Ty 7 
5a4 Lord Kelvin on Hlectrie 
distribution of equal density at equal distances from the 
centre, into which a neutralizing quantum of electrions could 
be shot and come to rest in such a configuration that, in the 
presence of other atoms or disturbing electrions, it w ould act 
as Radium does. ‘This is in fact done for the 8 rays of 
Radium in § 13 above: because the two atoms A, B there 
put together concentrically may be supposed fixed relatively 
‘to one another and called one atom. 
LI. On Electric Insulation in Vacuum” *. 
| By Lord Kevin fF. 
§1. JT has long been well known that difference of 
electric potential between conductors in a high 
racuum is maintained without appreciable current, even 
when the distance between them is a small fraction of a 
millimetre. Fitty or sixty years ago, when we had no experi- 
mental knowledge of what is now called a high vacuum, it 
was a vexed question whether vacuum is an insulator ora 
conductor. In a Royal Institution Friday evening lecture 
of May 18th, 1860 {, I find that I made the following state- 
ment :—‘“ It has i supposed, indeed, that outside the 
‘“earth’s recognised atmosphere there exists something or 
‘nothing in space which constitutes a perfect insulator ; but 
‘this supposition seems to have no other foundation than a 
“* strange idea that electric conductivity is a strength or a power 
‘of matter, rather than a mere non-resistance.” 
§ 2. The labours of many experimenters during the last 
fifty years, and the comparatively modern atomic “theory of 
electricity, have thoroughly confirmed the view that the 
_ space of our best modern vacuum, and interstellar and inter- 
planetary space, and generally, space occupied only by the 
all-pervading luminiferous ether, is a very perfect non-resister 
of electricity passing through it. 
ie 3. Hence we see that the insulation of electricity in 
vacuum ”’ is to be explained, not by any resistance of vacant 
space or of ether, but by a resistance of glass or metal or 
other solid or liquid against the extraction of electrions from 
it, or against the tearing away of electrified fragments of its 
own substance. The kathode torrent of resinously electritied 
particles, discovered in 1871 by Varley, rediscovered eight 
years later by Crookes, and generally accepted as a truth 
* By “vacuum” I mean space occupied only by the luminiferous ether. 
+ Communicated by the Author. 
t Sir William Thomson's ‘ Electrostatics and Magnetism,’ § 281. 
