274 BELL SYSTEM TECHNICAL JOURNAL 



There are actually two grand experiments, which I tried to dis- 

 tinguish above in a sentence by saying that the energetic particles 

 must be easy to isolate and easy to detect. " Isolate " is not a very happy 

 word: the fact is, that if so energetic they must be able to fly right 

 out of the bombarded sheet of uranium (unless they start too deep 

 beneath its surface) — thus, if some sort of a collector is placed across 

 from the uranium and not too far away, they must assemble on it and 

 there they should be found together with all their descendants. 

 Joliot published this experiment before the end of January. He found 

 radioactive substances on his collector even when more than two 

 centimetres of air * had separated it throughout from the uranium. 



The experiment has been performed by many, some introducing 

 new refinements. Meitner and Frisch for instance used a bowl of 

 water for collector, and then could concentrate the radioactive bodies 

 by letting the water evaporate, or by precipitating various salts which 

 in advance they had dissolved in it. This last is the chief technique 

 for finding out the chemical nature of the radioactive products, to wit, 

 the elements of which they are unstable isotopes; but we have not 

 space for entering into the details of the technique, already practiced 

 these five years. Glasoe, McMillan and others modified the method 

 by piling very thin foils of very light substances — aluminium, filter- 

 paper, cigarette-paper — on the uranium. Some of the radioactive 

 matter is found embedded in each of the first few foils, and one may 

 study thus their "distribution-in-range," an almost self-explanatory 

 term. In McMillan's experiment the utmost perceptible range was 

 slightly above 2.2 cm of air. 



Already in the first experiment Joliot observed that in respect of 

 its decay in time, the radioactivity on the collector was very like that 

 remaining on the uranium. Later more accurate work has merely 

 strengthened that conclusion, and Segre in particular affirms that out 

 of many radioactive bodies there are only two which are found in the 

 bombarded uranium itself and not on the distant collector also. On 

 the distant collector there are found, in particular, the substances 

 once classed as "trans-uranic elements." This is very important, for 

 in the theory of the trans-uranic elements there occurs no stage in 

 which the fragments of the uranium nucleus (or any other) are thrown 

 apart with so tremendous energies. Were these elements trans-uranic, 

 they should not be able at all to escape from the bombarded uranium 



^To give the thickness of air (of the density corresponding to 15° C. and one 

 atmosphere of pressure) which can just be traversed by a charged particle is the 

 ordinary way of stating the "penetrating power" of the particle. Often some other 

 substance than air is used in the tests; it is then not the actual thickness of the 

 substance, but the "air-equivalent" thereof, which is ordinarily stated. Joliot 

 appears to have used actual air in the experiments. 



