A special property of the X-ray is one also possessed by the rays from radium. It is that of ionizing the air and so rendering the air a conductor of electricity as may be demon strated by the discharge of an electroscope. This property is made the basis of measuring the radio-active power of radium and has been occasionally used to measure the intensity or the dosage of the X-ray.
The X-ray is ordinarily made up of different rates of vibrations having as different proper ties as the different rates of vibration in ordi nary light. X-rays of very rapid vibration are highly penetrating and are called hard rays; they are produced when the tube presents a high resistance to the passage of electricity and it takes a high voltage, perhaps 90,000, to send a current through the tube. This high resist ance occurs when the vacuum of a gas-filled X-ray tube is high, that is most of the gas is removed; with an electron discharge X-ray tube like the Coolidge tube the resistance is high when the filament is at a low degree of incandescence. Rays of a low degree of pene tration or soft rays are produced in greatest abundance by an X-ray tube whose resistance is so low that about 40,000 volts will send elec tricity through the tube. As a rule, the rays are not all of the same wave length, hut the radiation is mixed. The average penetration is determined by the regulation of the resistance of the tube. Under special conditions, however, homogeneous X-rays are obtainable. The parallel spark-gap is the distance that the high tension current will pass across in the open air in preference to passing through the X-ray tube. It is frequently employed as a means of measuring the resistance of the X-ray tube. In a rough way. each inch of parallel spark gap curresponds to 10,0110 volts of high tension current required to excite the X-ray tube and ver% •oughk to one Benoist unit of penetration. Direct measurement of the aserage degree of penetration of the rays generated by an X-ray tube is generally by the Benoist Radloch-o mometer. There is a central disc of silver 0 11 millimeter thick, surrounded by sector. of aluminum from I to 12 millimeters thick The number of millimeters of aluminum which cast a shadow of the same density as that of 011 millimeters of silver gives the degree of pene tration. Soft rays are number three or four Benoist, and very hard rays number 10 or 12 Benoist X-rays are partly absorbed by every substance through which they pasts. The den ser the substance and the softer the X-rays the greater proportion is absorbed by a layer of a given thickness. Very soft rays are so al sorb able that even the petals of flowers cast shadows and produce beautiful pictures, while extreme,. hard rays are employed to look for flaws in side of steel rails. Medium degrees of pene
tration produce X-ray pictures of the human body in which the bones cast a darker shadow than the flesh, air spaces like the pneuma:-c sinuses of the face and like the lungs cast a lighter shadow and dense foreign substances like a bullet in a wound or an opaque meal in the stomach and intestines or a stone in the kidney cast dense shadows. Except in the case of a broken hone or a bullet, the interpre tation of a radiograph requires a knowledge of the normal X-ray lights and shadows of the part depicted.
The effects of the X-ray upon the bar= body were first noticed as X-ray turns and loss of hair. Later it was found that these could be prevented and that the X-ray had marml ously beneficial effects upon cancer and !cone mia and tubercular glands and other diseases. Skin cancers are usually permanently cured without the necessity for an operation and moderately soft rays are employed which wdl be absorbed by the skin with very little deep effect. Deep-seated lesions require rays of great penetration so that a large fraction will reach the deep tissues. This is partly accane phshed by regulating the X-ray tube to a high resistance so that it generates chiefly hard rays and also by interposing a filter between the X-ray tube and the patient. An eighth of m inch or more of aluminum will arrest rays which would be absorbed by the skinTa danger of burning, and which would greatly limit the quantity of the accompanying pene trating rays that could be safely applied The intensity varies inversely as the square of the distance, so even rays of the proper penetra tion are much stronger at the surface toward the X-ray tube than in the deep tissues me siderably further away. And even the pene trating rays are rapidly weakened by passing through the tissues. To secure sufficient effect upon a deep-seated organ without too muds effect upon the skin a cross-fire system is neces sary. One exposure is made shielding the skin with heasy lead except for an opening though which the X-ray passes to the deep organs. For another exposure, the person is in a dif ferent position and the X-ray reaches the Name deep organ through a different part of the skin. Sometimes as many as eight ports of entry are employed, the deep-seated organ receis mg eight times the quantity o: X-rays that could have been safety a from a single direction. And so tlwt the X-ray is weakened by distance and by ahsorp non in passing through the overlying tissues, a really beneficial total dosage is applied to the deep organ. All hut the successive ports of entry must be shielded by thick lead It is not sufficient to merely change the position of the X-Ray tube and the patient.