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XX. Contribution by Lord KEtvin to the Discussion on the 
Nature of the Emanations from Radium which was opened 
by Professor E. Rutherford at the Meeting of the British 
Association last September *. 
KT us first consider the mere fact, now known as a result 
of observation and experiment, thatradium has been 
found to emit three types of rays :— 
a. Positively electrified, and largely stopped by solid, 
liquid, or gaseous screens. 
8. More penetrative than a, and negatively electrified. 
y. Electrically neutral, and much more penetrative than 
either a or @; passing with but little loss through a lead 
screen 1 centimetre thick, which is an aimost perfect screen 
against # and @ rays. 
A simple prima facie view is to regard the ‘ y rays’ as 
merely vapour of radium. The ‘8 rays’ seem certainly 
to be atoms of resinous electricity—electrions, as I have 
called them (to specialise Johnstone Stoney’s ‘electron,’ 
which might be either a vitreous or resinous atom of 
electricity, or an atom of matter deprived of its natural 
quantum of electricity). The ‘a rays, according to my 
proposed atomic resuscitation of Aepinus’s doctrine +, are 
atoms or molecules of matter, probably atoms of radium, 
or perhaps molecules of bromide of radium ; either deprived 
of electrions, or having less than their neutralising quantum. 
The electro-etherial hypothesis, referred to in my commu- 
nication of last Thursday to Section A f, affords a ready ex- 
planation of the relative penetrativities of the three radiations, 
and of the fact that each one of them makes its existence 
known to us by conferring electric conductivity on air or any 
ordinary gas in which it is present. 
Taking the y rays first, we have to explain the free pene- 
tration of unelectrified radium molecules through dense 
liquid or solid matter. An easy assumption suffices : let the 
Boscovichian mutual forces (that is, the chemical affinities 
and the repulsions) between an atom of radium and the atoms 
of lead and other permeable substances be small enough to 
allow the known permeation. 
Taking, next, the a radiation. The apparent great absorp- 
tion of the vitreous electric emanation from radium is only 
apparent ; it means that an atom shot from radium with less 

* Communicated by the Author. 
+ Baltimore Lectures, Appendix C. Reprinted from the Jubilee 
Volume presented. to Prof. Boscha of Leyden in November, 1901. 
Phil. Mag. 1902 (1st half year). 
t Phil. Mag. Oct. 1903, 

