Read Ebook: Radio-Activity by Rutherford Ernest

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The preparations of emanium are in some cases luminous, and a spectroscopic examination of this light has shown a number of bright lines.

The distinctive character of the emanation of actinium, as well as of the other radio-active products to which it gives rise, coupled with the permanence of its activity, renders it very probable that actinium will prove to be a new radio-active element of very great activity. Although very active preparations of actinium have been obtained, it has not yet been found possible to free it from impurities. Consequently, no definite observations have been made on its chemical properties, and no new spectrum lines have been observed.

A more complete discussion of the radio-active and other properties of actinium is given in later chapters.

Sublimation in a vacuum. The active sulphide is more volatile than that of bismuth. It is deposited as a black substance at those parts of the tube, where the temperature is between 250 and 300? C. In this way polonium of activity 700 times that of uranium was obtained.

Precipitation of nitric acid solutions by water. The precipitated sub-nitrate is much more active than the part that remains in solution.

Precipitation by sulphuretted hydrogen in a very acid hydrochloric acid solution. The precipitated sulphides are much more active than the salt which remains in solution.

For concentration of the active substance Mme Curie has made use of method . The process is, however, very slow and tedious, and is made still more complicated by the tendency to form precipitates insoluble either in strong or weak acids. After a large number of fractionations, a small quantity of matter was obtained, enormously active compared with uranium. On examination of the substance spectroscopically, only the bismuth lines were observed. A spectroscopic examination of the active bismuth by Demar?ay and by Runge and Exner has led to the discovery of no new lines. On the other hand Sir William Crookes states that he found one new line in the ultra-violet, while Berndt, working with polonium of activity 300, observed a large number of new lines in the ultra-violet. These results await further confirmation.

The polonium prepared by Mme Curie differs from the other radio-active bodies in several particulars. In the first place the radiations include only very easily absorbable rays. The two penetrating types of radiation given out by uranium, thorium, and radium are absent. In the second place the activity does not remain constant, but diminishes continuously with the time. Mme Curie states that different preparations of polonium had somewhat different rates of decay. In some cases, the activity fell to half value in about six months, and in others, about half value in eleven months.

It may be of interest here to consider briefly the suggestions advanced at various times to account for the temporary character of the activity of polonium. Its association with bismuth led to the view that polonium was not a new active substance, but merely radio-active bismuth, that is, bismuth which in some way had been made active by admixture with radio-active bodies. It was known that a body placed in the vicinity of thorium or radium became temporarily active. The same action was supposed to take place when inactive matter was in solution with active matter. The non-active matter was supposed to acquire activity by "induction," as it was called, in consequence of its intimate contact with the active material.

There is no proof, however, that such is the case. The evidence points rather to the conclusion that the activity is due, not to an alteration of the inactive body itself, but to an admixture with it of a very small quantity of intensely active matter. This active matter is present in pitchblende and is separated with the bismuth but differs from it in chemical properties.

The polonium, if obtained in a pure state, should initially be several hundred times as active as pure radium. This activity, however, is not permanent; it decays with the time, falling to half value in about six months.

The absence of any new lines in the spectrum of radio-active bismuth is to be expected, for, even in the most active bismuth prepared, the active matter exists in a very small proportion.

The active substance was found to consist mainly of tellurium, and for this reason Marckwald gave it the name of radio-tellurium. In later work, however, Marckwald has shown that the active constituent has no connection with tellurium, but can always be separated completely from it by a simple chemical process.

In order to obtain a large amount of the active substance, 2000 kilos. of pitchblende were worked up. This yielded 6 kilos. of bismuth oxychloride, and from this was separated 1?5 grams of radio-tellurium. The tellurium present was precipitated from a hydrochloric acid solution by hydrazine hydrochloride. The precipitated tellurium still showed some activity, but this was removed by repeating the process. The active matter then remained in the filtrate, and, after evaporation, the addition of a few drops of stannous chloride caused a small quantity of a dark precipitate which was intensely active. This was collected on a filter and weighed only 4 milligrams.

When plates of copper, tin or bismuth were dipped into an hydrochloric acid solution of this active substance, the plates were found to be covered with a very finely divided deposit. These plates were intensely active, and produced marked photographic and phosphorescent action. As an illustration of the enormous activity of this deposit, Marckwald stated that a precipitate of 1/100 milligram on a copper plate, 4 square centimetres in area, illuminated a zinc sulphide screen so brightly that it could be seen by an audience of several hundred people.

The active substance of Marckwald is very closely allied in chemical and radio-active properties to the polonium of Mme Curie. Both active substances are separated with bismuth and both give out only easily absorbed rays. The penetrating rays, such as are given out by uranium, radium or thorium, are completely absent.

There has been a considerable amount of discussion as to whether the active substance obtained by Marckwald is identical with that present in the polonium of Mme Curie. Marckwald stated that his active substance did not sensibly diminish in activity in the course of six months, but it is doubtful whether the method of measurement used was sufficiently precise.

The writer has found that radio-tellurium of moderate activity, prepared after Marckwald's method and sold by Dr Sthamer of Hamburg, undoubtedly loses its activity with time. The radio-tellurium is obtained in the form of a thin radio-active deposit on a polished bismuth rod or plate. A bismuth rod was found to have lost half its activity in about 150 days, and a similar result has been recorded by other observers.

This polonium can be separated temporarily from the lead by suitable chemical methods, but the radio-lead still continues to produce polonium, so that a fresh supply may be obtained from it, provided an interval of several months is allowed to elapse.

It will be calculated later that in all probability the radio-lead would lose half of its activity in an interval of 40 years.

The constituent present in radio-lead has not yet been separated, but it will be shown that, in the pure state, it should have an activity considerably greater than that of radium itself. Sufficient attention has not yet been paid to this substance, for, separated in a pure state, it should be as useful scientifically as radium. In addition, since it is the parent of polonium, it should be possible to obtain from it at any time a supply of very active polonium, in the same way that a supply of the radium emanation can be obtained at intervals from radium.

Hofmann and Strauss have observed a peculiar action of the cathode rays on the active lead sulphate separated by them. They state that the activity diminishes with time, but is recovered by exposure of the lead for a short time to the action of cathode rays. No such action is shown by the active lead sulphide. This effect is due most probably to the action of the cathode rays in causing a strong phosphorescence of the lead sulphate and has nothing to do with the radio-activity proper of the substance.

Recent work of Hofmann and Zerban and of Baskerville, however, certainly tends to show that the element thorium is itself non-radio-active, and that the radio-activity observed in ordinary thorium compounds is due to the admixture with it of an unknown radio-active element. Hofmann and Zerban made a systematic examination of the radio-activity of thorium obtained from different mineral sources. They found generally that thorium, obtained from minerals containing a large percentage of uranium, were more active than those obtained from minerals nearly free from uranium. This indicates that the radio-activity observed in thorium may possibly be due to a transformation product of uranium which is closely allied chemically to thorium and is always separated with it. A small quantity of thorium obtained from the mineral gadolinite was found by Hofmann to be almost inactive, whether tested by the electric or by the photographic method. Later Baskerville and Zerban found that thorium obtained from a Brazilian mineral was practically devoid of activity.

If, as we have suggested, thorium itself is not active, it is certainly a matter of surprise that ordinary commercial thorium and the purest chemical preparations show about the same activity. Such a result indicates that the methods of purification have not removed any of the radio-active constituent originally present.

Whatever the radio-active constituent in thorium may ultimately prove to be, it is undoubtedly not radium nor actinium nor any of the known radio-active substances.

In later chapters, the radio-activity of thorium will, for simplicity, be discussed on the assumption that thorium is itself a radio-active element. The analysis of the changes which occur will thus not refer to thorium itself but to the primary radio-active substance usually found associated with it. The conclusions to be drawn from an examination of the radio-active processes are for the most part independent of whether thorium is itself radio-active or whether the radio-activity is due to an unknown element. If thorium is not radio-active itself, it is not possible to draw any conclusions upon the question of the duration of the primary radio-activity associated with it. Such a deduction cannot be made until the quantity of the radio-active element present in thorium has been definitely determined.

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