CONTEMPORARY ADVANCES IN PHYSICS 177 



great extent, as electrons or protons of 10 or 20 times 10^ equivalent 

 volts would be." 



Unfortunately this, like Mott-Smith's, estimate is based upon a 

 tentative answer to a question, one of the most perplexing in physics, 

 though not so familiar as it should be to many who are interested in 

 radiation, ionization of gases, and such phenomena: it is the ancient 

 conundrum, IMiat is the strength of the magnetic field inside of a magnet- 

 ized solid.'' ^^ j\Iott-Smith and Rossi both assume that the said field 

 strength is equal to the induction B, admitting however that another 

 assumption might be made. If instead one were to put it equal to the 

 "magnetizing field" //, the lower limit inferred for the energy of the 

 corpuscles would sink to a value so small, that the experiments would 

 scarcely even be interesting. Considering this, it is especially im- 

 portant to recall the experiment of Curtiss which preceded the two 

 others. He used two counters only, so foregoing the advantage of the 

 narrow vertically-descending beam to which Mott-Smith confined his 

 study; but between them he placed, not the iron of a magnet, but the 

 space between the pole-pieces thereof; and he did get a positive result. 

 The fieldstrength was 7000 gauss, the field extended over 24 cm. of the 

 distance between the counters; vertically-descending electrons trav- 

 ersing the upper counter should have been deflected away from the 

 lower, had their energy been 10^ or less; actually "making allowance 

 for accidental coincidences, a decrease of the order of 25 per cent has 

 been observed." 



We now take up a great but very thorny question : what do counters 

 testify about the penetration and absorption of the cosmic rays in air, 

 in water, and in metals? 



Counting-tubes have been taken to great heights of air by Piccard, 

 lowered to great depths of water by Regener. Neither has published 

 (so far as I know) a full account of his data; but Regener implies that 

 the curve of number-of-discharges-per-unit-time (it seems that one 

 tube alone was used) versus depth-of-water agrees in shape with the 

 curve of Fig. 3, especially in respect of its lowest end with the least 

 value of tJL. Bothe and Kolhorster took a pair of counters four hundred 

 metres down into a mine, and found — if I read correctly — that there 

 were no coincidences at all, apart from casual ones; a valuable result! 

 The most striking work heretofore published was done in another way: 

 by putting a thick piece of lead (or iron, or gold) either above or 

 between a pair of counters set up in the same vertical line, and ascer- 

 taining in what proportion the coincidences are cut down. 



The first who tried this out were Bothe and Kolhorster. In the 



1^ I have discussed this question (without professing to solve it) in this journal, 6, 

 pp. 295-310 (1927). Or, see any textbook of magnetism. 



