234 COMPOUNDS OF THE RARE ELEMENTS.
plate that is connected with the earth should be supported.
Charge the electroscope, and measure the conductivity of the
layer of air between the two plates by determining the time
required for the two leaves of the electroscope to reach the zero
position. It is sufficient to observe over how many scale divi-
sions the leaves pass within a certain length of time, e.g., the
scale divisions per minute. To measure the activity, spread an
amount of the substance, weighed to an accuracy of two figures,
upon a piece of paper, and place the paper on the lower metal
plate. Adjust the second plate in position, charge the electro-
scope, and make the readings. The quotient obtained by divid-
ing the number of scale divisions traversed per minute by the
weight of the substance, gives the specific activity. The observed
rate may be corrected by subtracting from it the velocity with
which the leaves come together in a blank experiment with none
of the active preparation. If it is desired to compare the values
obtained with those that have been published in the literature,
the apparatus may be standardized by using uranyl nitrate.
Of the active radiations, the part consisting of the so-called
a-rays is held back by a sheet of paper. If, therefore, the experi-
ment is repeated exactly as above, except that the preparation is
covered with filter paper, the value then obtained corresponds to
the activity of the rays that pass through the paper, that is, of
the so-called /9-rays. The difference between the two values gives
the activity of the a-rays. Radioactive lead preparations emit
principally a-rays.- Uranium Compounds.
As the starting material in the preparation of uranium compounds, either
uranium nitrate prepared from pitchblende according to the following pro-
cedure, or the commercial product, may be used.
Uranyl Nitrate, UO 2 (NO 3 ) 2. Heat 50 g. of finely powdered
and sifted pitchblende in an evaporating dish with 150 c.c. of
30% nitric acid until the excess of acid has been expelled.
Extract the mass with hot water, and evaporate the filtered solu-
tion to dryness on the water bath. Boil the dry residue repeat-
edly with ether in a small flask, and evaporate the ethereal extract
to dryness, taking great care that the vapor does not take fire.
1
Recrystallize the last residue from water. Yield, about 35 g.
(^1) Cf. footnote 2, p 214.