The evidence for artificially produced elements has so far been based entirely on radioactive methods (fluorescent screen, electrometric device, or Wilson chamber); either the rays accompanying the process of transmutation have been observed, or, in the case of artificial radio-elements, the rays emitted by the products of transmutation have been used for their detection and the study of their behaviour. The amount of new matter is usually so small that there is no possibility of discovering it by any non-radioactive method; attempts to detect hydrogen spectroscopically have failed, but with helium where the limit of identification is so low (
see
I) there was more hope of success. There are at present several processes of artificial transmutation of which helium is known to be a product. In high-voltage apparatus it is difficult completely to exclude the possibility of contamination by helium from the air, or from glass walls; but bombardment by radioactive sources can be carried out under conditions much better suited to our purpose. Chadwick and Goldhaber and Fermi and co-workers have found that under the impact of slow neutrons lithium and boron produce helium, the latter according to the reaction :
5
10
B +
0
1
n
→
2
4
He +
3
7
Li. (1)
Manometric measurement of human sphincter of Oddi length.
