See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

Consequently an old mineral containing loco kilos of uranium should contain 340 milligrams of pure radium. In addition to radium and polonium, a number of other radioactive substances have been found in uranium minerals. With the exception of the radium emanation, none of these have yet been isolated in a pure See also:

• STATE
• STATE, GREAT OFFICERS OF

In later work, he showed that the new substance could be chemically separated from tellurium. By treating the residues from 15 tons of Joachimsthal pitchblende, Marckwald (If) finally obtained 3 milligrams of intensely active material—far more active weight for weight than radium. It has been definitely settled that the active substance of Marckwald is identical with polonium. Both substances give out a type of easily absorbed a rays and both lose their activity at the same rate. The activity of polonium decays in a geometrical progression with the time and falls to See also:

• HALF

Preparations of ionium have been obtained several thousand times as active as uranium. Ionium emits a rays of See also:

• SHORT, FRANCIS JOB (1857– )

Bodies which phosphoresce under X rays usually do so under the rays from radioactive matter. See also:

• BARIUM (symbol Ba, atomic weight 137'37 [0=16])

On the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

On the See also:

• ASSUMPTION, FEAST OF

These three types of radiations all set up secondary radiations in passing through matter. A pencil of /3 rays falling on matter is widely scattered in all directions. This scattered radiation is some-times called the secondary (3 rays. They rays give rise to secondary rays which consist in part of scattered 7 rays and in part electrons moving with a high velocity. These secondary rays in turn produce See also:

• TERTIARY

This emanation can he carried away by a current of air and its- properties tested apart from the substance which produces it. A little later Dorn showed that radium possesses a similar property, while Giesel and Debierne observed a similar effect with actinium. These emanations all possess the property of ionizing a gas and, if sufficiently intense, of producing marked photographic andphosphorescent See also:

• ACTION

The emanations have all the properties of radioactive gases. They can be transferred from point to point by currents of air. The emanations can be separated from the air or other gas with which they are mixed by the action of extreme See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

The See also:

• SURFACE

The active deposits obtained on a See also:

• PLATINUM [symbol Pt, atomic weight 145.0 (0=16)]

The atoms of the emanation are far more unstable than the atoms of radium, and break up at a much quicker rate. We shall now consider the law of radio-active transformation according to this theory. It is experimentally observed that in all simple radioactive substances, the tensity of the radiation decreases in a geometrical progression with the time, i.e. I/Io = e-' where I is the intensity of the radiation at any time t, Io the initial intensity, and X a See also:

• CONSTANT, BENJAMIN (1845-1902)

As far as our observation has gone, the law of radioactive See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

Suppose that initially radium is completely freed from emanation. In consequence of the steady supply, the amount of emanation present increases, but not at a constant rate, for the emanation is in turn breaking up. Let q be the number of atoms of emanation produced by the radium per second and N the number present after an interval t, then dN/dt = q-AN where A is the radio-active constant of the emanation. It is obvious that a steady state will ultimately be reached when the number of atoms of emanation supplied per second are on the average to the atoms which break up per second. If No be the maximum number, q=AN0. Integrating the above See also:

• EQUATION (from Lat. aequatio, aequare, to equalize)

Suppose n be the number of atoms of A initially present. From the law of radioactive change it follows: P =ne-'See also:

• FIT

(c) The matter A is supplied at a constant rate from a primary source. Required to find the number of particles of A, B and C present at any time t later, when initially A, B, and C were absent. This is a converse case from case (2) and the solutions can be obtained from general considerations. Initially suppose A, B and C are in equilibrium with the primary source which supplied A at a constant rate. The source is then removed and the amounts of A, B and C vary according to the equation given in case (2). The source after removal continues to supply A at the same rate as before. Since initially the product A was in equilibrium with the source, and the radioactive processes are in no way changed by the removal of the source, it is clear that the amount of A present in the two.parts in which the matter is distributed is unchanged. If Pi be the amount of A produced by the source in the time t, and P the amount remaining in the part removed, then P1+P=P0 where P, is the equilibrium value. Thus PI/Po = i -P/P0. The ratio P/P0 can be written down from the solution given in case (2). Similarly the corresponding values of Ql/Qo, R1/Ro may be at once derived. It is obvious in these cases that the curve plotted with P/Po as ordinates and time as abscissae is complementary to the corresponding curve with PI/P0 as ordinates.

This simple relation holds for all recovery and decay curves of radioactive products in general. We have so far considered the variation in the number of atoms of successive products with time when the periods of the products are known. In practice, the variation of the number of atoms is deduced from measurements of activity, usually made by the electric method. Using the same notation as before, the activity of any product is proportional to its rate of breaking up, i.e. to X1P where P is the number of atoms present. If two products are present, the activity is the sum of two corresponding terms X1P and X2Q. In practice, however, no two products emit a or 13 particles with the same velocity. The difference in ionizing power of a single a particle from the two products has thus to be taken into account. If, under the experimental conditions, the ionization produced by an a particle from the second product is K times that from the first product, the activity observed is proportional to X1P+KX2Q. In this way, it is possible to compare the theoretical activity curves of a mixture of products with those deduced experimentally. Analysis of Radioactive Changes.—The analysis of the successive changes occurring in uranium, thorium, radium and actinium has proved a very difficult matter. In order to establish the existence of a new product and to See also:

• FIX, THEODORE (180o-1846)

It must be remembered that in most cases the amount of radio-active matter under examination is too small to detect by weight, but its presence is inferred from its characteristic radiations and rate of change. In some cases, a separation may be effected by ordinary chemical methods; for example thorium X is separated from thorium by precipitation of thorium with See also:

• AMMONIA (NH3)

This property was independently investigated by Russ and Makower (21) and by See also:

• HAHN, AUGUST (1792–1863)

In each case, the half period of transformation is given and the type of radiation emitted. If the product emits a rays, the range of ionization of the a particle in air is given. Half Period Range Product. of Rays. of Rays Transforma- in Air in tion. Cms. URANIUM— 5 X lo' years a 3'5 Uranium X 22 days 0+7 Ionium . ? a 2.8 RADIUM— 176o years a 3'5 Ra Emanation. 3.86 days a 4'33 Radium A 3 mins. a 4'83 Radium B 26 mins. slow ,B 7.06 Radium C 19 mins. a+ +y Radium D 17 years slow # Radium E 5 days 13 Radium F 140 days a 3.86 Radium G =lead? about IOl° yrs. 3.5 THORIUM— (Th. I) 5'5 years rayless Mesothorium (Th. 2) 6.2 hours /3+7 Radiothorium . 737 days a 3'9 Thorium X 3.6 days a 5.7 Th Emanation 54 secs a 5'5 Thorium A Io•6 hours slow 13 5.0 Thorium B 55 mins. a Thorium C very short? a 8.6 Thorium D 3 mins. 13+7 ACTINIUM— ? rayless Radioactinium .

19.5 days a-FP 4.8 Actinium X 11.8 days a 6.55 See also:

• ACT (Lat. actus, actum)

Hahn (31) found that a very active substance emitting a rays, which gave rise to thorium X, could be separated from thorium minerals. This active substance, called radiothorium, has been closely examined by Hahn and See also:

• BLANC, (JEAN JOSEPH CHARLES) LOUIS (1811-1882)
• BLANC, MONT

For many purposes active preparations of these substances would be as valuable as radium itself, and the amount of active matter from this source would be greater than that at present available from the separation of radium from uranium minerals. Actinium.—The transformations observed in actinium are very analogous to those in thorium. Actinium itself is a rayless product which changes into radioactinium, an a ray product of period 19.5 days, first separated by Hahn (32). This changes into actinium X, of period 10.2 days, first separated by Godlewski (33). Actinium X is transformed into the emanation which in turn gives rise to three further products, called actinium A, B and C. Although very active preparations of actinium have been prepared, it has so far not been found possible to separate the actinium from the rare earths with which it is mixed. We do not in consequence know its atomic weight or spectrum. Origin of Radium.—According to the transformation theory, radium, like all other radioactive products, must be regarded as a changing element. Preliminary calculations showed that radium must have a period of transformation of several thousand years. Consequently in order that any radium could exist in old minerals, the supply must be kept up by the transformation of some other substance. Since radium is always found associated with uranium minerals, it seemed probable from the, beginning that uranium must be the primary element from which radium is derived. If this were the case, in old minerals which have not been altered by the action of percolating See also:

• WATERS, TERRITORIAL

This must evidently be the case, for in a state of equilibrium the rate of breaking up of radium must equal the rate of supply of radium from uranium.