VOt,. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 473 



easily to change its acquired state of electricity, leaves the metal without de- 

 priving it of that quantity of fluid which it had acquired. 



If these two coatings, separated from the glass, are brought near each other, 

 they attract each other ; a spark, ensues, because the coating, which has acquired 

 a superabundant quantity of electric fluid, imparts it to the other, which had 

 lost as much ; and thus a perfect equilibrium is restored between them. If both 

 these coatings be applied as before on the same glass, a positive spark may be 

 obtained from the upper coating, and a negative one from the other. If they 

 be separated again from the glass, as in the first case, the upper coating will 

 afford a negative spark, and the under a positive ; and these alternate sparks 

 may be continued a very long time. 



This explanation or theory agrees perfectly with the experiments exhibited by 

 our deceased member, the late Mr. Canton, with elder pith balls hanging by 

 linen threads from a wooden box, which balls are excited either negatively or 

 positively by a piece of excited glass. 



XLIX. Observations and Experiments fending to confirm Dr. Ingenhouszs 



Theory of the Ekclrophorus ; and to shew the Impermeabilily of Glass to 



Electric Fluid. By JVilliam Henly, F. R. S. p. IO49. 



Dr. Franklin has observed, " That there is a great quantity of the electrical 

 fire in glass ; that what it has it holds ; and that it has as much as it can hold : 

 that what is already in it, refuses or strongly repels any additional quantity : that 

 when an additional quantity is applied to one surface of a phial (for instance by 

 the atmosphere of an excited tube) a quantity is repelled or driven out of the 

 inner surface of that side into the vessel, returning again into its pores, when 

 the excited tube with its atmosphere is withdrawn ; and that the particles of that 

 atmosphere do not themselves pass through the glass." The following experi- 

 ments remarkably illustrate this, by showing that bodies are very differently 

 afifected by a fluid acting immediately on them through glass ; or by acting on 

 them immediately by the glass, as abovementioned. 



Exper. — A circular box, 3 or 4 inches in diameter, and ^ of an inch deep, is 

 furnished with a thin glass for a top. In this box scatter some very small steel 

 filings, or sift them into it through a piece of writing paper, which has a number 

 of holes pricked through it with a pin. Then apply one of the ends of a mag- 

 netic bar to the upper surface of the glass ; the filings will be instantly attracted 

 to the glass, and remain there as long as the magnet is thus supended over 

 them ; but the moment it is removed, the filings Jail to the bottom of the box, 

 and there remain at rest. The glass then being made perfectly clean and warm, 

 let a fine piece of amber, or sealing wax, &c. be strongly excited and applied to 

 it, as the magnet was in the former experiment ; the filings will be instantly in 



VOL. XIV. 3 P 



