A system of physical chemistry - Index of

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A system of physical chemistry - Index of

120 A SYSTEM OF PHYSICAL CHEMISTRY

the positive sphere, by substituting for it the nucleus idea of Rutherford.

The difficulty, as already pointed out, consists in the fact that an atom

consisting of a nucleus surrounded by an atmosphere of magnetons

would be an unstable system. The problem has been discussed in a

preliminary manner by D. L. Webster {/. Amer. Chem. Soc, 40, 375,

From what has been said it will be obvious that the magneton

Parson is still in a rudiment-

theory of atomic structure as developed by

ary stage. Its advantages, however, are such, particularly in regard to

its

applicability

to chemical phenomena, that it cannot be discarded

without a serious attempt being made to overcome the main difficulty

of the scattering of the a particles.

High Frequency Spectra of the Elements. Moselefs Relatmt.

By the high frequency spectrum is meant the X-ray spectrum which

is obtained from a substance by bombarding the substance with /?-rays.

It has been pointed out in Vol. I., Chap. II., that the investigation of the

structure of crystals by means of their X-ray spectra affords a means of

determining with precision the wave-length of any homogeneous X-ray

employed. X-rays only differ from light in possessing very much shorter

wave-lengths (of the order io~^ cm.). The substance to be used as a

source of X-rays forms the anticathode in a vacuum tube, and the beam

of X-rays emitted as a result of bombardment by the cathode rays is

characteristic of the substance forming the anticathode. In general

the X-rays thus excited are not homogeneous. They consist of two

or more wave-lengths which can be analysed and measured by means

of the crystal acting as a grating. As a rule, however, one frequency

is more prominent than the others, that is, the intensity of the resulting

spectrum is greatest for one type, and for the sake of simplicity we

shall consider these single frequencies as characteristic of the X-rays

emanating from the substance examined. When the anticathode is

of nickel, the characteristic wave-length of the X-ray produced is

i-io X io~^ cm. It follows, therefore, that a this type contains energy in amount 1*78

single quantum hv of

x io~^ ergs. It is of in-

terest to compare this quantity with the energy of the electron of the

cathode stream which gave rise to the X-ray. Whiddington {Proc.

Canib. Phil. Soc, 1910) has shown that an electron of the cathode

stream will not excite X-ray radiation unless its velocity exceeds a

certain critical value, which varies with the nature of the material

composing the anticathode. If the latter be of nickel the limiting

velocity of the electron striking it is6'i7 x 10^ cm. per sec, or about

one-fifth of the speed of light. Taking the mass of the electron to be

o'9 X lo"^'' gram, the kinetic energy of the electron possessing the

limiting speed is 17 x io~^ erg, a quantity which is very nearly the

same as the energy in a single quantum of the X-ray produced.

Further, the investigation of the reverse process, i.e. the emission of an

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