, and the representative curve has become straight. From this moment the activity diminishes by one-half during each period of twenty-eight minutes.
Figure V
If the duration of the action of the emanation is not so long, the law of the variation of the radiation during its loss is much more complex. In Fig. V. are represented the results of experiments for different times of action, as indicated on the corresponding curves. We see, for example, that for a time of excitation of five minutes, the intensity of the radiation during the loss of activity first falls rapidly to a minimum; then it increases, and again begins to fall. Finally, the law of loss of activity tends towards a simple exponential law which is the same as the limiting law after prolonged excitation. These complex phenomena can be explained by assuming that on the excited plate the radioactive energy is in three distinct states, but the demonstrations relative to this subject are too long to have a place in this article.
The emanation from radium causes the energetic phosphorescence of a large number of substances. Glass reservoirs containing air charged with the emanation are luminous, Thuringian glass being the most sensitive. Phosphorescent sulphide of zinc is particularly sensitive to the action of the emanation from radium, and then gives an intense light.
If in a closed space substances become more active the greater the amount of free gas around them. When plates placed parallel to one another, and a short distance apart, are put into a closed space with radium, the faces of each plate become excited in proportion to its distance from the others. When glass tubes of different diameters are filled with the emanation and communicate with one another, the walls of those of greatest interior diameter become most active. These tubes are also most luminous. To interpret these facts it must be assumed that the air charged with the emanation acts on the walls by a radiation that arises in every part of the gaseous mass, and that the radioactivity induced upon a wall is proportional to the flow of exciting radiation received by that wall.
The Slow Evolution of Induced Activity.—A solid body acquires a very feeble, persistent, induced activity when it remains a month or less in contact with the radiation from radium. A substance withdrawn from the influence of the emanation after having been subjected to it for a long time loses its activity rapidly at first, according to the laws given. But the activity of the radiation does not disappear completely. There remains a radiation several thousand times more feeble than it was at first. This radiation is given off with extreme slowness and continues for several years. (The radiation passes through a minimum, then slowly increases for several months, but always remains very slight.)
Occlusion of the Emanation of Radium by Solids.—All solids when excited by contact with the emanation from radium acquire the property of themselves emitting the emanation in very small quantity. They preserve this power for only twenty, minutes from the time they are removed from the space containing the emanation. Nevertheless certain substances as caoutchouc, paraffin, and celluloid have the property of being saturated with the emanation, and of emitting it afterwards in abundance for several hours, or even days.
Induced Activity of Liquids.—When a liquid is placed in a closed space with radium it becomes radioactive. Water, salt solutions, petroleum, etc., can thus be excited. The liquids dissolve a certain amount of the emanation. When an excited liquid is separated from the radium and sealed up in a tube it slowly loses its activity according to the law of the destruction of the emanation (decrease to one-half in four days). When the liquid is placed in a flask open to the air it loses its activity very rapidly, and the emanation spreads into the surrounding air.
Variations of the Activity of Solutions of Radium Salts and of the Solid Salts of Radium.—When a solution of a salt of radium is exposed to the air of a room in an open vessel it becomes almost inactive. The solution emits an emanation that spreads into the room and causes induced radioactivity of the walls. The radiation of the radium is thus externalized. If the solution be enclosed in a sealed tube its activity increases little by little, and tends toward a limiting value that is reached only after several months. No doubt the emanation produced by the radium accumulates in the tube until the velocity of its spontaneous destruction is equalized by the supply from the radium.