Photographs of the “spark” spectra of A, Helium as extracted from the mineral Clevelandite of B, the Radium “emanation” after a year’s enclosure in the tube used and of C of Hydrogen gas: copied from the paper by Mr. F. Giesel in the Berichte der Deutschen Chemischen Gesellschaft, vol. xxxix, part 10.
The three photographs are accurately super-imposed so as to show the coincident lines.
The spectrum B of the tube containing radium emanation is the one which we are comparing with the other two. When the radium emanation was first enclosed there was only a small quantity of helium developed in it, but after keeping for a year the quantity has greatly increased. After five minutes “sparking” (passage of the electric spark through the tube) the chief lines of helium become evident but faint in intensity. The present photograph B was obtained after forty minutes sparking, and one result of that longer “sparking” has been that a minute quantity of water vapour in the tube has been broken up—so as to yield the hydrogen spectrum, which is accordingly seen accompanying the now strong and brightly developed helium spectrum.
The lines of the spectrum B which correspond with those of hydrogen are at once recognised by the juxtaposition (below) of the pure Hydrogen spectrum from another tube—C: the lines in B belonging to and indicating helium are also recognised by comparison with the pure helium spectrum of the tube A juxta-posed above. A very few of the lines in B must be due to other minimal impurities as they are not present either in A or C.
Thirteen lines of the helium spectrum are thus photographed and recognised in the radium emanation.
The following lines are present in the photographic but invisible spectrum of radium (not given in fig. 10), viz. at 381·47 µµ (the strongest line in the radium spectrum) and at 364·96 (a strong line).
In the photographic but invisible spectrum of helium there are three very faint lines between wave-length 447·2 and 443·7 (appearing as two only in our photograph); a moderately strong one at 438·8; others at 414·4, at 412·1, at 402·6, and 396·5; a very strong one is present at 388·9, and a very faint one at 381·9. All these are seen in the photograph A and also in B. Special treatment and spectroscopes reveal four other very faint lines in the helium spectrum—the one furthest in the invisible direction (that is of highest refrangibility and lowest wave-length) being placed at 318·6 (Soddy).
It must be obvious from all the foregoing that radium is very slowly, but none the less surely, destroying itself. There is a definite loss of particles which, in the course of time, must lead to the destruction of the radium, and it would seem that the large new credit on the bank of time given to biologists in consequence of its discovery has a definite, if remote, limit. With the quantities of radium at present available for experiment, the amount of loss of particles is so small, and the rate so slow, that it cannot be weighed by the most delicate balance. Nevertheless it has been calculated that radium will transform half of itself in about fifteen hundred years, and unless it were being produced in some way all of the radium now in existence would disappear much too soon to make it an important geological factor in the maintenance of the earth’s temperature. As a reply to this depreciatory statement we have the discovery by Rutherford and others that radium is continually being formed afresh, and from that particular element in connection with which it was discovered—namely, uranium. Hypotheses and experiments as to the details of this process are at this moment in full swing, and results of a momentous kind, involving the building-up of an element with high atomic weight by the interaction of elements with a lower atomic weight, are thought by some physicists to be not improbable in the immediate future.
The delicate electric test for radio-activity has been largely applied in the last few years to all sorts and conditions of matter. As a result it appears that the radium emanation is always present in our atmosphere; that the air in caves is especially rich in it, as are underground waters. Tin-foil, glass, silver, zinc, lead, copper, platinum and aluminium are, all of them, slightly radio-active. The question has been raised whether this widespread radio-activity is due to the wide dissemination of infinitesimal quantities of strong radio-active elements, or whether it is the natural intrinsic property of all matter to emit Becquerel rays. This is the immediate subject of research.