of residual effect is not difficult if the effect be large, but if it be vanishingly small, as 
it was with the sulphur, and if it be required to assign a limit to this small effect, 
