December 26, 191 2] 



NATURE 



403 



produced those showing on these latter specimens 

 (that is, of course, when they have obviously not been 

 detached by fortuitous blows). 



(3) I find that by bringing pressure to bear upon 

 a tiint with a sharp edge resting upon a rounded 

 pebble a "bay" can be produced upon the uppermost 

 stone which has the appearance of a "hollow scraper " 

 made by man. 



By carefully watching one of these sharp-edged 

 stones in the process of being flaked, it was seen that 

 so long as the pressure was applied the flaking was 

 continued. 



The flint was evidently breaking along the lines of 

 least resistance, and very thin flakes being removed, 

 the hollow produced having a totally different appear- 

 ance from one made by blows. 



The reason for this difference was seen to be owing 

 to the fact that as it is impossible to strike the flint 

 near enough to its edge to remove flakes as thin as 

 are detached by pressure, the hollow scraper produced 

 by blows has a rougher appearance because each indi- 

 vidual flake has cut deeper into the flint (Figs. 9 and 

 10). The hollow scrapers which are derived from the 

 pre-river-drift deposits have obviously been produced 

 by blows, delivered at a constant angle to the edge, 

 and it is therefore concluded that man has made them. 



It is, of course, possible that these early men in 

 some cases may have edge-flaked their flints by pres- 

 sure applied with another stone or a bone point, as 

 the later Neolithic people did, but it does not seem 

 likely that this was the case. Experiments were con- 

 ducted with flint flakes covered by an inch or inch 

 and a half of fine sand in an iron dish, and it was 

 found that the greatest pressure obtainable with the 

 differential screw-press was unable to break them. 



.\ similar experiment w-as also conducted under the 

 same conditions except that no iron dish was used and 

 the sand was allowed to flow under the pressure. Here 

 again, however, no fracturing of the flint took place. 

 T think these results should induce caution in assert- 

 ing that large stones lying under many feet of fine 

 sand have been broken by pressure. I found that with 

 pressure-flaking the small fissures in the flint, which 

 are so common in flaking by percussion, are very rare, 

 and I think this is due to the different methods of 

 fracture. .Also the surface of a pressure-flake is verv 

 seldom so glossy as that produced by a blow, which 

 fact can perhaps be explained on the same hvpothesis. 



I may say that my experiments were carried through 

 a great number of times with all sorts of flints, and 

 the same results obtained. 



In conclusion, I would like to state that specimens 

 demonstrative of all the foregoing experiments are 

 housed in the department of ethnologv of the British 

 Museum (BIoomsbur\'1, and can be seen and handled 

 by anyone who wishes to do so. 



,T. Reid MoiR. 



12 St. Edmund's Road, Ipswich, November 25. 



Excitation of 7 Rays by « Rays. 



In a paoer shortly to be published in The Philo- 

 sophical Magazine, one of us has shown that when 

 the a rays from radium C impinge upon matter, they 

 excite a small but detectable amount of 7 radiation. 

 In continuation of this work a systematic investiga- 

 tion of the radiations from bodies which expel a rays 

 has been commenced. So far the radiations from 

 ioniurn, radio-thorium, and radio-actinium have been 

 investigated. \\'orking with a very strong source of 

 ionium, we find that, after all radio-active products 

 likely to emit fl or 7 rays have been removed by 

 chemical treatment, ionium emits, in addition to its 

 a rays, a certain amount of 7 radiation, but no detect- 

 able amount of radiation. The amount of 7 radia- 

 NO. 2252, VOL. 90] 



tion compared with the total a radiation is much 

 smaller than that emitted by a typical 7 ray product 

 like radium C. The amount, however, is of about the 

 same order as that e.xcited by the a ravs of radium C 

 me.xternal matter. Since there is no evidence of the 

 e.xistence of a product accompanying the ionium and 

 emitting 7 rays only, it is natural to suppose that 

 these 7 rays are excited either in the ionium, or in 

 the thorium which is mixed with it, by the a rays. 



Analysis of this radiation by means of absorption 

 measurements gave the interesting result that it con- 

 sists of three types at least. The least penetrating of 

 these consists of a radiation, the absorption coefficient 

 divided by the density (,u/D) of which has a value in 

 aluminium of about 400 (cm.)->, the second of about 

 82, and the third type, which has not been investigated 

 in detail owing to the weakness of the source, of 

 about o'i5, i.e. it is of about the same order of pene- 

 trating power as the hard 7 rays from radium C, viz. 

 004. It will be noticed that the second type has 

 approximately the same value of /i/D as the char- 

 acteristic radiation of series L excited by X-rays ii 

 thorium, as found recently by Chapman. It is there- 

 fore natural to suppose that all three types are char- 

 acteristic radiations of ionium of different series. 



We find also that radio-thorium emits y rays, and 

 also a small amount of e radiation. This radiation 

 has not been studied in as much detail, owing to the 

 rapid formation by the radio-thorium of thorium X 

 and subsequent products, expelling intense and 

 7 rays. The ratio of the amount of 7 to a radiation 

 emitted by radio-thorum is approximately the same as 

 the corresponding ratio for ionium. 



The results obtained with radio-actinium, which is 

 the product in the actinium series corresponding to 

 radio-thorium in the thorium series, and to ionium 

 in the uranium series, are very different. Dr. Hahn 

 has' shown that radio-actinium emits, in addition to 

 a rays, some soft 3 rays, and also a radiation which 

 is either a hard 3 or a soft 7 radiation. We have 

 repeated his work, and find that it e.xpels, in addition 

 to soft $ rays, 7 radiation of two types, the more 

 penetrating of which is of the same order of penetrat- 

 ing power as the hard rays from radium C. 

 The amount of and 7 radiation emitted by radio- 

 actinium, however, is much too large to be ascribed 

 to a rays alone. 



It has hitherto been supposed that radio-actinium 

 is a single product, having a period of ig'~, days, but 

 we have succeeded in showing that it consists of two 

 successive products. The parent product has the 

 period of i9'5 days, as found by Hahn, and emits 

 little or no penetrating 3 or 7 radiation, and verv 

 probably no a rays. The second product expels a, /3, 

 and 7 rays, and has a period of about thirteen hours. 

 So far we have not succeeded, by means of a single 

 chemical operation, in separating" completely one pro- 

 duct from the other, but, by means of a series of 

 operations, we have been able to obtain a fraction of 

 either product free from the other. It is of interest 

 to note that Dr. Geiger and Mr. Nuttall predicted, 

 from their well-known relation between rate of trans- 

 formation and range of o rays, that radio-actinium 

 probably consists of two successive products, the first 

 of these having the period of ig's days, as found bv 

 Hahn, and the second giving the a rays and having a 

 period of about one day. It is seen that this predic- 

 tion was surprisingly accurate. 



We intend to continue this work by investigating 

 the_7 and B radiations expelled by intense sources of 

 radium, polonium, thorium X, and other a ray pro- 

 ducts. J. Ch.^dwick. 



A. S. Russell. 

 Physical Laboratories, Manchester L'niversity, 

 December 16. 



