done in the past. Fortunately, other methods
have been found by which this may be confirmed. For instance, the
stopping and scattering power of the element for alpha particles has
been suggested and successfully used.

Use of X-Ray Spectra

A most interesting method is due to Moseley's observations upon the
_X_-ray spectra of the various elements. It has been found that
crystals, such as those of quartz, have the power of reflecting and
defining the _X_ rays. The spectra given by these rays can be
photographed and the wave lengths measured. These _X_ rays are emitted
by various substances under bombardment by the cathode rays (negative
electrons) and have great intensity and very minute wave lengths.
Moseley made use of various metals as anti-cathodes for the production
of these rays. These metals ranged from calcium to zinc in the
Periodic System. In each case he observed that two characteristic
types of _X_ rays of definite intensity and different wave lengths
were emitted. From the frequency of these waves there is deduced a
simple relation connected with a fundamental quantity which increases
in units from one element to the next. This is due to the charge of
the positive central nucleus. The number found in this way is one less
than the atomic number. Thus the number for calcium is 19 instead of
20 and that for zinc is 29 instead of 30. So, by adding 1 to the
number found the atomic number is obtained.

The atomic weight can usually be followed in fixing the atomic number,
but where doubt exists the method just given can be resorted to. Thus
doubt arises in the case of iron and nickel and cobalt. This would be
the order according to the atomic weights. The _X_-ray method gives
the order as iron, cobalt, and nickel, and this is the accepted order
in the Periodic System.

Changes Caused by Ray Emission

On studying the properties of the elements in a transformation series
in connection with the ray emission which produced them, it was seen
that these properties were determined in each case by the nature of
the ray emitted from the preceding transformation product or parent
element.

Atomic Weight Losses

Each alpha particle emitted means a loss of 4 in the atomic weight.
This is the mass of a helium atom. Thus from uranium with an atomic
weight of 238 to radium there is a loss of three alpha particles.
Therefore, 12 must be subtracted from 238, leaving 226, which agrees
closely with the atomic weight