radio-active properties. This activity is due to the presence of a very small proportion of an active constituent called radio-lead, which has chemical properties identical with those of ordinary lead. The bulk of the lead obtained from radio-active minerals differs in atomic weight from ordinary lead and appears also to be different according to whether its source is a thorium or a uranium mineral.

A large number of other radio-active substances have been separated and some of their properties determined, but these were found by different means and will be noted in their proper place. They number in all more than thirty. The sources or parents of these are the original uranium or thorium, and the products form regular series with distinctive properties for each member.

CHAPTER II

PROPERTIES OF THE RADIATIONS

The activity of these radio-active bodies consists in the emission of certain radiations which may be separated into rays and studied through the phenomena which they cause.

One of these phenomena is the power of forming ions or carriers of electricity by the passage of the rays through a gas, thus ionizing the gas. The details of an experiment will serve to make the meaning of this ionization clear.

Fig. 1.—Ionization of Gases.

When this apparatus is set up a minute current will be observed without the introduction of any radio-active matter. This, as Rutherford says, has been found due mainly to a slight natural radio-activity of the matter composing the plates. If radio-active matter is spread on plate A, which is connected with one pole of a grounded battery, and if plate B is connected with an electrometer which is also connected with the earth, a current is caused which increases rapidly with the difference of potential between the plates, then more slowly until a value is reached that changes only slightly with a larger increase in the voltage.

According to the theory of ionization, the radiation produces ions at a constant rate. The ions carrying a positive charge are attracted to plate B, while those negatively charged are attracted to plate A, thus causing a current. These ions will recombine and neutralize their charges if the opportunity is given. The number, therefore, increases to a point at which the ions produced balance the number recombining.

When an electric field is produced between the plates, the velocity of the ions between the plates is increased in proportion to the strength of the electric field. In a weak field the ions travel so slowly that most of them recombine on the way and consequently the observed current is very small. On increasing the voltage the speed of the ions is increased, fewer recombine, the current increases, and, when the condition for recombination is practically removed, it will have a maximum value. This maximum current is called the saturation current and the value of the potential difference required to give this maximum current is called the saturation P.D. or saturation voltage.

The picture, then, is this. The radiations separate the components of the gas into ions, or carriers of electricity, half of which are charged negatively and half positively. In the electric field those negatively charged seek the positive plate and those positively charged seek the negative plate. If time is given, these ions meet and recombine, their charges are neutralized, and there is no current.

This theory of the ionization of gases has been most interestingly confirmed by direct experiment. For instance, the ions may form nuclei for the condensation of water, and in this way the existence of the separate ions in the gas may be shown and the number present actually counted.

When air saturated with water vapor is allowed to expand suddenly, the water present forms a mist of small globules. There are always small dust particles in air and around these as nuclei the drops are formed. These drops will settle and thus by repeated small expansions all dust nuclei may be removed and no mist or cloud will be formed by further expansions.

If now the radiation from a radio-active body be introduced into the condensation vessel, a new cloud is produced in which the water drops are finer and more numerous according to the intensity of the rays. On passing a strong beam of light through the condensation chamber, the drops can readily be seen. These drops form on the ions produced by the radiation.

If the condensation chamber has two parallel plates for the application of an electric field like that already described, the ions will be carried at once to the electrodes and disappear. The rapidity of this action depends upon the strength of the electric field and experiment shows that the stronger the field the smaller the number of condensation drops formed. If there is no electric field, a cloud can be produced some time after the shutting off of the source of radiation, showing that time is required for the recombination of the ions.

If the drops are counted (there being special methods for this) and the total current carried accurately measured, then the charge carried by each ion may be calculated. This has been determined. The mass of an ion compared with the mass of the molecules of gas in which it was produced can also be approximately estimated. In the study of these ions the view has been held that the charged ion attracted to itself a cluster of molecules which surrounded the charged nucleus and traveled with it. It is roughly estimated that about thirty molecules of the gas cluster around each charged ion.

Fig. 2.—Photograph of the Track of an Ionizing Ray.

Utilizing the fact that these ions with their clusters of molecules form nuclei for the condensation of water vapor, C. T. R. Wilson has by instantaneous photography been able to photograph the track of an ionizing ray through air. The number of the ions produced, and hence the number of drops, is so great that the trail is shown as a continuous line. In the copy of this photograph it will be seen that at some distance from its source the straight trail is slightly but abruptly bent. Near the end of its course there is another abrupt and much sharper bend. These bends show where the ionizing ray, in this case an alpha particle, has been deflected by more or less direct collision with an atom. These collisions and the final disappearance of the ray will be discussed later.

Taking up now other means of examining these radiations, it is well to consider their action upon a photographic or sensitive plate. It will be recalled that this was the method by which their existence was originally detected. To illustrate the method, the following account of how one such photograph was taken may be given.

The plate was wrapped in two thicknesses of black paper. The objects were placed upon this and