Protection of the skin from U.V. rays is brought about mainly by swelling and thickening of the horny layer of the epidermis. The pigment which results from sunburn chiefly protects the blood from over-heating by visible rays. By absorbing of visible rays, nerve endings in the epidermis are excited, and this provokes sweating which cools the skin by evaporation. The naked pig mented races have a thin skin, and, sweating readily in the sun, lose heat easily in the shade. The pigmented skin of such races can be inflamed by U.V. rays. So long as the skin is kept cool by a flow of water, visible rays, even when concentrated by a lens (burning glass), have no sunburn effect on the skin.
The U.V. rays, on the other hand, sunburn the cold skin. Glacial burns are often very severe. It is possible to sensitize the skin to visible rays by injection of various dyes, eosin, erythrosin, and by a derivative of haemoglobin, viz., haematoporphyrin. The white skin thus sensitized, reacts to visible rays as it does to U.V. rays. Black skins are immune. Certain foods may have this sensitizing effect. Thus, only black pigs can be kept in tropical America, where the red mangold grows. There are rare cases of unfortunate people sensitive to visible rays. Exposed to light, their skin be comes inflamed and necrosed. They have to live like night animals. In some of these, haematoporphyrin is found in the urine.
Ultra-violet rays have great power to kill microbes, but as their penetrating power is very small, they only sterilize surfaces. In a skin disease such as lupus, these rays effect cure, not by directly killing the infecting tubercle bacilli, but by provoking an inflam matory reaction of the tissues which results in killing of the microbes and repair of the damage. Ultra-violet rays benefit the health of chicks and provoke egg-laying and fertility of fowls kept indoors under glass. They also, in suitable dosage, benefit growth of vegetables and fruits under ordinary glass. A particular kind of glass, called vitaglass, is transparent to the U.V. rays of the sun, and this is used for skylights with advantage. At the new monkey house at the London Zoo, the animals are given warm shelves, in candescent lamps, vitaglass skylights, louvres always open in the roof, and free access to open air playgrounds through openings closed by hinged flaps. Bathing pools are set in the playgrounds. Food rich in vitamins, with plenty of fruit and raw vegetable, is provided. These are conditions which, if provided in creches in cities, would raise an Al population.
Light-baths are very useful for treatment of rickets and tubercu losis, particularly tuberculosis of the skin (lupus), glands and joints ; also many skin and eye diseases, and some forms of bald ness. They may benefit chronic infections, e.g., chronic bronchitis and asthma. Combined with visible and infra-red rays, or red rays, they may benefit forms of chronic rheumatism. Used in win ter months, light-baths act as a tonic, and prevent rickets and debility of children.
The sources of U.V. rays are the sun and arc lamps. The U.V.
of the sun is limited to rays longer than about 30o owing to absorption of shorter rays by a layer of ozone placed miles high in the sky.
The artificial sources of U.V. rays are :—(1) The mercury va
pour lamp ; (2) the short flame arc ; (3) the long flame arc. The last two may have plain carbon, or metal-cored carbon poles. Metal poles, such as tungsten or iron may be used with a short flame arc. The mercury vapour lamp is a powerful but relatively cold source of U.V. rays. A ring of incandescent lamps added to this source affords visible and infra-red rays for heating. The short flame arc, with plain carbon poles, gives much visible and infra red and some U.V. rays as the sun does. The long flame arc, with metal-cored carbon poles gives plenty of visible and infra-red, and is at the same time a powerful source of U.V. rays. Given equal energy, the U.V. from the long flame arc may at right angles to the flame be 14 times that of the short flame arc. Much of the U.V. rays come from the flame, and to effect the best output, the posi tive pole must be placed below and be metal-cored (iron). The negative pole, placed above, can be plain carbon.
Half a dozen people can sit round a long flame arc, taking 2,500 Watts; at two feet distance, a dose of five minutes suffices. One half of the body can be bathed at a time, and two baths a week given. For intense local treatment, e.g., required for lupus, the Kromayer, or water-cooled mercury vapour-lamp is most valuable, the dose given being five minutes, with the quartz glass of the lamp pressed on the skin—a dose sufficient to produce strong reac tion and desquamation. The dose is repeated when the desquama tion has passed away and the skin is again susceptible. The mer cury vapour lamp or the short flamed arc with metal poles can be run off the ordinary house circuit and afford efficient sources. By means of a frosted Vitreosil screen the powerful mercury vapour lamp can be reduced to an intensity safe for domestic use.
With a screen of Chance's dark glass which transmits almost only long U.V. rays the fluorescence of substances can be studied in a dark room. This is a most useful method of identification. Thus, hair affected with ring-worm can be identified.
The far U.V. rays merge into the soft X-rays. The penetration of the skin increases again with the shortening of wave length. The X-rays produce inflammatory reaction after a long latent period, e.g., two to three weeks. They are used to check the growth of cancer cells and to produce sterility. Acting on embryoes, deformi ties in growth and monsters may be produced by their means. Owing to the great penetration of the soft tissues by X-rays in comparison with bones, photographs of fractures and disease of bones can be obtained. The shorter the X-ray the greater is the penetration. General mild doses of X-rays have been used in treatment of asthma, and to prevent recurrence of cancer after removal of the primary growth.
X-rays are used for cure of skin diseases such as ring-worm. Great care must be used, for X-ray burns have a tendency to turn into cancer. The gamma rays of radium resemble very short ("hard") X-rays, and have the same effect as these. Tubes con taining radium emanation are buried in cancers which cannot be removed with the knife, with the view of stopping growth. In certain accessible sites this treatment may be successful.