November 30, 191 1] 



NATURE 



143 



On October 6, 182 1, Victor Emmanuel I. instituted the 

 first Italian public chair of higher physics, and in November 

 Avogadro was appointed to it. Owing to political troubles 

 the post was suppressed two years later, and Avogadro, 

 after a brief interval, was given a post at the Regia 

 Camera dei Conti, where he continued with undiminished 

 activity hit cherished studies. In January, 1832, Carlo 

 Alberto restored the chair Avogadro had occupied, but 

 appointed to it the famous Cauchy, who, it will be re- 

 membered, had been compelled to leave the College de 

 France and his seat in the National Institute of France 

 owing to the Revolution of 1830. Cauchy had been 

 elected, amidst a storm of indignation, to the seat in the 

 institute vacant by the expulsion of Monge, and he was 

 now in his turn suffering the vicissitudes of fickle fortune. 



Towards the end of 1833 Cauchy left Turin to take 

 charge of the education of the young princes at Prague, 

 and Avogadro was again appointed to the position for 

 which he was so well fitted. He remained professor of 

 physics until 1850, when he resigned owing to advancing 

 age, and was succeeded by his favourite pupil, Felice Chi6. 

 His scientific work was continued almost to the end of 

 his life ; and he died at Turin on July 9, 1856, having 

 nearly reached the age of eighty. 



Avogadro 's memoirs, numbering some forty or fifty, 

 ppeared in the Journal de Physique, the Annales de 

 Chimie et de Physique, BrugnatelWs Journal, and other 

 periodicals. Among his works was a huge compilation of 

 3700 pages, in four volumes, on physics, which was pub- 

 lished at the expense of Carlo Alberto. His principal 

 studies, as we have seen, related to the physical properties 

 and the internal structure of bodies, and the law — that 

 equal volumes of gas contain the same number of mole- 

 cules — to which his name is attached perpetuates his 

 memory. Ampere independently enunciated the same 

 doctrine ; but in point of priority Avogadro 's claim is 

 beyond dispute. 



He was a man esteemed in private life for his kindness, 

 affability, and sincerity. His habits were noted for their 

 simplicity, and he was as modest as he was learned. The 

 year following his death a marble bust in his honour was 

 placed in the University of Turin. In the Scuola Pro- 

 fpssionale at Biella is another bust, with the inscription — 



I GAS A PARI CONDIZIONI 

 DI PRESSIONE E DI TEMPERATURA 

 IN EGUALI VOLUMI CONTENGONO 

 UN EGUAL NUMERO DI MOLECOLE. 



Last year a project was formed to celebrate the centenary 

 of the enunciation of his law, but I do not know whether 

 the scheme was carried through or not.' The above details 

 have been obtained from the short biography of Avogadro 

 by Alfonso Cossa, for a copy of which I am indebted to the 

 kindness of Prof. Naccari, the present occupant of the 

 chair of physics at Turin. Edgar C. Smith. 



Hong Kong, October 23. 



Characteristic Ronteren Radiations. 



In the September number of The Philosophical Magazine 

 there appears a paper by Prof. Barkla, in which are 

 tabulated the results of some further experiments on 

 characteristic (fluorescent) Rontgen radiations. It appears 

 that some of the elements of high atomic weight give " two 

 lines in the fluorescent spectrum." Optical analogy sug- 

 gests that there may be some simple relation connecting 

 these spectral lines. Such a relation can, indeed, be readily 

 obtained, and I should like to be allowed to direct attention 

 to it. 



In a paper on the production of characteristic Rontgen 

 rays (Roy. Soc. Proc, iqii), I showed that for the elements 

 from Al upwards (which gave characteristic rays in series 

 K) the radiation they emitted could be defined, ind'rcctly, in 

 terms of their atomic weight. It was shown that the 

 radiation characteristic of an element of atomic weight w is 

 the same (when tested by penetrating power) as the most 

 penetrating of the Rontgen rays emitted from an anti- 

 kathode bombarded by kathode rays of velocity equal to 

 kw, where fe=io*. 



' *•><• Nature of Oct-^lier 26, p. 557.— Ed. 

 NO. 2196, VOL. 88] 



Thus the radiation characteristic of Se would be emitted 

 by kathode rays of velocity 7-92x10° cm. /sec. (•u; = 79-2), 

 that characteristic of Cu by kathode rays of velocity 

 6-36xio° cm. /sec. (■i£; = 63-6), and so on. Such kathode 

 rays as would produce a characteristic Rontgen-radiation 

 may be conveniently termed equivalent kathode rays. Thus 

 we can define a radiation the \/p of which is known, in 

 terms of the velocity of the equivalent kathode rays, and 

 so indirectly in terms of the atomic weight. 



Prof. Barkla tells us that Bl gives out (in series L) a 

 radiation of very nearly the same penetrating power as that 

 from Se in series K (the actual values of \/p are 190 and 

 18-9). From this it follows that the equivalent velocity for 

 Bi (series L) is the same as for Se (series K), i.e. 7-92x10'. 

 Thus, if we adopt series K for our standard, we can say 

 that Bi behaves (for series L) as though it were an element 

 of atomic weight 79. 



The actual atomic weight of Bi is 208, and it behaves (for 

 series L) as if its atomic weight were w' = l{w — ^o) = jg; 

 thus the two possible spectral lines for Bi are defined in 

 terms of the two equivalent kathode rays of velocities 

 2-08x10"' cm. /sec. and 7-9X10" cm. /sec. 



The expression 



w' = ^(w — 50) 



holds fairly well over the greater part of the range studi>?d 

 by Prof. Barkla, as the following table shows. 



The calculated values of \/p have been obtained by 

 interpolation' of w' in a w, \/p graph. 



The observed value of \/p for Sb is almost certainly too 

 high, since in some experiments with Sb (August, 1910, 

 Proc. Camb. Phil. Soc.) I found that the equivalent velocity 

 was about 3-6x10' cm. /sec. (compare w' above). The 

 agreement otherwise is as good as could be expected. 



In conclusion^ formulas of the type to which attention has 

 just been directed (•«;' = A.iy+B) may possibly prove useful 

 to experimental investigators in suggesting new " lines " 

 and indicating where to look for them. 



R. Whiddington. 



St. John's College, Cambridge, November 16. 



A Suggested Reform in Palaeobotany. 



In a paper in the Annals of Botany for October (pp. 

 903-7, text figures) I brought forward conclusive botanical 

 evidence that the Cretaceous fossils from the Amboy Clays 

 of North America, hitherto known as Ophioglossuni 

 granulatum, Heer, are not only not Ophioglossums, but 

 are not ferns at all, and belong to the widely distinct 

 family of Gymnosperms, in the genus Pinus. 



This result, in itself of not much importance, forms thf 

 text of a general recommendation to palaeobotanists, whicli 

 is as follows ; — In the interests of the sciences of palaeo- 

 botany, geology, and botany, I " urge that the lists pub- 

 lished by palaeobotanists should be printed in two forms, 

 and that the names of species of leaves, stems, &c., of 

 which there is a reasonable security of determination, 

 should be difl'erentiated from those in which there is no 

 guarantee at all that the actual nature of the plant has 

 been discovered. Any tri-nomial system is cumbrous ; but 

 those who publish on fossil plants might print their names 

 in type of two kinds, which would indicate which species 

 are doubtful. I should like to suggest that, instead of 

 using italics or ordinary capitals as is usual in printing 

 the names of species and genera, such doubtful plant- 

 impressions should be printed in Gothic lettering. This 

 would indicate that our knowledge about them is mediaeval 

 — of the Dark Ages — and would further save the incon- 

 venience of tri-nomials, while it would indicate immedi- 

 ately the difference between the established and the 

 doubtful determinations. As information accrued about a 



