The use of X-rays for the treatment of cancer and other tumours in the human body has raised a scientific question of great difficulty concerned with the accurate measurement of dosage. This is a matter of great complexity, as so many important fac tors are involved. Not only must the X-rays be known with great accuracy, both with regard to their quality (wave-length) and their quantity (intensity), but the intensity must be measured at the exact spot where the rays are intended to operate. A unit of X-ray intensity has been defined as that quantity of X-rays which would produce an ionization current of one electrostatic unit in each cubic centimetre of air. Methods of measuring intensity have been various. Some methods seek to transform the energy of an X-ray beam into heat energy and measure it with delicate thermopiles ; but the most precise method is to utilize the ioniz ing properties of the rays and to measure the ionization produced in a given quantity of gas. A rough-and-ready method which finds considerable favour with the medical profession, is to ob serve the change in colour produced in "pastilles" of barium platinocyanide by the rays. The colour produced by any par ticular X-ray intensity is compared with a standard. The in formation so obtained, however, is not absolute but only relative.
Another method of measuring X-ray intensity has been developed by Fiirstenau, which depends upon the property possessed by X-rays of causing the element selenium to change its electrical resistance ; but here again the results are only relative.
The use of X-rays as a curative agent has increased enormously since their value was first recognized in medicine, and their im portance in this sphere is no longer second to their great value in medical and surgical diagnosis. In this branch they are not only used to examine bones and the coarser structures of the body, but the technique has so improved that the circulatory and respiratory systems are now investigated systematically by radio graphical methods. By the administration of an opaque "meal" which usually consists of a barium salt in a palatable form, com plicated processes of digestion may be studied by the physician as they proceed.
The question of localization of foreign bodies and foci of disease has received a considerable amount of attention and has developed into a high state of accuracy. The work was first carried out by the late Sir James Mackenzie Davidson, who developed ingenious geometric methods of localization. Stereoscopic X-ray pictures are also used in this connection and, when viewed in a suitable holder or stereoscope, afford a remarkably graphic method.
Another application of X-rays which is now in universal use is concerned with dentistry. By the development of small flexible
apparatus and small X-ray tubes it has been possible so to com mercialize these units that most dentists possess one as part of their ordinary equipment and the diagnosis of the condition of the roots of teeth has, by the help of X-rays, become a matter of absolute precision. Yet another practical biological applica tion is to be found in veterinary practice, where X-rays have been shown to have considerable value. The Royal Army Veterinary Corps in England possess a specially designed installation which is in constant use.
The industrial applications of X-rays fall mainly into two divi sions, the first being radiography or the photographic method. Under this heading we will also consider the visual examination by means of a fluorescent screen, which has obvious advantages over the photographic method in many instances. The second main division is concerned with the more difficult technique known as X-ray crystal analysis. We will consider these two spheres of usefulness in sequence.
In the first place it is in the science of engineering that X-rays have been shown to have the most important place. Engineering materials are constantly a source of weakness. Flaws and cracks in castings are always liable to occur and very often are only dis covered when expensive machining has been done; they may then have to be scrapped and the work is wasted. If X-rays could be used to examine all castings immediately they would be univer sally employed, but unfortunately there is a limiting thickness of metal beyond which X-rays cannot penetrate. In the year 1928 this limiting thickness is about 5 in. of steel. Metal ingots and castings below this thickness are all capable of X-ray inspec tion although, owing to the complicated shape of many castings, their examination by X-rays is not always a practical thing. The illustrations give some idea of what a radiograph of a casting looks like. The white lines and patches indicate that in these places the metal departs from its normal homogeneity. (Plate I., fig. 9.) They represent flaws, either blowholes, inclusions, or cracks.
A certain number of patches or cracks may occur in a casting and still not be serious enougk to entail its rejection. The actual significance of the X-ray picture in terms of mechanical strength is a matter for experience in interpretation. The positions and dimensions of metallic flaws may be calculated with great accuracy by stereoscopic methods. Interpretation is very quickly learnt by engineers and the radiograph becomes an infallible guide as to the soundness of material.