174. The investigations into the nature of the radio-active emanations have thus led to the following conclusions:—The radio-elements thorium, radium and actinium continuously produce from themselves radio-active emanations at a rate which is constant under all conditions. In some cases, the emanations continuously diffuse from the radio-active compounds into the surrounding gas; in other cases, the emanations are unable to escape from the material in which they are produced, but are occluded, and can only be released by solution or by the action of heat.
The emanations possess all the properties of radio-active gases. They diffuse through gases, liquids, and porous substances, and can be occluded in some solids. Under varying conditions of pressure, volume, and temperature, the emanations distribute themselves in the same way and according to the same laws as does a gas.
The emanations possess the important property of condensation under the influence of extreme cold, and by that means can be separated from the gases with which they are mixed. The radiation from the emanation is material in nature, and consists of a stream of positively charged particles projected with great velocity.
The emanations possess the property of chemical inertness, and in this respect resemble the gases of the argon family. The emanations are produced in minute amount; but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum. With regard to their rates of diffusion, the emanations of both thorium and radium behave like gases of high molecular weight.
These emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character. These radiations consist entirely of α rays, i.e. particles, projected with great velocity, which carry a positive charge and have a mass about twice that of the hydrogen atom. The emanations do not possess the property of permanently radiating, but the intensity of the radiations diminishes according to an exponential law with the time, falling to half value, from actinium in 4 seconds, from thorium in one minute, and from radium in about four days. The law of decay of activity does not seem to be influenced by any physical or chemical agency.
The emanation particles gradually break up, each particle as it breaks up expelling a charged body. The emanation after it has radiated ceases to exist as such, but is transformed into a new kind of matter, which is deposited on the surface of bodies and gives rise to the phenomena of excited activity. This last property, and the connection of the emanation with it, are discussed in detail in the next chapter.
CHAPTER VIII.
EXCITED RADIO-ACTIVITY.
175. Excited radio-activity. One of the most interesting and remarkable properties of thorium, radium, and actinium, is their power of “exciting” or “inducing” temporary activity on all bodies in their neighbourhood. A substance which has been exposed for some time in the presence of radium or thorium behaves as if its surface were covered with an invisible deposit of intensely radio-active material. The “excited” body emits radiations capable of affecting a photographic plate and of ionizing a gas. Unlike the radio-elements themselves, however, the activity of the body does not remain constant after it has been removed from the influence of the exciting active material, but decays with the time. The activity lasts for several hours when due to radium and several days when due to thorium.
This property was first observed by M. and Mme. Curie[[269]] for radium, and independently by the writer[[270]] for thorium[[271]].
If any solid body is placed inside a closed vessel containing an emanating compound of thorium or radium, its surface becomes radio-active. For thorium compounds the amount of excited activity on a body is in general greater the nearer it is to the active material. In the case of radium, however, provided the body has been exposed for several hours, the amount of excited activity is to a large extent independent of the position of the body in the vessel containing the active material. Bodies are made active whether exposed directly to the action of the radio-active substance or screened from the action of the direct rays. This has been clearly shown in some experiments of P. Curie. A small open vessel a ([Fig. 62]) containing a solution of radium is placed inside a larger closed vessel V.