June 14, 1900] 



NA TURE 



151 



■ 



sometimes be removed in this way ; but if we consider that a 

 greatly increased supersaturation (six-fold instead of four-fold) is 

 necessary, and that the production of ions is continually going 

 on, so that negative ions as well as positive are always pre- 

 sent, we can hardly consider it a likely occurrence. What then 

 is the subsequent history of the positive ions after being car- 

 ried up out ot reach of the drops formed on the negative ions? 

 They will, under the action of the electric field produced by this 

 separation, tend to travel downwards relatively to the air with 

 a velocity of the order of one centimetre per second for a field of 

 ICK3 volts per metre, as the measurements of Rutherford and 

 others have shown. After being carried beyond the region of 

 ascending air-currents, they will travel downwards towards the 

 earth's surface ; but long before reaching it they will become 

 attached to cloud particles or to the dust particles of the lower 

 layers of the atmosphere, where the positive charge will 

 accumulate. 



It is not claimed that the process described above is the only 

 source of rain or the only source of atmospheric electricity. It 

 should be pointed out, for example, that another way in which 

 rain may possibly acquire a negative charge is by falling through 

 ionised air. For according to Zeleny {Phil. Mag. vol. xlvi. 

 p. 135) a body suspended in a current of ionised air becomes 

 negatively charged in virtue of the slightly greater velocity of the 

 negative than of the positive ion under a given force. Elster 

 and Geitel make use of this difference between the positive and 

 negative ions to account for the normal positive electrification 

 of the atmosphere, by the passage of air through the vegetation 

 on the earth's surface. Whether, however, the charged parti- 

 cles, the presence of which near the surface of the earth their 

 experiments seem to prove, are really free ions whose velocity 

 under a given force is that of the ions produced by Rontgen 

 and other rays and not comparatively slow-moving masses 

 (the nuclei called dust particles by Mr. Aitken) to which ions 

 have attached themselves remains as yet undecided. In air 

 charged with dust even to the extent to which clear air near 

 the surface of the ground is shown by Mr. Aitken's observations 

 to be, it is likely, since the rate of ionisation in the atmosphere 

 is certainly slow, that an ion would become attached to some 

 •dust particle in a time very short compared with what the 

 average life of an ion would be in dust-free air, where it is 

 determined merely by the rate of recombination of the ions. 



In conclusion, it must be confessed that if the rate at which 

 the electric field of the earth is being destroyed by leakage 

 ihrough the air is anything like so great as is given by Elster 

 and Geitel's interpretation of their experiments {i.e. of the order 

 of I per cent, per minute), no theory which attributes the normal 

 fine weather electricity to the effect of precipitation at a dis- 

 '.ance is sufficient to explain tl\e facts. C. T. R. Wilson. 



Cambridge Laboratory, Cambridge, May i6. 



Specimens of " Dromaeus ater." < 



In reference to Prof. Giglioli's note (^///w, ^age 102), I may 

 perhaps be allowed to remark that Bullock's Museum appears 

 to have contained a specimen of the extinct Dromaeiis ater. 

 The twelfth edition of the ' ' Companion " to that Museum, 

 published in 18 12, has the following entries (page 80) : — 



" Great Emea, or New Holland Cassowary . . . 



" Lesser Emea, not half the size of the above, and a distinct 

 species." 



At the dispersal of his collection the sale Catalogue includes 

 both specimens as lots 97 and 98 on the eleventh day of the 

 sale (May 18, 1819), the latter as 



" Lesser Emew, a distinct species from the last," 

 and my annotated copy of the Catalogue shows that both were 

 bought by the Linnean Society — for 10/. io.r. and 7/. \os. 

 respectively. I have tried to trace the latter specimen, but in 

 vain. It may still exist unrecognised. Alfred Newton. 



Magdalene College, Cambridge, June 4. 



Effect of Iron upon the Growth of Grass. 



Some years ago Nature published a short letter of mine from 

 India, noticing the way in which laying out iron (famine) tools 

 on the ground brought on grass upon very dry surfaces. Any 

 one who looks now under the rows of iron chairs, and round the 

 railings, of the band-stand on the east side of the Green Park, 

 a ill see the same stimulating effect produced. A. T. F, 



London, June 4. 



NO. 1598, VOL. 62] 



SOURCES AND PROPERTIES OF BECQUEREL 



RAYS. 

 T N the following article a general account is given of 

 -^ a few of the more striking phenomena connected 

 with Becquerel rays, including some of the recent develop- 

 ments of the subject at the hands of Becquerel, M. and 

 Mme. Curie and others. 



Among a large number of papers which have lately 

 been published, dealing with properties of these rays, 

 two are worthy of especial notice, as giving a compre- 

 hensive view of the phenomena. For those who propose 

 to study the subject more fully, no better guide can be 

 found than Prof. Bister's report in Eder's Jahrbuch fUr 

 Photographic und Reproduciionstechntk for 1900. The 

 footnote references to original papers form a complete 

 bibliography of the literature of the subject existing 

 at the time when the article appeared, and it is sur- 

 prising that Prof. Elster should have succeeded in sum- 

 marising so large an amount of matter in eleven very 

 small pages. Dr. B. Walter's article in the Fortschritte 

 auf dem Gebiete der Rontgensirahlen is somewhat less 

 condensed and more popular ; the chief phenomena, 

 especially the photographic and fluorescent properties, 

 are dealt with at greater length, and the article is illus- 

 trated by a plate of radiographs showing the difference 

 between the actions of Becquerel and Rontgen rays. 

 Already Walter's paper, and, to a less degree, Bister's 

 report, have become out of date on the subject of magnetic 

 deviation, and for this and other later developments no 

 better guide could be found than the well-condensed 

 summaries contained in the current monthly parts of 

 Scietice Abstracts. 



The discovery of these rays in 1896 was a natural 

 sequence of the discovery of the Rontgen rays, and was 

 led up to, on the one hand, by the attempts of M. Henry 

 to mtensify the action of Rontgen rays by the use of 

 phosphorescent substances ; and, on the other hand, by 

 the theory, since abandoned, that the Rontgen rays were 

 themselves the result of phosphorescence of the vacuum 

 tube. Becquerel and other physicists made numerous 

 experiments to test whether phosphorescent substances 

 emitted rays capable of acting on a photographic plate 

 that was enveloped in opaque paper, and it was found 

 that rays which produce actinic action were emitted by 

 the phosphorescent salts of uranium, not only when these 

 salts had been exposed to the action of sunlight or of 

 Rontgen rays, but even after they had been kept in the 

 dark for months, the "radio-activity" showing no 

 perceptible falling ofif. 



The next step was the discovery, by Mme. Curie, 

 that Bohemian pitch-blende — a black, shiny ore of 

 uranium — possessed a higher degree of radio-activity 

 than uranium itself, and this result naturally suggested 

 the view that the ore contained, besides uranium, some 

 other substance to whose presence the increased action 

 was due. By separating the pitch-blende into its con- 

 stituents, M. and Mme. Curie were led to discover the 

 existence of two sources of radio-activity, one associated 

 with the compounds of bismuth, and the other with those 

 of barium occurring in the ore. Seeing that barium and 

 bismuth obtained from other sources do not emit 

 Becquerel rays, these radiations were attributed to the 

 existence of two new substances, that associated with 

 bismuth being named polonium, a name derived from 

 the Polish nationality of Mme. Curie, while the other 

 substance associated with barium chloride was called 

 radium. The separation of these two substances has led 

 to the production of rays of sufficient intensity to e.xcite 

 fluorescent screens, discharge electrified conductors, and, 

 indeed, to reproduce, with differences, most of the 

 properties of Rontgen rays. A third radio-active sub- 

 stance, produced from the residues of pitch-blende, is 

 recorded by Debierne, who names it actinium. It is 

 precipitated by the principal agents for titanium, and it 



