April 14, 1905.] 



SCIENCE. 



565 



6. Lead-Cased Fog-Chamher.— To inter- 

 pret these surprising results it will be 

 necessary to surround the fog-chamber 

 with a casket of lead, having a lid on the 

 side fronting the X-ray bulb; for even 

 though the lead plates above may efficiently 

 cut off the primary rays, they would leave 

 the secondary radiation free to enter later- 

 ally through the broadsides of the fog- 

 chamber. When this was done the results 

 reduced the penetrability of lead to a more 

 reasonable figure as may be seen from the 

 following example of results when the dis- 

 tance between bulb and fog-chamber was 

 2 meters. 



Thickness of lead penetrated = .14 .28 .42 cm. 

 iVxlO-= = 77 10 7 5 



i. e., 14, 9 and 7 per cent, of the total in- 

 tensity passes one, two and three plates 

 respectively. A glass plate 7 mm. thick 

 and an iron plate .5 mm thick allowed 

 about 90 per cent, to pass, when the casket 

 was left open and the lead plate placed 

 near the bulb 17 per cent, of the total 

 radiation was effective, the excess being of 

 secondary origin. The passage through a 

 plate of tinned iron may be observed for 

 a bulb 6 meters distant as follows: 



Thickness of plate .05 .10 .20 cm. 



NX 10-' 36 28 11 7 ' 



It follows then that in the above ex- 

 amples (§5) nearly one half of the total 

 radiation was derived from secondary 

 sources since the primary radiation was 

 certainly stopped off to within 10 per cent, 

 by the lead plates. 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 (ob- 

 servationally 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 sec- 



ondary radiation here in question may be 

 identical; for the corpuscles may come 

 from the circumambient air molecules shat- 

 tered by the shock of gamma rays. The 

 latter would in turn be traceable to the 

 atomic disintegration of the anticathodal 

 platinum while under bombardment by the 

 cathode torrent. 



The fog-chamber, if open at the end 

 toward the bulb, shows the same total in- 

 tensity; but in. such a case the inner walls 

 of the casket, etc., become the source of 

 secondary rays. The closed lead casket, 

 however, sometimes introduces a discrep- 

 ancy, for the coronas on second exhaustion 

 are fainter, but nearly as large as on the 

 first. Hence the lead itself is radioactive 

 or it becomes so after becoming energized 

 by the X-rays. This recalls § 3 on the be- 

 havior of radium in the sealed tubes as 

 specified in my last paper. 



7. Ordinary Dust-free Air an Aggregate 

 of Nuclei.— Th.Q steam jet shows that nuclei 

 of small relative size, but, nevertheless, 

 large as compared with the molecules of 

 air must normally be present in dust-free 

 air: for the axial colors may be kept per- 

 manent at any stage by fixing the super- 

 saturation. Such nuclei may be called col- 

 loidal molecules. Moreover, the available 

 nuclei to be reckoned in millions per cubic 

 centimeter increase with enormous rapidity 

 with the supersaturation, in proportion as 

 the molecular dimensions are approached. 

 But even when the yellows of the first order 

 vanish, condensation probably still takes 

 place on the colloidal molecules specified. 

 It is natural to associate these extremely 

 fine nuclei with the existence of a very 

 penetrating radiation, Icnown to be present 

 everywhere. Moreover, the occurrence of 

 many nuclei with but few ions is not con- 

 tradictory, if the latter are only manifest 

 when the former are made or broken. 



8. Conclusion.— It has been shown that 

 for very short exposures (§3), the nuclea- 



