From the time of their discovery, an out standing feature of X-rays has been that they pass freely through certain substances which are opaque to light (black paper, soft parts of the human body), but are stopped by certain transparent substances (for example, glass). It has been found (L. Benoist, 1901) that the opacity to X-rays increases with the atomic weight of the substance It is owing to this that most organic substances (notably those which constitute the tissues of animals and vegetables), which consist entirely of ele ments of low 2 atomic weight, are very trans parent to X-rays, as are the light metals, such as aluminium and magnesium and their On the other hand, bone, which contains ele ments of atomic weight, and the heavy metals, such as lead and platinum, are opaque to X rays in thicknesses which decrease as the atomic weight increases.
The general properties of X-rays (speed, method of propagation, geometric laws) are the same as those for light, hut certain of them, notably reflection, refraction, and interference, have for a long time escaped investigation, as the minuteness of the wave-length requires very delicate experimental apparatus.
Results analogous to those obtained in the spectrum analysis of light by diffraction gratings have been obtained with X-rays by using crystals which play the part of three-dimensional gratings, or by using, at the grazing angle, gratings ruled on glass. It has thus been pos sible to determine precisely the wave-lengths of the various radiations which are grouped together under the generic name of X-rays.
An X-ray generator cannot produce X-rays of a wave-length (measured in Angstrom units) less than 12,350/V, where V represents the difference of potential between the electrodes ; 4 e.g. the shortest wave-length of X-rays which can be emitted with a potential difference of 123,50o volts is 0.1 AU. At the same time a large proportion of rays of greater wave-lengths is produced ; the distribution of the various rays throughout the complete range depending to a large extent on the material of which the anti cathode is made.
Rays of long wave-lengths are usually called soft rays, or rays of low penetration, and are emitted when the voltage is low ; when the potential difference is great, rays of great pene tration are emitted ; these are of very short wave-length and are known as hard rays.
When a beam of X-rays passes through a body, a small proportion of the total continues its rectilinear course ; a portion is diffused with out appreciable alteration in the composition of the radiation, as is the case when light passes through a translucent medium. A very small
fraction of the total X-radiation is absorbed, with the formation of a corresponding amount of X-radiation of longer wave-length (secondary radiation), in a manner comparable with the formation of visible light when ultra-violet light is absorbed by a fluorescent material.
The penetration of the secondary rays is greater (and the wave-length is correspondingly shorter) as the atomic weight of the substance constituting the irradiated body (the secondary radiator) is higher. In addition to this mixed radiation, bodies exposed to the action of X-rays of medium penetrating power emit rays which are characteristic of the elements present. These rays are particularly troublesome in photo graphy when they are emitted by such metals as iron, nickel, and copper, and the direct irradiation of these metals by X-rays should. always be avoided. The radiations character istic of aluminium are not troublesome, since they have so weak a power of penetration that they can be absorbed by a single thickness of black paper. For this reason aluminium is usually used as a filter to absorb secondary rays of low penetration, and as a material for the construction of the cassettes used for holding sensitive plates or films.
926. Physiological Properties of Everyone is familiar with the numerous serious effects of X-rays on the earlier radiologists at a time when the dangers of frequent exposure to the rays were not suspected.
X-rays provoke, notably, lesions of the skin (radio-dermatitis), frequently developing into cancer ; slow alterations in the blood,' which result in death through acute anaemia, and deep lesions (bones, genital glands, etc.).
The risk of accident obviously increases in proportion to the progress made in the produc tion of apparatus capable of producing more penetrating radiation, against which protective devices, considered safe for less powerful instal lations, are ineffective.
927. Methods of Protection. The protective measures required in a radiographic laboratory are of three types— Protection against the risk of electrocution by the exceedingly high tension current. Effective ventilation.