X-RAYS: A SKETCH OF THEIR HISTORY. 
large percentage of the total radiation. (V. Fig. 7.) For ytterbium will 
absorb strongly not only all long wave-length radiation but also all shorter 
than that of Angstrom units for which wave-length its own characteristic 
radiation is excited. Hence the continuous part of the X-ray spectrum 
is sharply limited on the short-wave side of the characteristic radiation 
-by the selective absorption of ytterbium and also reduced by general 
absorption as the longer wave-lengths are approached. 
FIG. 7.—Spectrum of X-rays from tungsten, showing effect of ytterbium 
filter in producing nearly monochromatic radiation. ‘ 
In radio-therapy the use of a homogeneous beam of X-rays is much to 
be desired. For the treatment of deep-seated tumours, in especial, it is 
obviously necessary to have radiation of sufficient penetration, and this 
radiation should not be mixed with too large a proportion of soft radiation, 
which would act injuriously upon the more superficial tissues. At present 
the practice of radio-therapists is to operate the tube with a long spark-gap, 
i.e., high voltage, thus securing the requisite penetrating quality, and to 
cut down the amount of softer rays by “ filtering,” through suitable thick- 
nesses of aluminium, a metal whose region of K-selective absorption lies in 
the extreme long-wave part of the spectrum, and which, therefore, exercises 
only a general absorption which is much greater for the softer than for the 
harder rays. ~ See 
By the use of different metals in the target and selectively absorbing 
_ filters we have seen that much more truly monochromatic radiations might 
be obtained; whether the advantages to be thus gained will outweigh the 
practical difficulties yet remains to be seen. rete 
We turn now to the consideration of an important development in the 
technics of X-ray production. It has long been known that an electrically- 
insulated piece of metal loses a charge of electricity, particularly a negative 
charge, much more rapidly if its temperature be at or above a red-heat than 
it does at ordinary temperatures. This effect had been traced to a 
spontaneous emission of electrons by the hot metal. Edison discovered that 
a piece of insulated metal plate or wire enclosed in a’vacuum tube thus became 
negatively electrified, and his discovery was turned to good account by 
Fleming in the vacuum-valve used in its later forms so widely for the 
‘reception of wireless signals. Whether this property of electronic emission: 
depended on the metal alone or on the chemical action of residual gases 
upon it was, until comparatively recently, a matter of dispute. The verdict 
of physicists both in Kurope and in America was probably inclined to the 
chemical explanation, when Dr. Irving Langmuir, of the Research Laboratory 
of the General Electric Co., U.S.A., took up the matter on its experimental 
side, and, using the unrivalled facilities which the laboratory offered for 
the production of the highest vacua, succeeded in establishing beyond 
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