Nagaoka, studying in detail certain satellite lines in the spectra of mercury and of bismuth, came to the conclusion that these lines were best explained by the hypothesis that the nuclei of these metals contain a single proton slightly detached from the main nucleus, and capable of executing coupled vibrations with it, that is, a proton which, while forming part of the nucleus, is more loosely bound than any other proton or electron of the nucleus. He concluded that it might be possible to strike the proton out of the nucleus, and thus effect a transmutation. He used purified mercury for his experiments, the method being to pass intense discharges, from an induction coil capable of giving a spark 4 ft. in length, between a tungsten wire and a mercury surface. To keep the terminal voltage high with the short spark gap used, the mercury was covered with paraffin oil or other suitable oil; the discharge was passed for some hours. Nagaoka found gold in the pasty mass of oil and mercury formed, using the ruby glass test for the detection. No estimate of the amount of gold so formed was made.
Miethe was led to his experiments on transmutation as the result of the observation that when a mercury-vapour lamp, of Jaenicke type, was run with too strong a current a black deposit was formed, in which he found gold. There was no particular theoretical reason for anticipating that the experimental con ditions adopted would force an electron into the nucleus, but Miethe came to the conclusion that there must be some special state of ionization of the mercury atom produced which was favourable to the transmutation, and, with his assistant, Stamm reich, carried out many experiments, in which currents of 12 amperes or so, at a terminal voltage of I7o volts, were passed for many hours through mercury vapour lamps burning at atmos pheric pressure. These experimenters reported yields of gold up to one-tenth of a milligramme, and also, a fact which is hard to explain on the most fantastic theory, a formation of silver. In later experiments Miethe adopted the method of constantly interrupting the arc, a procedure which he considered to exercise a favourable influence on the formation of gold. Although the amounts of gold found were very small, crystals recognizable under the microscope were obtained, and various tests, such as the streak of fine gold, gave a satisfactory result.
Smits and Karssen endeavoured to effect a transmutation of lead by Miethe's method, and built for the purpose a special lead arc, on the pattern of a mercury vapour lamp, the discharge passing between surfaces of molten lead enclosed in a vessel of quartz glass. Special devices were needed to prevent the lamp breaking when it cooled after running. After the lamp had been in operation for some hours, with a heavy current passing, the spectrum of mercury was observed. With certain conditions thallium was also detected. Smits and Karssen stated that the lead was shown spectroscopically to be free from mercury and thallium before the experiment, but admitted that sometimes the experiment does not succeed, and that the precise conditions for a positive result are not known. In one successful experiment they
reported mercury in quantities of from •I to -2 milligrammes. In the latest communication of Smits (Nature, vol. cxx., p. 475, 1927) he describes sparking experiments in which carbon di sulphide was used as a dielectric. The traces of mercury obtained in the mixture of dispersed lead and carbon obtained by this method were afterwards traced by him to mercury contaminations in the carbon disulphide.
Another attempted transmutation is that of hydrogen into helium, on which a long series of particularly careful experiments has been made by Paneth and Peters. Calculation shows that this transformation, which demands the combination of four hydrogen nuclei with two electrons to form the helium nucleus, should be attended by a very great liberation of energy, namely 7X Doi" gramme calories per 4 grammes of helium formed. Paneth and Peters have perfected a spectroscopic technique, which per mits, in favourable cases, c.c. of helium (2X grammes) to be detected. They attempted to see if helium could be pro duced by passing electric discharges of various types through hydrogen, without success. They then looked for a formation of helium in the presence of a catalyst, for which purpose they either passed large amounts of hydrogen through heated palladium, or used finely divided palladium to absorb the hydrogen, and after an interval burnt the hydrogen with pure oxygen, at the surface of the catalyser, and examined the residual gases. Helium was found in small quantities, and at first the experimenters be lieved that it had been formed from the hydrogen. Atmospheric air contains minute traces of helium, but a small air leak was ruled out as a source of the gas, as no neon was detected.
Pushing their enquiries further, Paneth and Peters discovered that glass in contact with the atmosphere takes up minute quan tities of helium, which afterwards leave the glass surface if it be heated in the presence of hydrogen. Once the helium has been driven off in this way the glass is no longer dangerous as a source of helium contamination, but if the glass be exposed once more to the air it again becomes contaminated. Thus the apparent production of small quantities of helium, not only in Paneth and Peters' own work, but also in the experiments of Collie and Patterson and other workers, including the more recent publica tions of Riding and Baly (1925) is explained. There is no trans mutation, but merely a transfer of atmospheric helium into the apparatus by the intermediary of the glass. Paneth and Peters now (1928) state definitely that they have repeated the experi ments of all previous experimenters on the formation of helium, with the necessary precautions, and that there is no production of helium amounting to c.c. in any of them. It may be noted that there is about 5X c.c. of helium in 1 c.c. of air, so that the c.c, which can be detected by Paneth is the amount con tained in only one fifty-thousandth of a c.c. of air.