620 



NA TURE 



[October i8, 1906 



conspicuous, the result in many cases being a remarkable 

 alternation of larger and smaller coronas, corresponding to 

 variations in the number of the drops, for successive ex- 

 pansions of equal amount. It is easily seen how, under 

 the appropriate conditions, such an alternation may arise, 

 for the second expansion may remove the greater number 

 of the residual nuclei due to the first, so that the third 

 rakes place under conditions similar to those of the first 

 expansion, h. large amount of space is given to the study 

 of these alternations, and they are finally traced to their 

 true source after many hypotheses have been suggested for 

 their explanation, " the solutional enlargement " of the 

 nucleus, as the author calls it, being then apparently re- 

 garded as a new discovery. Besides incidental references 

 to these residual nuclei in earlier papers, he would have 

 found them described in Thomson's " Conduction of 

 lilectricity through Gases," p. 139, or in a review of the 

 subject of condensation nuclei presented to the International 

 Electrical Congress of St. Louis in 1904, and a great deal 

 of labour might have been saved. That small drops of 

 pure water might be expected to cease to evaporate, even 

 in an unsaturated atmosphere, beyond a certain minimum 

 size (related to the thickness of minimum surface tension 

 of thin films) is pointed out by Thomson in the same 

 chapter, p. 153 ; and a theory (having a similar basis), 

 which explains the permanence of certain slow-moving ions 

 requiring a negligible degree of supersaturation to make 

 water condense upon them, has been given by Langevin 

 and Bloch. 



By exposing to intense X-rays the moist air in a 

 " rectangular condensation chamber of wood impregnated 

 with resinous cement," the front and rear faces being of 

 plate glass, persistent nuclei requiring only a very slight 

 expansion to cause water to condense upon them were 

 obtained. The only nuclei hitherto observed in dust-free 

 air exposed to X-rays require large expansions to capture 

 them. That such nuclei should, under the appropriate 

 conditions (the occurrence of chemical action giving rise 

 to soluble products), grow into larger bodies is what might 

 be expected ; such a growth has, for example, been observed 

 in the case of the ions arising from a point discharge. It 

 is quite likely that sufficiently intense X-rays or radium 

 rays might bring about in moist air the chemical action 

 necessary for such a growth of the nuclei, as intense ultra- 

 violet light certainly does ; but results, obtained with a 

 chamber of wood impregnated with resinous cement and 

 not rigorously shielded from all possible direct electrical 

 effects from an X-ray bulb placed a few cm. from it, are 

 not free from ambiguity. 



.\part from this effect of very intense radiation, the con- 

 clusions arrived at by the study of the effect of X-rays 

 and radium rays appear to differ from those of other 

 observers. Prof. Barus holds original views, not onlv 

 upon the relation of " nucleation " to ionisation, but as 

 to the nature of the radiation from an X-ray tube. These 

 are best given in his own words : — 



Chapter vi., p. 133 : " Let the X-radiation to which the 

 dust-free air is exposed be relatively weak, so that the 

 density of ionisation may remain below a certain critical 

 value. The nuclei observed on condensation are then verv 

 small, and they require a high order of exhaustion, 

 approaching but always below the fog limit of non- 

 energised air. They are usually instantaneouslv generated 

 (within a second) by the radiation, so that their number 

 is definite independent of the time of exposure. Thev decay 

 in a few seconds after the radiation ceases, i.e., roughly, 

 to one-half their number in 2 seconds to one-fifth in 20 

 seconds, in the usual way. I fancy that these nuclei are 

 what most physicists would call ions ; but nevertheless the 

 particles are not of a size, the dimensions depending on 

 the intensity of the penetrating radiation to which thev 

 are usually due, and they pass continuously into the per- 

 sistent nuclei, as shown in the next paragraph, where 

 decay of ionisation and of nucleation are very different 

 things. They are abundantly produced by the 7 ravs, 

 which though weak ionisers, become_ from this point of 

 view strong nucleators." 



Chapter vi., p. 142 : " While the phosphorescent, photo- 

 graphic, and electric effects of X-radiation decrease rapidly 

 with the distance, D, from the tube, the nucleating effect 

 (N, nuclei generated per cubic centimetre, instantly) is 



NO. 1929, VOL. 74] 



nearly constant over relatively enormous distances. Thu^ 

 to give two examples among many t9/> = 25 cm.) : — 



The law of inverse squares would predicate a reduction 

 of 10,000 to I between these limits; and in fact, at 6 cm. 

 the phosphorescent screen is intensely luminous, at 200 cm. 

 very dim, at 600 cm. quite dark as in the case of any 

 ordinary illumination. The leaves of an electroscope 

 within a glass bell jar collapse in a time which is directly 

 as the square of the distance from the energised X-ray 

 bulb. The result obtained with nuclei is astonishing ; the 

 nuclei-producing radiation would, at first sight, seem to be 

 of an extremely penetrating kind, akin to the gamma rays 

 of radium, and distinct from the ordinary phosphorescence- 

 producing X-rays." 



Chapter vi., p. 144 : " To the eye of the fog chamber 

 therefore the walls of the room are aglow with radiation, 

 and no matter in what position the bulb may be placed 

 (observationally from 6 cm. to 6 m. between bulb and 

 chamber) the X-illumination as derived from primary and 

 secondary sources is constant everywhere. It is to be 

 understood that the X-illumination here referred to may be 

 corpuscular. In fact, so far as I see, the primary and 

 secondary radiation here in question may be identical ; for 

 the corpuscles may come from the circumambient aii 

 molecules shattered by the shock of gamma rays." 



Chapter vi., p. 145 : " It has been shown that for very 

 short exposures (sections loi and 102) the nucleation is 

 the same, whether the bulb is placed at 6 cm. or 6 m. 

 from the fog chamber. But only in the former case 

 (D = 6 cm.) is the effect cumulative; only for very short 

 distances will persistent or very large nuclei appear if the 

 exposure is prolonged several minutes. I have therefore 

 suspected that the radiation from the X-ray bulb is twofold 

 in character ; that the instantaneous effect (fleeting nuclei) 

 is due to a gamma-like ray, quick moving enough to pene- 

 trate several millimetres of iron plate appreciably even 

 for D = 6 metres; furthermore that the cumulative effect 

 (persistent nuclei) is due to X light, properly so called, 

 which produces the usual effects subject to the laws of 

 inverse squares ; but it is noteworthy that while the pene- 

 tration of X-rays is relatively small, and the distance effect 

 negligible (section loi), thev are both large for the radi- 

 ation from radium (section 104)." 



The conclusion that the nucleus-producing radiation from 

 an X-ray bulb is constant over distances varying from 

 6 cm. to 6 m. (or as elsewhere expressed that " the whole 

 medium within the room is almost equally energised 

 throughout ") is somewhat startling. One would not 

 expect the number of nuclei present at a given moment in 

 any case to fall off inversely as the square of the distance ; 

 the number of ions might under suitable conditions be 

 expected to vary inversely as the distance ; but the fact 

 that there is no falling off at once suggests that there 

 is something wrong with the experiments or the interpret- 

 ation put upon them. Possibly the observed constancy is 

 partly due to the failure of the method to deal with more 

 than a limited number of nuclei. Some of the results, 

 however, suggest that it may have been partly due to the 

 failure to shield off the rapidly changing electric field pro- 

 duced by the working of the coil. 



There is more danger of the statements of the first 

 paragraph quoted above leading to confusion. The ex- 

 pression " fog-limit " apparently indicates the smallest 

 pressure fall which produces a sufficiently large number 

 of drops to admit of a corona being observed. Previous 

 expansion experiments, in which a sudden deTinite volume 

 change was produced, have shown three critical or limit- 

 ing values of the expansion (measured by the ratio of the 

 final to the initial volume). These are 1-38. beyond which 

 dense fogs begin to be produced in dust-free air under 

 normal conditions ; 125, the least expansion required for 

 the capture of negative ions ; and an intermediate one in 

 the neighbourhood of i-?i, the least expansion required 

 for the capture of positive ions. Certain apparently un- 

 charged nuclei require an expansion of about the same 

 amount as do the positive ions. Ions of both kinds are 

 always present in small numbers in the air of a closed 



