March 1 1, 1909] 



NA TURE 



^7 



kinds of ions, dependent on and varying witli atmosplieric 

 conditions, such as humidity. 



The recent interesting worlc of Townsend proves that 

 under some conditions the positive ion may have a double 

 charge, two negative ions appearing at its formation. 

 Hence it is possible that in the atmosphere a newly 

 generated positive ion may for a short time be the more 

 mobile, and the apparent excess of positive electricity may 

 not improbably be traced to this cause, as some preliminary 

 experiments seem to indicate. Even if that is so, the fact 

 remains that the quantities of positive and negative elec- 

 tricity in the atmosphere do not differ, at least not to the 

 large extent usually recorded. A. S. Eve. 



Montreal, February 17. 



The Absorption of X-Rays. 



Tin; results of experiments that have been made by a 

 number of investigators on the absorption of X-rays, 

 secondary X and y rays, are so complicated by a variety 

 of conditions that few general conclusions can be drawn 

 from them. It is apparent that a knowledge of the simple 

 laws governing the absorption of X-rays, and the emission 

 of secondary rays, would in many cases enormously simplify 

 the explanations, and save much fruitless labour. 



Bv the use of homogeneous beams of X-rays, and by a 

 studv of secondary X-rays, we have been enabled to arrive 

 at the following conclusions : — 



Many elements — possibly all — when subject to a suitable 

 Rontgen radiation emit at least one homogeneous beam 

 of X-radiation, which is characteristic simply of the sub- j 

 stance emitting it. 



When a radiation which is of more absorbable type than 

 the radiation characteristic of a certain substance is 

 Incident on that substance, it does not appreciably excite 

 that characteristic secondary radiation. 



When the incident radiation is of more penetrating type 

 than that characteristic of the exposed substance, that 

 characteristic secondary radiation is excited. 



The absorption of the radiation not sufficiently pene- 

 trating to excite the homogeneous secondary radiation 

 characteristic of an absorbing element is governed by very 

 simple laws, the ratio of the absorption coefficients in 

 elements A and B (say) being constant. That is, Aa/'Ib 

 is approximately a constant for any radiation experimented 

 upon which is not more penetrating than the radiation 

 characteristic of A or B. 



When the incident radiation is of more penetrating tvpe, 

 the absorption is greater than would be given by this law, 

 additional absorption being evidently essential to the 

 fmission of the characteristic secondary radiation. As the 

 general penetrating power of the incident radiation in- 

 creases, the intensity of secondary radiation increases, and 

 the absorption by this particular element increases, and 

 finally for more penetrating primary rays the intensity of 

 secondary radiation and absorption of primary rays decrease 

 again in the ordinary way. 



The beam emerging from the absorbing plate consists of 

 :i weakened primary beam proceeding in its original direr- 

 lion, a little scattered radiation, and a homogeneous radia- 

 tion uniformly distributed (except for internal absorption). 

 There is no evidence of anv other kind of transformation — 

 speaking purely of X-rays. 



We may, therefore, by a proper choice of primary radia- 

 tion and absorbing element observe anv of the following: — 

 d) Incident and emergent beams of identical penetrating 

 power. 



(2) Incident beam, homogeneous ; emergent beam a 

 mixture of two homogeneous beams, the ratio of the 

 intensities of which asvmptoticallv approaches a constant 

 value, as the thickness of the absorber increases. 



(t,) Incident beam, homogeneous ; emergent beam a 

 mixture of two, the radiation of incident type ultimately 

 vanishing and leaving a completely transformed radiation. 



\ homogeneous radiation from an element appears 

 specially penetrating to that element and to elements of 

 neighbouring atomic weight, because it is of less pene- 

 trating type, or only just more penetrating than the radia- 

 tions from these elements. 



The change in the character of an ordinary heterogeneous 



beam of X-rays in transmission through an element is 

 due to (i) the general selection of rays of the more absorb- 

 able type ; (2) the special selection of those rays of greater 

 general penetrating power than the radiation characteristic 

 of the absorbing substance ; (3) the emission of secondary 

 rays, which are more generally absorbable than the radia- 

 tions which produced them, but which may be more pene- 

 trating to the element emitting them. 



The energy of primary radiation transformed into 

 secondary rays is so great that the secondary X-rays pro- 

 ceeding from the antikathode of a Rontgen tube constitute 

 a considerable portion of the heterogeneous beam. 



Many of Mr. Kaye's experiments on so-called primary 

 rays, for example, are obviously experiments on secondary 

 ravs, verifying our previous results. A comparison of the 

 absorption coefficients shows the identity of the two. 



.\ fuller treatment of the subject of absorption will 

 shortly be published. We wish, however, to point out the 

 great simplification that results from the application of 

 these simple laws to many of the phenomena which have 

 recently been described in a variety of papers on X-rays 

 and secondary X-rays. Probably the laws may be extended 

 to include also the 7 rays. 



C. G. Barkla. 

 C. A. Sadler. 



University of Liverpool, March 5. 



The Rays of Uranium X. 



Is continuation of the work published in a letter to 

 Nature of January 28, p. 366, I have now carried out 

 under more favourable conditions a second series of observa- 

 tions designed to detect the growth, if any, of a feeble 

 a radiation during the decay of the intense ;3 radiation 

 of uranium X. I used the preparations, obtained from 

 50 kilograms of uranyl nitrate, employed by Mr. Russell 

 and myself in the study of the 7 rays (Nature, March 4, 

 p. 7). The preparation was placed i-6 cm. from the thin 

 aluminium foil, forming the base of an electroscope, in a 

 magnetic field of 10,800 units, so that no ;8 rays with a 

 value for Hp less than 8640 could enter the electroscope. 



Under these conditions, although the /3 radiation from 

 the preparation was sufficiently intense to show luminosity 

 on an X-ray screen in a fully lighted room, the leak in 

 the electroscope was small enough for accurate measure- 

 ment. About one-fourth of the leak was due to 7 rays, 

 and the remainder to still undeflected rays. Initially 

 the leak was not measurably altered by covering the pre- 

 paration with a layer of thin tin foil sufficient to absorb 

 any o radiation. In a few hours after preparation a 

 decided difference was noticed, pointing to a growth of 

 a radiation from the preparation, as the considerations' out- 

 lined in mv previous letter had led me to expect. Instead, 

 however, of this absorbable radiation growing with the 

 time according to the function i — e-*'. where \ is the 

 radio-active constant of uranium X, contrary to all expecta- 

 tion the absorbable radiation very quickly reached a maxi- 

 mum, and has since remained constant. About one-half 

 the maximum was reached after the lapse of one day, 

 while after 2-5 days no further increase was observed. At 

 this stage the absorbable radiation was about one-fifth of 

 the total. 



The observations have now been in progress for one 

 month. These observations and the whole of those 

 previously made indicate that this a radiation remains 

 constant after the maximum is reached oyer a period of 

 several years. This points to the existence of a new body, 

 presumably somewhere in the uranium series, with a period 

 of the order of one day, the product of which gives a rays, 

 and has a very long period of life. 



I have thought it well to record these observations before 

 being in a position fully to explain them, as there has just 

 come to hand the announcement by M. Danne (f-i? Radium. 

 February, p. 42) that, working with 20 kilograms of 

 uranvl nitrate, he has succeeded in effecting the partial 

 separation of the parent of uranium X, which he terms 

 radin-uranium. From his description it appears that the 

 new body is very closely allied to uranium in chemical 

 nature. So far as can be iud<?ed, it appears improbable 

 that in preparing the uranium X for these experiments anv 

 of the radio-uranium was separated. One may conclude at 



NO. 2054, VOL. 80] 



