plate was wrapped in two thicknesses of black paper. The objects
were placed upon this and the radio-active ore, separated by a board
one inch thick, was placed above. The exposure lasted five days. The
action is much less rapid and the result not so clearly defined as in
the case of photographs taken by _X_ rays. Of course, the removal of
the board and the use of more concentrated preparations of radium
would give quicker and better results. The method, however, on
account of time consumed and lack of definition is ill adapted to
accurate work.

[Illustration: FIG. 3--PHOTOGRAPH OF VARIOUS OBJECTS TAKEN
BY MEANS OF PITCHBLENDE]

Discharge of Electrified Bodies

The radiations from radio-active bodies can discharge both positively
and negatively electrified bodies by making the air surrounding them a
conductor of electricity. To demonstrate this, use is made of an
electroscope. If the hinged leaf of such an instrument be electrically
charged and a radio-active body be brought into its neighborhood, the
electricity will be discharged and the leaf return to its original
position. The rapidity of this discharge is used to measure the degree
of activity of the body giving off the radiation.

[Illustration: FIG. 4.--GOLD-LEAF ELECTROSCOPE.

The gold-leaf _L_ is attached to a flat rod _R_ and is
insulated inside the vessel by a piece of amber _S_ supported
from the rod _P_. The system is charged by a bent rod _CC'_
passing through an ebonite stopper. After charging, it is
removed from contact with the gold-leaf system. The rods _P_
and _C_ and the cylinder are then connected with the earth.]

Scintillations on Phosphorescent Bodies

It was found by Crookes that a screen covered with phosphorescent zinc
sulphide was brightly lighted up when exposed to the radiations. This
is due to the bombardment of the zinc sulphide by a type of ray called
the alpha ray. Under a magnifying glass this light is seen to be made
up of a number of scintillating points of light and is not continuous,
each scintillation being of very short duration. By proper subdivision
of the field under the lens, the number of scintillations can be
counted with close accuracy.

A simple form of apparatus called the spinthariscope has been devised
to show these scintillations. A zinc sulphide screen is fixed in one
end of a small tube and a plate carrying a trace of radium is placed
very close to it. The scintill