22 
DR. J. HOPKINS ON ON ELECTROSTATIC CAPACITY OF GLASS. 
First Experiment.- — Plates in contact with glass. 48 elements in battery. 
S 2 drawn out, S x =95. 
Plate removed, condensers again equal when R=3 60. 
Hence K=10'28. 
Second Experiment. — Plates in contact with glass. 7 2 elements in battery. 
S 2 drawn out, S x =55 when slide is positive. 
= 95 ,, negative. 
S x =75 is equivalent to R ( — ) = 3‘6l 
R (+) = 3*69 
K=10*07. 
The latter result is probably much the best ; take 10T as most probable value. 
In the next two glasses the determinations were made first with plates in contact 
with glass, second with a space of air between the glass and the upper plate ; the 
results suggested the experiments of § 3. In each case 72 elements were used. 
Dense flint (the glass generally used in the objectives of telescopes). — Density about 
3’66, thickness = 16’58 turns of the screw, diameter =230 millims. 
First Experiment. — Plates in contact with glass, S 2 drawn out. 
When the slide is positive, S x = 205, on removal of glass plate this equals 
R (-) = 5‘50. When the slide is negative, S X =175, on removal of glass plate 
R (+) = 3-50. 
Hence K=7‘34. 
The mean zero reading being now IT 5. 
Second Experiment. — R is put at 18T4 with glass between the plates. 
S 2 drawn out when the slide is negative. 
Sj= 10 on removing glass equals R (+) 3*78, when the slide is positive. 
S 2 =40 on removing glass equals R ( — ) 379, i.e. glass 16’58 and air 0 - 32 are 
equivalent to air 2’525 or K=7'45. 
Mean =7 ’4. 
A very light flint. — Density about 2 - 87, thickness =127 turns of the screw, 
diameter =235 millims. 
First Experiment. — Plates in contact with glass S 2 drawn out, when the slide 
is negative. 
S t =380 on removing glass equals R (+) 3*20, when the slide is positive. 
S x =440 on removing glass equals R ( — ) 378. 
K=6-6. 
Second Experiment. — R was put at llfiO, S 2 was drawn out when the slide 
is positive. 
S x =80 on removing glass equal to R (— ) 371, when the slide is negative. 
