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Rh Production of Helium. It was stated in the earlier article that, since the particle is an atom of helium, all radioactive matter which emits a particles must produce helium. This has been found to be the case for every a-ray product that has been examined. The rate of production of helium by radium in equilibrium has been measured with accuracy by Dewar, Bolt- wood and Rutherford. In terms of the International Radium Standard, the rate of production of helium by one gram of radium in equilibrium with its three a-ray products has been found to be 164 cub. mm. per year. This value is in excellent accord with that calculated from the rate of emission of a particles, viz. 163 cub. millimetres. The rate of production of helium by the radium emanation, ionium and polonium has been found by Boltwood to be in fair agreement with calculation. Soddy has observed the production of helium by purified uranium, while Strutt showed that the rate of production of helium in uranium and thorium minerals accorded with calculation.

Strutt has made a systematic examination of the amount of helium present in many minerals and rocks which contain minute quantities of radium and has utilized the results to estimate the age of the geological deposits. On account of the tendency of the helium to escape from minerals in the course of geologic ages, this method gives only a minimum estimate of the age of the mineral, except in the case of very dense and compact specimens. The measurement of the lead content should ulti- mately prove a more reliable method of estimating the age.

Heat Emission of Radioactive Matter As was stated earlier, there is no doubt that the evolution of heat by radium and other radioactive matter is mainly a secondary phenomenon, resulting mainly from the energy of the absorbed radiation. Since the particles have a large kinetic energy and are easily absorbed by matter, all of these particles are stopped by the radium itself or by the envelope surrounding it and their energy of motion is transformed into heat. The evolution of heat from any type of radioactive matter is thus proportional to the energy of the expelled a particles, together with the energy of the /3 and 7 rays absorbed in the envelope. The energy supplied by the recoil of the radioactive atom after the expulsion of an a particle is about 2 % of the energy of the a particle.

These conclusions have been confirmed by the measurements of Rutherford and Robinson, who found that each of the a-ray prod- ucts gave a heating effect proportional to the energy of the a particle and absorbed /3 and y rays. The emanation and its products when removed from radium were responsible for three-quarters of the heating effect of radium in equilibrium. The heating effect of the radium emanation, radium A and radium C decayed at the same rate as their activity. From their measurements they found that the total heating effect of radium in equilibrium surrounded by sufficient material to absorb the a rays was 134-7 gram-calories per hour per gram. Of this, 123-6 gram-calories were due to the a particles, 4-7 to the /3 rays and 6-4 to the y rays. The energy of the /3 and y rays comes from radium B and radium C, but on account of their great penetrating power it is difficult to measure the /3 energy with accuracy. The results, however, show that the energy of the y rays is even greater than that of the /3 rays, and the two together are equal to about 28% of the energy of the a particles from radium C.

Measurements have been made of the heating effect of radium, uranium and thorium and of uranium and thorium minerals. In each case the evolution of heat is of about the magnitude to be expected from the energy of the radiations.

Radioactivity of Ordinary Mailer. Apart from the well- known radioactive elements of high atomic weight, only two other elements have been shown to exhibit the property of radioactivity to a detectable degree, viz. potassium and rubid- ium. Campbell showed that these elements emit only ft rays and in amount small compared with uranium. This property appears to be atomic, but no evidence has been obtained of any subsequent changes. If the ft particle comes from the nucleus of the atom, potassium should be transformed into an isotope of calcium, and rubidium into an isotope of strontium.

Radium and thorium have been found to be distributed, but in very minute amount, in the surface rocks and soil of the earth. The emanation from the soil diffuses into the atmosphere and causes a small ionization which can be readily measured. A penetrating 7 radiation, no doubt due to the presence of radium and thorium in the earth's crust, has been observed near the earth's surface, but becomes very small over a lake or the sea.

BIBLIOGRAPHY. Mme. Curie, Traite de Radioactivite (2 vols. 1910); E. Rutherford, Radioactive Substances and their Radiations (1913) ; St. Meyer and E. V. Schweidler, Radioaktivitat (1916) ; F. Soddy, Chemistry of the Radioelements, parts I. and II. (1914-5); see also under " Radioactivity " in annual Reports of the Chemical Society. (E. Ru.)

RADIOTHERAPY. Since 1910 there have been notable developments, extending the practice of X-ray treatment (see 28.887) m t the wider field now included in radiotherapy, a term which had not then come into general use. Strictly speaking, under this term should be included treatment by all kinds of rays; thus treatment by heat, by sun's rays, by ultra-violet rays, by X-rays and by the rays of radio-active substances, all come under the etymological term of radiotherapy. In practice, however, it is restricted to the application of ultra- violet rays, X-rays and radium rays. Amongst radiologists, the term has undergone an even sharper definition, so that radiotherapy is applied by them to treatment with X-rays alone, the terms radiumtherapy (or, in France curietherapy, in honour of the discoverer of radium) being applied to treat- ment with the rays of radium and other radio-active substances. Treatment by means of high frequency currents and diathermy are included rather under the term electrotherapy.

Ultra-violet Rays. These rays to a large extent are the es- sential feature of those forms of medical treatment which depend upon exposure to sunlight (heliotherapy). Probably this is not the whole story. Even though heat rays may also play some part, experience of the treatment of wounds by sunlight in France during the World War indicated that a degree of benefit arises from exposure to sunlight which cannot be entirely attributable to warmth and ultra-violet rays. On the other hand, in the Finsen light treatment of lupus and in the treat- ment of tuberculosis at high altitudes, ultra-violet rays probably play a predominant part. It is uncertain how these rays act; they penetrate but a fraction of a millimetre into the epithelium and yet the fact that in tropical countries where sunlight is great, the white races show a proverbial irritability which does not characterize the pigmented native races, suggests that in the one, effects are produced by the ultra-violet rays which the pigment of the other is able to eliminate. Certain it is that under ultra-violet light, persons vary in the appearances they present, those who freckle or tan easily when exposed to sun- light showing the potential freckles or bronzing of their skin by dark marks which are absent from the skins of those who do not freckle or tan easily. In this connexion, it is noteworthy that those tuberculous persons are said to derive greater benefit from a sojourn at high altitudes who normally tan easily under sunlight, than those who do not. The rays are bactericidal, but whether part of their action lies in this direction, is unknown.

X-rays. The X-rays which were discovered by Rontgen in 1895 are employed in medicine in two ways, firstly, as an aid to diagnosis when they form those branches of the subject known as radioscopy and radiography, and secondly, for the actual treatment of conditions when diagnosed. Thus by means of radioscopic or radiographic examination it may be found that there is a tumour in the chest, and as a result of that diagnosis it may be decided to institute treatment (radiotherapy) by means of X-rays or radium rays or the two combined.

X-radiation has the advantage that considerable doses can be employed. It has the disadvantage that the X-rays are frequently not of sufficiently penetrating power to serve for the treatment of tumours deep within the body. Three varieties of X-rays are used, the difference consisting in variations in wave lengths and in penetrating power. These varieties are known clinically as " soft," " medium " and " hard " X-rays, the soft rays being those of longer wave length and less pene- trating power, and the hard rays being those of shorter wave length and greater penetrating power. The softest X-rays are not used clinically; those employed in the treatment of ring- worm for example are " medium soft " since it is necessary for penetration to reach as deeply as the hair follicles. Medium