424 



NA TURE 



[March 5, 1908 



more Ukui oiip ihousruid million years, it will r.ot be profit- 

 able at the niomcrit to try and trace back the lamily 

 further. 



It api)ears almost certain that, from the radio-active 

 point of view, uranium and thorium must be considered 

 as two independent elements. The case of actinium is 

 different, for Boltwood has shown that the amount of 

 .utinium in minerals, like the amount of radium, is pro- 

 portional to the amount of uranium. This indicates that 

 actinium stands in a genetic relation with uranium. 

 L'nless our experimental evidence is at fault, it does not 

 appear probable that actinium belongs to the main line 

 of descent of uranium, for the activity of actinium separated 

 from a mineral compared with radium is only about one- 

 quarter of what we should expect under such conditions. 

 I think that a suggestion which I put forward some time 

 ago may account for the obvious connection of actinium 

 with uranium, and at the same time for the anomaly 

 observed. This supposes that actinium is a branch descent 

 from some member of the uranium family. It does not 

 appear improbable that at one stage of the disintegration 

 two distinct substances may be produced, one in greater 

 quantity than the other. After the expulsion of an 

 a. particle, it may happen that there are two possible 

 arrangements of temporary stability of the residual atom. 

 The great majority of the atoms may fall into one arrange- 

 ment, and the remainder into the other. Actinium in this 

 case would correspond to the substance in lesser quantity. 

 It would act as a distinct element, and would break up in 

 a different way from the main amount. It is probable 

 that a large amount of accurate work will be required 

 before the position of actinium in the scheme of changes 

 can be fi.xsd with certainty. It is a matter of remark how 

 closely actinium resembles thorium in its series of trans- 

 formations. It would appear that the atom of actinium 

 has many points in common with thorium, or rather with 

 its product, mesothorium. 



The recent observations on the growth of radium offer 

 a very simple and straightforward method of determining 

 experimentally the period of radium. Suppose that we 

 take a uranium mineral and determine by the emanation 

 method the quantity of radium contained in it. If the 

 immediate parent of radium {i.e. ionium) is next completelv 

 separated from the uranium and radium, it will begin to 

 grow radium at a constant rate. Now the rate of grow'th 

 of radium observed is a measure of the rate of breaking 

 up of the radium parent in the mineral, since before' 

 separation the rate of production was equal to the rate of 

 breaking up. Now the growth of radium observed for a 

 short interval, for example, a year, divided by the quantitv 

 present in the mineral, gives the fraction of the radiurn 

 breaking up per year. Proceeding in this way, Boltwood 

 found that the fraction breaking up per vear is about 

 1/3000, and that the period of radium is about 2000 vears 

 — a value which lies between the most probable values 

 deduced from quite distinct data. 



From an inspection of the radio-active families, it will 

 be seen that out of twenty-six radio-active substances that 

 have been identified, seventeen give out a rays or o and 

 5 rays, four give out only & rays, and five emit no ravs 

 at all. The rayless and /3-ray products are transformed 

 according to the same law as the a-ray products, and 

 there is the same sudden change of physical and chemical 

 properties as the result of the transforrriation. In the case 

 of the substances which throw off atoms of matter in the 

 form of ct particles, there are obvious reasons for antici- 

 pating a change in properties of the substance, but this is 

 not the case for the rayless or /3-ray products. We must 

 either suppose that the mass of the atom is not appreciablv 

 changed by the transformation, which consists in an 

 internal rearrangement of the parts of the atom, or that 

 the atom expels a particle at too low a velocitv to be 

 .appreciated bv the electrical methods. Unfortunatelv, it 

 is very difficult to study the rayless products with care, as 

 in practically every case they are succeeded by a ray pro- 

 duct of comparatively rapid transformation. The ravless 

 products are of great interest as indicating the possibilitv 

 of transformations which can occur without anv delectable 

 radiation. 



In the course of the analvsis of radio-active changes, 

 special methods have been developed for the separation of 

 NO. 2001, VOL. 77] 



the Viu'ious products from each other. It is only in a few- 

 cases, however, that we can hope to obtain a sufficient 

 quantity of the substance to examine by means of the 

 balance. It should be possible to obtain workable quanti- 

 ties of actinium, radium D (radio-lead), and radium G 

 (polonium), but the isolation of these substances in any 

 quantity has not yet been effected. Sir William Ramsay 

 and Mr. Cameron have made a number of important in- 

 vestigations of the properties and volume of the radium 

 emanation, freed so far as possible from any traces of 

 known gases. The remarkable initial contraction of the 

 volume due to the emanation shows that there is still much 

 to be done to obtain a clear understanding of the behaviour 

 of this intensely radio-active gas when obtained in a pure 

 state. 



.Simultaneously with the work on the analysis of radio- 

 active changes, a large number of investigations have 

 been made on the laws of absorption by matter of the 

 three primary types of radiation from active matter, viz. 

 the a, ;3, and 7 rays, and the secondary radiations to 

 which they give rise. It has generally been accepted for 

 some years that the 7 rays are a type of penetrating- 

 X-rays. The latter are supposed to consist of electro- 

 magnetic pulses in the ether, set up by the impact or 

 escape of electrons from matter, and akin in many respects 

 to very short waves of ultra-violet light. Recently, how- 

 ever, Bragg has challenged this view, and has suggested 

 that the 7 rays (and probably also the X-rays) are mainly 

 corpuscular in character, and consist of uncharged particles 

 or " neutral pairs," as he terms them, projected at a higli 

 velocity. Such a view- serves to explain most of the 

 experimental observations equally well as the pulse 

 theory ; Bragg has recently brought forward additional 

 evidence, based on the direction of the secondary radiation 

 from the 7 rays, which he considers to be inexplicable by 

 the pulse theory. We must await further data before this 

 important question can be settled definitely, but the theory 

 of Bragg, which carries many important consequences in 

 its train, certainly deserves very careful examination. 



From the radio-active point of view, the a. rays are by 

 far the most important type of radiation emitted by active 

 matter, although their power of penetration is insignificant 

 compared with the ^ or 7 rays. They consist of veritable 

 atoms of matter projected at a speed, on an average, of 

 6000 miles per second. It is the great energy of motion 

 of these swiftly expelled masses that gives rise to the 

 heating effect of radium. In addition, they are responsible 

 for the greater part of the ionisation observed near art 

 uncovered radio-active substance. On account of their 

 importance in radio-active phenomena, I shall devote some- 

 little attention to the behaviour of these rays. The work 

 of Bragg and Kleeman, of Adelaide, first gave us a clear 

 idea of the nature of the absorption of these rays by 

 matter. The a particles from a very thin film of any 

 siniple kind of radio-active matter are all projected at an 

 identical speed, and lose their power of ionising the gas 

 or of producing phosphorescence or photographic action 

 after thev have traversed exactly the same distance, which 

 may conveniently be called the " range " of the a particle. 

 Now everv product emits a. particles at an identical speed 

 among themselves, but different from every other product. 

 For example, the swiftest a particles from the radium 

 familv, viz. that from radium C, travels 7 cm. in air 

 under ordinary conditions before it is stopped, while that 

 from radium itself is projected at a slower speed, travelling 

 onlv 3-5 cm. V<e may regard the o particle as a pro- 

 jectile travelling so swiftly that it plunges through every 

 molecule in its path, producing positively and negatively 

 charged ions in the process. On an average, an a particle 

 before its career of violence is stopped breaks up about 

 100,000 molecules. So great is the kinetic energy of the 

 a projectile that its collisions with matter do not sensibly 

 deflect it, and in this respect it differs markedly from the 

 particle, which is apparently easily deflected by its 

 passage through matter. At the same time, there is un- 

 doubted evidence that the direction of motion of some of 

 the a particles is slightly changed by their passage through- 

 matter. 



The sudden cessation of the ionising power produced by 

 the a particle after traversing a definite distance of air 

 has been show^i by Bragg to be a powerful method of 



