VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. 235 



the cap. The consequence was, that the electricity, meeting with scarcely any 

 resistance, passed from the top to the bottom of the tube, and electrized the air- 

 pump as before : and it was a most delightful spectacle, when the room was dark- 

 ened, to see the electricity in its passage; to be able to observe, not, as in the 

 open air, its brushes or pencils of rays an inch or 2 in length, but here the co- 

 ruscations were of the w hole length of the tube between the plates ; viz. 32 

 inches, and of a bright silver hue. These did not immediately diverge as in the 

 open air, but frequently, from a base apparently flat, divided themselves into less 

 and less ramifications, and resembled very much the most lively coruscations of 

 the aurora borealis. 



At other times, when the tube has been exhausted in the most perfect man- 

 ner, the electricity has been seen to pass between the brass plates in one con- 

 tinued stream of the same dimensions throughout its whole length ; and this, 

 with a subsequent observation, seems to demonstrate, that the cause of that very 

 powerful repulsion of the particles of electrical fire one to the other, which we 

 see in open air, is more owing to the resistance of the air than to any natural 

 tendency of the electricity itself; as we observe that its brushes from blunt bodies, 

 when the electricity is strong, diverge so much, as to form, when seen in the 

 dark, an almost spherical figure. This figure seems therefore to arise from the 

 electricity's endeavouiing to insinuate itself between the particles of air. The 

 figure that an elastic fluid of less density must form, when let loose, and equably 

 compressed by one more dense and more elastic, must necessarily approach to 

 that of a sphere. 



On admitting a very small quantity of air into the tube, these phenomena dis- 

 appeared ; not so much from the small quantity of air admitted, as from the 

 vapours which insinuated themselves with it. These lined the sides of the glass, 

 and conducted the electricity imperceptibly from one end of the tube to the 

 other. These experiments seem to evince, that however great the vacuum 

 could be made, the electrical coruscations would pervade it through its whole 

 length. 



Hence it appears that our atmosphere, when dry, is the agent by which we 

 are enabled to accumulate electricity on non-electrics: as in the experiment 

 before us, on the removal of it, the electricity passed off into the floor through 

 a vacuum, of the greatest length we have hitherto been able to make, became 

 visible in this vacuum, and manifested itself by its effects on the air-pump, being 

 the non-electric substance, which terminated that vacuum ; whereas, when the 

 air is not taken away, the dissipation of the electricity is from every part of the 

 prime conductor. We see here also, contrary to what we have found hitherto, 

 that an originally-electric body, viz. a dry glass tube, puts on the appearance of 



HH 2 



