CONTEMPORARY ADVANCES IN PHYSICS 181 



intensities, and values of ix depending on the frequencies; in principle, 

 one might resolve the curve into these terms; but there is a great 

 difference between "in principle" and "in practice." If instead of 

 water we consider the atmosphere, there is reason to infer that the 

 range R of the ionizing corpuscles discovered in cloud-chamber and 

 counting-tube experiments may be greater than the thickness of the 

 atmosphere itself! In this case, however, there should be a maximum 

 of ionization lower down in the air, than the heights already attained by 

 balloons; such a maximum has not been found. The possibilities are 

 enormously complex; and after mentioning that Steinke has made a 

 number of measurements with sheets of metal of thicknesses apparently 

 inferior to the range of the electrons in the metal, with significant 

 results (he speaks of Uehergajigseffekte or "transitional effects"), I will 

 pass to the description of an experiment by Rossi meant as a contri- 

 bution to this problem. ^^ 



I have already quoted the value which Rossi obtained for the 

 percentage drop in the number of coincidences occurring when a plate 

 of lead is slipped between two counters — 16 per 100. When he slipped 

 the same lead plate above the upper counter, the drop was less— 12 per 

 100. Let us imagine that photons coming from above were releasing 

 fast forward-flying electrons from atoms in the plate of lead, these 

 passing in the latter case through both of the counting-tubes, in the 

 former through only one — causing coincidences therefore in the latter 

 case, not in the former. The numerical values then oblige us to 

 suppose that when 100 fast electrons dash from above into the upper 

 counter, 16 of them can be stopped by ten centimetres of lead, but 

 accompanying photons will generate four new ones in the slab, leaving 

 a net decrease of 12 per 100. It is, then, this twelve per cent which is 

 the decrease in the ionizing-power of the mixed beam, produced by 

 interposing ten cm. of lead; Rossi thinks that the data prove that the 

 photons are less penetrating than the electrons. 



If so, how can it be that the charged particles responsible directly 

 for the ionization have so great a penetrating-power? The electrons 

 expelled from atoms by X-rays, and by the gamma-rays of known 

 wave-length emerging from radioactive substances, are definitely less 

 penetrating than the photons which set them on their ways. No 

 theory gives the slightest indication that this relation should eventually 

 be reversed as the frequency and penetrating power of the gamma-rays 



'' I take equation (3) from Rossi's paper in the Zeitschrift fiir Phvsik, where the 

 reader can also find equations for cases somewhat more complicated but probably 

 not more plausible. The region beyond R is often called the region where "the 

 primary beam is in equilibrium with its secondaries." Notice also the argument of 

 Kulenkampff, mentioned in footnote 11. I am indebted to Dr. P. M. Morse for a 

 discussion of these questions. 



