304 Dr. J. Joly on the Motion of 
about 6 ems. long 
g, their surfaces lying in the same plane. 
A short piece of a fine needle is fixed by cement at the 
baie of the fibre, perpendicularly to it and in the plane of 
the vanes, so that when the needle-point is poised upon a 
smooth surface a radiometer-like instrument with vanes in a 
vertical plane is provided. A drop of alcohol is placed upon 
a face of one of the cover-glasses and about half the contents 
of a 5-milligramme tube of radium bromide poured over it. 
The alcohol and radium are carefully spread over the surface 
and warmed over a lamp till the radium is alone left adherent 
to the glass. ‘The second vane is treated in the same manner, 
but upon the alternative or opposite face—the, rotationally, 
similar face. 
A glass receiver is turned over the little mill, some calcium 
chloride being placed beneath. If now an electrified body, 
such as a rod of ebonite rubbed on cat’s skin or glass rubbed 
on silk, is brought up close to the receiver, there is a rotational 
motion imparted to the vanes, which becomes much more 
marked and decisive as to its direction if the pressure of the 
air under the receiver is reduced to about 5 or 6 centimetres; 
and may then become so violent as to cause the vanes to 
refuse to remain upon the support. It matters not what the 
sign of the electrification, the rotation is always in the one 
direction, and in such a sense that the radium-covered surface 
is repelled from the electrified body. A steady rotation is 
best obtained by placing the mill between metal plates (con- 
tained under the receiver), which can be connected with a 
small Wimshurst machine. A little consideration will show 
that this unidirectional rotation is a result of the effects 
described above. 
The rotation persists at pressures of a couple of millimetres, 
but, I think, more feebly. I have not extended the observation 
to high vacua. 
In order to examine the effect more closely a radium- 
coated cover-glass was attached to the beam of a Coulomb’s 
balance, which was modified in such a way that the fixed 
metal sphere could be charged from without. It was then 
found that sharp repulsion was produced when the sphere 
was opposite the radium-coated surface and charged with 
positive or negative electricity, and that there was equally 
definite attraction when the sphere was upon the naked side 
of the glass, whatever the sign of the charge. If, however, 
the char, ge given is very intense and the radium is very close 
to the sphere e, there may be attraction in every case. 
It is possible to frame more than one explanation of this 
peculiar behaviour of ‘a radium-coated body in an electric 
