Even so, there is a case in which the helium content is anomalous, if not altogether beyond explanation at the present stage. In examining a large number of minerals for helium, Strutt[136] found that some samples of beryl, a beryllium aluminium silicate, contain a relatively very large amount of helium, but only traces of thorium, and was altogether inactive. The absence of any active constituent renders untenable the ordinary explanations of the presence of such a surprising quantity of helium. Boltwood has put forward a suggestion which in the present state of our knowledge must be regarded as a provisional explanation. He conceives that in the concentration of beryllium from the parent magma, it may have become associated with some short-lived intermediate radioactive element, which had been altogether separated from its long-lived parent element in the process of concentration; this intermediate element, having collected in the crystallised beryl, decayed completely in the course of the great period which must have elapsed, leaving the helium to which it had given rise during its disintegration enclosed in the mineral. It is difficult to see how two substances which must be so intimately connected as a parent-element and its product could be completely separated in the process of cooling of a magma; but since so little is known of the process of crystallisation of minerals, the suggestion can hardly be rejected on geological grounds. In any case, we have here only one strongly marked exception to the very definite rule that in all cases in which helium occurs in minerals, it is accompanied by and undoubtedly produced from, a radioactive element or elements; and in the majority of cases, the helium in minerals is produced by disintegration of uranium or thorium and their products.
[136] Proc. Roy. Soc. 1908, A, 80, 572.
Strutt found that traces of helium are universal in the mineral world. His method of determining helium was approximate only. He obtained the gas content by heating the powdered mineral—a method which, as Wood has shown,[137] will only give all the gas when very high temperatures (up to 1000°C.) are employed. The gases were freed from oxygen and hydrogen by passing over a heated, partially oxidised, copper spiral, and from carbon dioxide by means of potash. Nitrogen was removed by sparking with excess of oxygen and shaking over potash; the excess of oxygen was removed by melted phosphorus. The inert gases so obtained were freed from all impurities by the use of the liquid alloy of sodium and potassium for the electrodes of the spectrum tube in which the gases were examined spectroscopically.[138] Argon, if present—it seems to be a universal constituent of igneous rocks, into which it may have been absorbed from the air—was removed by charcoal at a temperature of -80°C. The helium so left was examined spectroscopically, and measured in a MacLeod gauge.
[137] Proc. Roy. Soc. 1910, A, 84, 70.
[138] As soon as the discharge is started in such a tube, all the gases present other than those of the helium family are absorbed by these electrodes.
As stated, helium was found in traces in nearly all minerals, and its presence is to be attributed to traces of radium, which also appears universal. In minerals containing uranium or thorium, or rare earths (the latter are almost always accompanied by uranium and thorium), helium is found to a much greater extent, and Ramsay considers it possible that some fraction of the helium content may arise from the rare earth metals. There is, however, no positive evidence to support the conjecture. He found that the helium ratio, i.e. the volume of helium per gram of uranous oxide, UO₂, varies with the amount of thoria present; but where the latter is absent the variations are much less marked. If helium were produced in a mineral from uranium alone, and none escaped, it is obvious that the helium ratio would depend only on the age of the mineral. For minerals of about the same age, and containing no thorium, the helium ratio would be roughly constant, if no disturbing factor required consideration.
In 1905 Strutt pointed out that in all the minerals he had examined, thorium was never present unless accompanied by uranium and radium, whilst uranium and radium often occurred without thorium. He suggested that the present atomic weight of thorium, 232·5, was too low, and that it was really the parent of uranium (at. weight 238·5); he further supposed that the next permanent member in the line of descent was one of the cerium metals. These suggestions have been negatived by later work of Boltwood and Holmes. The former pointed out[139] that it was far more likely that thorium is a disintegration product of uranium of considerably longer life. On the whole, however, there is very little positive evidence to connect thorium with uranium.
[139] Boltwood, Amer. J. Sci. 1905, [iv.], 20, 256.
In the same year Boltwood (loc. cit.) drew attention to the persistent appearance of traces of lead, bismuth, barium, etc., in the radioactive minerals, and also pointed out that the variations of the ratio of helium to uranium in pitchblende might be used to determine the age of the mineral. In 1907 he suggested[140] that lead was the final product of the degradation of uranium, from which it follows that the ratio of uranium to lead should be constant for minerals of the same age (since, lead decays, if at all, at an infinitely slower rate than uranium). He collected all the available analyses, and classified the minerals dealt with into six groups according to the value of the ratio. The order given by the ratio was declared to be in accordance with the order of age as given by geological evidence.
[140] Amer. J. Sci. 1907, [iv.], 23, 77.