Radio broadcast .. (1922-30)

SEPTEMBER, 1927 A DISCOVERY THAT NEWTON MISSED 265 even larger quantities result if we substitute rods of iron for the carbons. When the iron arc is used in laboratories, it is necessary to protect the eyes from its light. Many a physicist making experiments with the iron arc has got as nice a case of sunburn as if he had been at the seashore. Other metals can be used instead of iron and even more copious quantities of ultra- violet light result, for example, if the elec- trodes are of silicon. In this case the visible light is reduced nearly to a minimum, and the output of the lamp is almost entirely invisible light. But mercury, or quicksilver, is most commonly used, particularly in the form of the mercury vapor lamp, where an electric discharge is made in an atmosphere of the vapor of mercury instead of air. As mercury vapor is a gas, it is necessary to completely enclose such a lamp in a transparent tube. Glass, of course, is often used, and the glass-tube mercury vapor lamp is a common light source in photo- graphic and movie studios, as well as in many factories, because it is a very efficient illuminating device. The incandescent gas mantle takes the equivalent of about nine watts of electric power to produce one candle power, the tungsten filament electric lamp about a watt, and the ordinary arc lamp about nine-tenths of a watt; but the glass tube mercury vapor lamp produces the same amount of light with about two- thirds of a watt. Of course the purple color of the light, and the lack of red rays, gives the skin a ghastly pallor, but the light is not harmful to the eyes and is said to be pleasant to work under after one gets used to it. Such a light doesn't give off much ultra- violet radiation, because these rays are absorbed by glass, but by making the tube of quartz, large amounts of ultra-violet radiation are emitted, and then the lamp gives one candle power for every quarter of a watt. This is the most convenient source of the rays, and is the one now most generally used in laboratories and hospitals. Such lamps are often used for sterilizing water and various food products as the action of ultra-violet light is fatal to many germs. A common use is in swimming pools, for the same water can be filtered and puri- fied by passing it and re-passing it through apparatus which exposes it to ultra-violet rays. The water in a pool so equipped can really be kept purer than if it were emptied and refilled daily. ULTRA-VIOLET RAYS IN PHOTOGRAPHY BECAUSE the ultra-violet rays are so short they have an important appli- cation in photography of minute objects through the microscope. In modern optical factories it is possible to make lenses for microscopes which will theoretically magnify almost without limit. Actually, such lenses are limited, because a structure must be as large as a wave of light is long in order to reflect it. If you toss a tennis ball against a wall it bounces back, but if you throw it against a spider web it passes right through. This is because the mass of the wall compared with the ball is very great, but that of the spider web is very small. The case is some- what analogous to light. The object that reflects it must compare favorably in size with the length of one of the waves. Since the waves of ultra-violet light are shorter than those of visible light, struc- tures that are too small to be seen under the microscope with ordinary illumination may be seen, or rather photographed, in the ultra-violet ray. It was with a method such as this that the English biologist, Gye, with the assistance of the expert microscopist, J. E. Barnard, was able to make photo- graphs through the microscope of the germs which caused cancer in chickens, and which were beyond the limits of the .ordinary microscopic methods. In metallurgical laboratories, ultra-violet microscopic appa- ratus is used in photographing steel and other metals. As glass stops most of the ultra-violet rays, such a microscope must use lenses made of quartz. Since ultra-violet light may be used so advantageously in photomicrography, a natural idea is that of using X-rays, the waves of which are much shorter than the ultra-violet. If X rays could be used, struc- tures could be photographed a thousand times smaller than with the visual rays, instead of, perhaps, half the size, as with the ultra-violet. But though X-rays can be deflected through crystals, they can't be focussed, like light, by means of a lens; so X-ray photomicrography is one of the yet unsolved riddles that are so numerous in science. One of the most important uses of ultra- violet light is its effect on the body. We hear a great deal nowadays about vitamins, those mysterious substances in food about which so little is known, but that are so necessary if we are to keep healthy. One of these vitamins prevents a disease which in some localities is very common among children, namely, rickets. Few people real- ize just how common this ailment is, but Dr. Alfred Hess, a prominent New York special- ist on children's diseases, has estimated that seventy-five per cent, of the children in New York City have at one time or another had at least a mild case of rickets. However, though there are not many diseases for which the medical profession knows any practically sure-fire remedies, rickets is one of them, for it may be cured with either cod liver oil, which is rich in the anti-rachitic vitamin, or with treat- ment by ultra-violet light. The ways of the vitamins, like the "heathen Chinee," are peculiar. But rickets is a disease of the bones, due to lack of salts of calcium, especially calcium phosphate, and the action of the anti- rachitic vitamin seems to be to hasten the deposition of calcium. Cod liver oil nor- mally contains the vitamin, and so it can be used as a cure. Other oils, like cotton- seed oil, or foods such as milk or flour, do not ordinarily contain it, but if they are exposed to ultra-violet light the vitamin seems to be formed, for then the anti- rachitic powers are bestowed on them. Then also, by exposing the child afflicted to the disease to the beneficial rays of the sun, or of the quartz tube mercury vapor lamp, the calcium deposition is also hastened and the disease relieved. Since egg shells consist largely of calcium, ex- posure of chickens to ultra-violet light hastens egg laying in the same way that it prevents or cures rickets in children. Such eggs also have anti-rachitic powers, so we may expect to see "eggs from sun-kissed hens" a common article in our markets of the future! In order that the ultra-violet rays may reach the body, it is necessary that they have an unobstructed path to the skin. Glass stops the rays, so that a sun bath indoors behind ordinary windows is of little value. Windows made of quartz, which can now be fused and made into a variety of shapes, including large windows, will let the rays through. Several kinds of glass have recently appeared on the market which are much less expensive than quartz, but still let the beneficial rays through in large quantity. Ordinary clothing also is opaque to the rays, so that at a sanitarium such as that of Dr. Rollier, in Switzerland, the patients, both children and older people, spend most of their time outdoors with a minimum of clothing. However, the so- called "artificial silk" or rayon, which is so much used, for hosiery and other articles of apparel, is partly transparent to the rays. We can conform with all the standards of modesty, and still get the ultra-violet rays. Unlike X-rays, ultra-violet rays are not very penetrating. They do not go very deep into the body, but reach only the outer skin, and it has been a puzzle how they have their effect on the bones. Doctor Hess has suggested an explanation which fits in pretty well with the observed facts. There is a substance known to the chem- ists as cholesterol, which is found in practi- cally every living animal, especially the skin and brain, a similar substance, phytosterol, being found in plants. Doctor Hess has found that when cholesterol is placed under ultra-violet rays, it achieves the power of preventing rickets, so he believes that this same process takes place in the skin when exposed to ultra-violet light. The radiated cholesterol is then carried by the blood to other parts of the body. To confirm this idea, Doctor Hess took pieces of animals' skins and fed them to rats, as their main article of diet. When the skin had been exposed to ultra-violet rays, the rats re- mained healthy but another group that were fed with skin that had not been radiated soon developed rickets. So another step has been made in under- standing how these rays work on the body. Beginning over two centuries ago with New ton, continuing with Ritter's discovery of the rays, our knowledge of them has gradually advanced. Though their medical uses are important, they are not the only applications, and the physicist constantly makes use of them. Just what part these rays play in the daily life of the physicist we shall see in a subsequent article.