Phosphorescent substances are altered by the prolonged action of the radium rays. They become less excitable and are less luminous under the influence of the rays. At the same time they change their color or become colored. Glass becomes violet, black or brown. Salts of the alkalies turn yellow, green or blue. Transparent quartz becomes smoky quartz. Colorless topaz turns yellow, orange, etc. Glass colored by radium is thermoluminescent. On heating it to 500° it emits light. At the same time it becomes colorless and returns to its original condition. It is then capable of being colored anew by the action of the rays of radium.
The salts of radium are spontaneously luminous. It might be said that they make themselves luminous by the action of the Becquerel rays they emit. Anhydrous chloride and bromide of radium are the salts that give the most intense luminescence. They may be obtained so luminous that the light can be seen in full daylight. The light emitted by the salts of radium recalls in tint that from a fire-fly. The luminosity of the radium salt decreases with time without ever completely disappearing, and those that were colorless at first become gray, yellow or violet.
Physiological Effects of the Radium Rays.—The rays from radium cause different physiological actions.
A salt of radium placed in an opaque box made of cardboard or metal acts on the eye and produces the sensation of light. To obtain this result, the box containing the radium is placed before the closed eye or against the temple. In this experiment the center of the eye becomes luminous by phosphorescence under the influence of the radium rays, and the light one sees has its source in the eye itself.
The rays of radium act on the epidermis. If we place on the skin for a few minutes a bulb containing radium no particular sensation is felt. But fifteen or twenty days afterward it produces a reddening of the skin, then a slough in the place where the bulb was applied. If the action of the rays be long enough there is finally formed a sore that takes several months to heal. The action of the rays from radium is analogous to that produced by the Roentgen rays. The attempt has been made to utilize this action in the treatment of lupus and cancer.
The radium rays also act upon the nervous centers and cause paralysis and death. (Danysz.) They seem also to act with especial intensity upon growing tissues. (Bohn.)
The Employment of Radium in the Study of Atmospheric Electricity.—The radium rays have also been utilized in the study of atmospheric electricity (Paulsen, Witkowski, Moureaux). A small quantity of a salt of radium placed at the extremity of a metallic rod serves as a point of contact for the potential. By this very simple arrangement we can avoid the use of flames or of water-dropping apparatus to measure the potential at any point in the atmosphere.
III. The Heat Given Off by the Salts of Radium.
The salts of radium continually give off heat. Its amount is sufficient for it to be detected by means of a crude experiment with two ordinary mercury thermometers. Two identical heat-insulating vacuum-bulbs are used. In one of these is placed a glass bottle containing 0.7 gram of pure radium bromide; in the second is placed a glass bottle containing some inactive substance, such as barium chloride. The temperature of the contents of each is shown by a thermometer placed with its bulb near the bottle. The mouths of the flasks are closed with cotton. Under these conditions the thermometer in the same flask as the radium always has a temperature 3° higher than that indicated by the other thermometer.
The quantity of heat given off can be estimated by means of a Bunsen ice calorimeter. By placing in the calorimeter a glass bottle containing a salt of radium, it is found that there is a continued supply of heat that stops as soon as the radium is removed. A determination made with a salt of radium that had been prepared a long time before shows that each gram of radium gives off 80 small calories an hour. Hence radium gives off enough heat to melt its own weight of ice every hour. Nevertheless the radium salt seems to remain in the same condition, and besides, not a single ordinary chemical reaction can be called in to explain such a continuous liberation of heat.