104 NATURE OF THE RADIATIONS [CH. 
layer 2°5 sq. cms. in area and 2 mms. thick, a current of the order 
of 10" amperes was observed after the rays had traversed a layer 
of aluminium ‘01 mm. thick and a layer of ebonite -3 mm. thick. 
The current was the same with discs of lead, copper, and zinc, and 
also when the ebonite was replaced by paraffin. 
Curie also observed in another experiment of a similar character 
that the radium itself acquired a positive charge. This necessarily 
follows if the rays carry with them a negative charge. If the 
8 rays alone carried with them a charge, a pellet of radium, if 
perfectly insulated, and surrounded by a non-conducting medium, 
would in the course of time be raised to a high positive potential. 
Since, however, the @ rays carry with them a charge opposite in 
sign to the @ rays, the ratio of the charge carried off by the two 
types of rays must be determined, before it can be settled whether 
the radium would acquire a positive or a negative charge. If, 
however, the radium is placed in an insulated metal vessel of a 
thickness sufficient to absorb all the a rays, but not too thick to 
allow most of the 8 rays to escape, the vessel will acquire a 
positive charge in a vacuum. 
An interesting experimental result bearmg upon this point 
has been described by Dorn'. A small quantity of radium was 
placed in a sealed glass tube and left for several months. On 
opening the tube with a file, a bright electric spark was observed 
at the moment of fracture, showing that there was a large differ- 
ence of potential between the inside of the tube and the earth. 
In this case the a rays were absorbed in the walls of the tube, 
but a large proportion of the 8 rays escaped. The inside of the 
tube thus became charged, in the course of time, to a high positive 
potential ; a steady state would be reached when the rate of escape 
of negative electricity was balanced by the leakage of positive elec- 
tricity through the walls of the tube. The external surface of the 
glass would be always practically at zero potential, on account of 
the ionization of the air around it. 
Strutt? has recently described a simple experiment to illus- 
trate still more clearly that a radium preparation acquires a 
positive charge, if it is enclosed im an envelope thick enough to 
1 Phys. Zeit. 4, No. 18, p. 507, 1903. 
2 Phil. Mag. Nov. 1903. 
