422 



NA TURE 



[March 5, 1908 



RECENT ADVANCES IN RADIO-ACTIVITY.^ 



IN 1904 I had the honour of giving an address at the 

 Roval Institution on the subject of radio-activity. In 

 the interval steady and rapid progress has been made in 

 unravelling the tangled skein of radio-active phenomena. 

 In the present lecture I shall endeavour to review very 

 shortly some of the more important advances made in the 

 last few years, but as I cannot hope to mention, even 

 briefly, the whole additions to our knowledge in the various 

 branches of the subject, I shall confine my attention to a 

 few of the more salient facts in the development of which 

 I have taken some small share. 



In my previous lecture I based the explanation of radio- 

 active phenomena on the disintegration theory put forward 

 in 1903 by Rutherford and Soddy, which supposes that 

 the atoms of the radio-active bodies are unstable systems 

 which break up with explosive violence. This theory has 

 stood the test of time, and has been invaluable in guiding 

 the e.xperimenter through the maze of radio-active com- 

 plications. In its simplest form, the theory supposes that 

 every second a certain fraction (usually very small) of the 

 atoms present become unstable and explode with great 

 violence, expelling in many cases a small portion of the 

 disrupted atom at a high speed. The residue of tire atom 

 forms a new atomic system of less atomic weight, and 

 possessing physical and chemical properties which markedly 

 distinguish it from the parent atom. The atoms com- 

 posing the new substance formed by the disintegration of 

 the parent matter are also unstable, and break up in turn. 

 The process of degradation of the atom, once started, pro- 

 ceeds through a number of distinct stages. These new 

 products formed by the successive disintegrations of the 

 parent matter are in most cases present in such e.xtremely 

 minute quantity that they cannot be investigated by 

 ordinary chemical methods. The radiations from these 

 substances, however, afford a very delicate method of 

 qualitative and quantitative analysis, so that we can obtain 

 some idea of the physical and chemical properties of sub- 

 stances existing in an amount which is far below the limit 

 of detection of the balance or spectroscope. 



The law that governs the breaking up of atoms is very 

 simple and universal in its application. For any simple 

 substance, the average number of atoms breaking up per 

 second is proportional at any time to the number present. 

 In consequence, the amount of radio-active matter decreases 

 in a geometrical progression with the time. The " period " 

 of any radio-active product, i.e. the time for half the 

 matter to be transformed, is a definite and characteristic 

 property of the product which is uninfluenced by any of 

 the laboratory agents at our command. In fact, the period 

 of any radio-active product, for example, the radium 

 emanation, if determined with sufficient accuracy, might 

 well be taken as a definite standard of time, independent of 

 all terrestrial influences. 



The law of radio-active transformation can be very 

 simply and aptly illustrated by an hydraulic analogy. Sup- 

 pose we take a vertical cylinder filled with water, with an 

 opening near the base through which the water escapes 

 through a high resistance.- When the discharge is started 

 the amount of water escaping per second is proportional 

 to the height of water above the zero level of the cylinder. 

 The height of water decreases in a geometrical progression 

 with the time in exactly the same way as the amount of 

 radio-active matter decreases. We can consequently take 

 the height of the column of water as representing the 

 amount of radio-active matter A present at any time. The 

 quantity of water escaping per second is a measure of the 

 rate of disintegration of A and also of the amount of the 

 new substance B formed per second by the disintegration 

 of A. The " period " of the substance is controlled by 

 the amount of resistance in the discharge circuit. A high 

 resistance gives a small flow of water and a long period 

 of transformation, and vice vcrsd. By a suitable arrange- 

 ment we can readily trace out the decay curve for such a 

 case. A cork carrying a light vertical glass rod is floated 

 on the water in the cvlinder. A light camel's hair brush 



1 A discourse ... „ 



by Prof. E. Ruiherford, F.R.S, 



- A short glass tube in which 

 suitable. 



d at the Royal Institution on Friday, January 31 

 placed a plug of glass wool is verj 



NO. 2001, VOL. yj] 



is attached at right angles, and moves over the surface of 

 a smoked-glass plate. A vertical line drawn on the glass 

 through the point of contact of the brush gives the axis 

 of ordinates, while a horizontal line drawn through the 

 brush when the water has reached its lowest level gives 

 the axis of abscisss. If the glass plate is moved with 

 uniform velocity from the moment of starting the discharge 

 a curve is traced on the glass which is identical in shape 

 with the curve of decay of a radio-active product, where 

 the ordinates at any time represent the relative amount 

 of active matter present, and the abscissae time. With 

 such an apparatus we can illustrate in a simple way the 

 increase with time of radio-active matter B, which is 

 supplied by the transformation of a substance A. This 

 will correspond, for example, to the growth of the radium 

 emanation with time in a quantity of radium initially freed 

 from emanation. Let us for convenience suppose that A 

 has a much longer period than B. In the hydraulic 

 analogy A is represented by a high head of water dis- 

 charging at its base through a circuit of high resistance 

 into the top of another cylinder representing the matter B. 

 The water from the cylinder B escapes at its base through 

 a lower resistance. Suppose that initially only A is pre- 

 sent. In this case the water in the cylinder B stands at zero 

 level. On opening the stop-cock connecting with A, water 

 flows into B. The rise of water with time in the cylinder 

 B is traced out in the same way as before by moving the 

 glass plate at a constant rate across the tracing brush. 

 If the period of A is very long compared with that of B 

 the water is supplied to B at a constant rate, and the 

 water in B reaches a constant maximum height when the 

 rate of supply to B equals the rate of escape from the 

 latter. The curve traced out in that case is identical in 

 shape with the " recovery curve " of a radio-active product 

 supplied at a nearly constant rate. The quantity of matter 

 reaches a maximum when the rate of supply equals its 

 own rate of transformation. The relative height of the 

 columns of water in .\ and B represents at any time the 

 relative amounts of these substances present. 



If the period is comparable with that of B, the height 

 of water in B after reaching a maximum falls again, since 

 as the height of .\ diminishes the supply to B decreases. 

 Ultimately, the height of B will decrease in a geometrical 

 progression with the time at a rate corresponding to the 

 longer period of the two. This is an exact illustration of 

 the way the amount of a radio-active substance B varies 

 when initially only the parent substance A is present. By 

 using a number of cylinders in series, each with a suitable 

 resistance, we can in a similar way illustrate in a quanti- 

 tative manner the variation in amount with time of a 

 number of products arising from successive disintegra- 

 tions of a primary substance. By suitably adjusting the 

 amount of resistance in the discharge circuits of the various 

 cylinders, the curves could be drawn to scale to imitate 

 approximately the variation in amount of the various pro- 

 ducts with time when the initial conditions are given. 



During the last few years a very large amount of work 

 has been done in tracing the remarkable succession of 

 transformations that occur in the various radio-active sub- 

 stances. The known products of radium, thorium, 

 actinium, and uraniurii are shown graphically below, 

 together with the periods of the products and the character 

 of the radiations they emit. It will be seen that a large 

 list of these unstable bodies are now known. It is prob- 

 able, however, that not many more remain to be discovered. 

 The main uncertainty lies in the possibility of overlooking 

 a product of rapid transformation following or succeeding 

 one with a very slow period. In tracing out the succession 

 of changes, the emanations or radio-active gases con- 

 tinuously evolved by radium, thorium, and actinium have 

 marked a very definite and important stage, for these 

 emanations can be easily removed from the radio-active 

 body and their further transformations studied quite apart 

 from the parent element. The analysis of the transforma- 

 tion of the radium emanation has yielded results of great 

 importance and interest. After passing through three 

 stages, r.idium .\, B, and C, of short period, a substance, 

 r.adium D, of long period, makes its appearance. This_ is 

 transformed through two stages E and F of short period 

 into radium G, of period 140 days. Meyer and Schweidlcr 

 have conclusively shown that radium D is the primary 



