KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1904. 



stance now called polonium, and the latter with the radium 

 which plays such an important part in modern science. 

 Neither polonium nor actinium have as yet been isolated 

 in the form of a salt. The former in many ways resembles 

 bismuth, and its nature still remains doubtful. Radium, 

 on the other hand, yields salts — e.g., a bromide, a chloride, 

 and a nitrate. Its combining weight has been fixed by 

 Madame Curie at 112-5, and 225 is suggested for its 

 atomic weight. Itexhibitsadefinite flame spectrum, which 

 has been recorded by Messrs. Runge and Precht, and 

 which is given in fig. 2, whilst its spectrum in the ultra 

 violet has been studied by Sir William Crookes and 

 others, and affords a means 'of idcntilying it. 





Fig. 2, — The Flame Spectrum of Radium. 

 The line <i has the wave length 4S26, b 6329, c 6349, d 6653. There 

 are bands about b c and <i as shown above. 



Radium is generally regarded as an element, but as the 

 total quantity of the pure radium salts yet made would be 

 insufficient to fill a small egg cup, this statement must 

 still be taken with some reserve. The so-called pure 

 salts of radium possibly may be mixtures, but, for the 

 present, in the absence of any evidence to the contrary, 

 we may assume them to be salts of a new and peculiar 

 elementary substance radium. 



Radium the element has not been isolated. Its salts 

 are so valuable, and the process of separating it probably 

 would be so wasteful, that it seems unlikely anyone will 

 attempt to prepare elementary radium at present. 



The process of purifying a radium saU has not been 

 very frequently described, but it is simple enough in 

 principle. The raw material is the residue left after the 

 uranium has been extracted from pitchblende. A ton of 

 this material suitably treated may yield 10 or more kilo- 

 grams of a mixture consisting chiefly of the sulphates of 

 barium, lead, iron, and calcium, with a trace of radium. 

 This is converted into carbonates by heating it with a 

 solution of carbonate of soda, and the carbonates are 

 dissolved in hydrochloric acid, which converts them into 

 chlorides. The lead and iron in the mixture of chlorides 

 thus produced are got rid of by means of sulphuretted 

 hydrogen and ammonium sulphide, and the remaining 

 barium, calcium, and radium are reprecipitated as car- 



• 



Fi<;. 3. — Kailiographs of radium salt and uranium taken 

 siniultaneouslv. 



bonates, again converted into chlorides, and then washed 

 with strong hydrochloric acid to remove the calcium 

 chloride. The residue consists of barium chloride con- 

 taining a trace of radium chloride. This is dissolved in 

 hot water and allowed to crystallize partially. The 

 crystals, which contain most radium, are separated 

 from the liquid portion, and the latter is then 

 evaporated to recover the remaining salt, and each of the 

 two portions thus produced is similarly fractionated. By 

 systematic work of this kind products were obtained first 

 nine hundred times as radio-active as uranium, then five 

 thousand times as active, then fifty thousand times, and 

 at last, it is said, a million times as active as the stan- 

 dard substance, the removal of the barium salt at the 

 later stages being facilitated by using solutions of hydro- 

 chloric acid in place of water for dissolving the mixture 

 of barium and radium chloride. 



Some idea of the difference between the activity of 

 uranium salts and of radium may be got from fig. 3. 

 On the reader's right is the silhouette a, given by 

 one-sixth of a grain of radium in fifteen minutes. The 

 area about the faint dark mark above b a little to 

 the left of this shows the effect of a much larger quantity 

 of a uranium salt, the two being exposed side by side 

 over the same plate. The uranium salt, as will be seen, 

 gave no sensible result at all. The small dark mark 

 above b was added to indicate the centre of the area 

 exposed to the uranium. 



The salts of radium, in their ordinary reactions, 

 resemble those of barium rather closely, but in other 

 respects they are remarkably different. Thus, they are 

 visible in the dark, and continuously evolve heat ; so 

 that a heap of a radium salt is always hotter than the 

 air around it. So great is the amount of heat evolved 

 that a gram-atom of radium gives out in a year as much 

 heat as a gram-atom of hydrogen when it is burnt in the 

 oxyhydrogen flame, and, moreover, as far as we know, 

 the radium would go on giving out heat at this rate for 

 many centuries. Its powers are destroyed to a great 

 extent if it is strongly heated (see emanation), but are 

 recovered spontaneously after a few weeks on cooling. 



Radium, or rather its radiations, are very destructive. 

 A piece of cambric placed above a box containing a 

 little radium salt was found by Lord BIythswood to be 

 pierced with holes after two or three days. A photo- 

 graphic film exposed to one-sixth of a grain of radium 

 bromide in the author's laboratory for four hours by Mr. 

 W. D. Rogers (who has kindly prepared many of the 

 figures given in this article) yielded no silhouette because 

 the film was completely disintegrated and its remains 

 washed away during the developing process ; and the 

 caustic powers of radium salts, as is well known, are 

 thought likely to prove useful in surgery, and have some- 

 times produced very unpleasant effects when specimens 

 of the salts have been kept too long near the human body. 

 Its power of making air conduct electricity is shown by 

 the way in which a tassel of silk electrified by rubbing 

 with india-rubber collapses when radium is brought near 

 it, and by the rapid collapse of the lea\-es of a charged 

 gold leaf electroscope under similar circumstances. But 

 the prettiest way of observing this property of radium 

 is as follows ; — 



Connect a spark gap at b, fig. 4, with an induction 

 coil and with a vacuum tube a — a large vacuum 

 tube gives the best result — as in the following figure. 

 Arrange matters so that the coil gives a very steady 

 discharge at the spark gap, and then draw back the 

 point and plate till the discharge just passes through the 

 vacuum tube, only an occasional spark crossing at b. 

 Then bring the radium close to the spark gap. When 



