56 



NATURE 



[March i6, 191 6 



Prof. Ford, shows the fresh material available for 

 research, and the progress that has been made 

 through new observations on established species 

 in the last six years. A special list is given of 

 literature on X-rays and crystal structure. 



Numerous studies on the thermal behaviour of 

 quartz and on its relations to cristobalite and tri- 

 dymite come within the period covered by this 

 appendix, and the attention of geologists may well 

 be directed to the references given on p. 66. t- 

 cross-reference to these would have been useful 

 under the heads of the other forms of crystalline 

 silica. Among the new species we may note bar- 

 bierite, which indicates that a monoclinic struc- 

 ture may be formed under certain conditions by 

 the well known felspathic molecule, NaAlSisOg. 

 Bazzite, a blue scandium silicate from Baveno, 

 and sevei'al vanadium minerals seem attractive 

 novelties. Sefstromite, among the latter, passes 

 away as a mixture. Metallic tantalum, first de- 

 scribed in 1909, forms an important record, and 

 specimens have already found their way from the 

 Urals into most collections. It is late to quarrel 

 with the makers of new names, but didymolite, 

 with no didymium, platynolite, suggesting plati- 

 num when pronounced, and Prof. Ford's own 

 pyroxmangite for "manganopyroxene," strike us 

 as unfortunate. The author of this appendix, how- 

 ever, will at once be gratefullv absolved. 



G. A. J. C. 



r/je structure of the Fowl. By Dr. O. C. Brad- 

 ley. Pp. xi+153. (London: A. and C. Black, 

 Ltd., 1915.) Price 35. 6d. net. 

 The author of this little volume has successfully 

 accomplished a somewhat difficult task in his 

 effort to produce a concise and not too elaborate 

 account of the structure of the fowl. The first 

 chapter deals with the zoological position of birds, 

 and includes a very interesting account of the 

 probable ancestry of the domestic fowl. This is 

 followed by chapters on the skeleton and muscular 

 system, both of which are of necessity dealt with 

 in a very elementary fashion. More detail is 

 entered into when the author deals in successive 

 chapters with the digestive system, the respira- 

 tory organs, the urinary organs, the reproductive 

 organs, and the circulatory system. The descrip- 

 tions of the macroscopic characters of these appa- 

 ratuses are so clear and lucid that they can be 

 well and easily followed by readers who have 

 received little or no previous anatomical training, 

 while the microscopic structure is dealt with in 

 such a masterly way as to render the book of the 

 greatest assistance to the student 6f comparative 

 histology and pathology. The illustrations in these 

 sections are excellent, and have very considerably 

 simplified the author's task. The nervous system, 

 the eye and its appendages, and the ear are briefly 

 considered, and a chapter is also devoted to the 

 skin and its appendages. 



Probably the best chapter is that on the de- 

 velopment of the chick, which is dealt with in 

 rather more detail, the various stages being well 

 illustrated. 



^^'hile this little book would appear to contain 

 X"0 9/1 9n VOT n7l 



little or nothing that is new, it is the only wor 

 with which we are acquainted that contains sue 

 an excellent general description of the structure c 

 the fowl. The illustrations must be regarded as 

 special feature. There are seventy-three of thenr 

 and many are full-page plates. 



There is a very complete index. G. H. W. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible fc 

 opinions expressed by his correspondents. Neithe 

 can he undertake to return, or to correspond wit 

 the writers of, rejected manuscripts intended fc 

 this or any other part of Nature. No notice i 

 taken of anonymous communications.^ 



The Structure of the Line of Wave-Length 4686 A.li 



Previous experiments by one of us (Nature, vo 

 xcii., p. 5; Phil. Mag., vol. xxix., pp. 284-297, 191: 

 have shown that the 4686 line could be obtained b 

 passing a condenser discharge through pure heliun 

 and it was concluded that the results supported 

 theory put forward by Dr. Bohr {Phil Mag., vol. xxvi 

 p. I, 1913). This theory, which was deduced by apph 

 ing the quantum hypothesis to Sir Ernest Ruthei 

 ford's atom-model, ascribed the line to helium. O 

 the other hand, Rydberg, assuming the Pickerin 

 lines to constitute the sharp series of hydrogen fror 

 analogy with the spectra of the alkali metals, obtaine 

 by calculation the value 4687-88 for the wave-lengt 

 of the first line of the principal series of hydrogen. 



The present experiments on the structure of th 

 line were commenced with the purpose of testing sti 

 further its chemical origin, and of obtaining result 

 which would throw further light on the mechanisr 

 of emission of spectrum lines. The importance c 

 accurate knowledge of the structure of hydrogen an 

 helium lines from the latter point of view has ahead 

 been shown by Bohr {Phil. Mag., vol. xxix., p. 33; 

 1915). It is well known that the hydrogen lines c 

 the Balmer series are not single lines, but clos 

 doublets, and it is therefore to be expected from bot 

 Rydberg's and Bohr's theories, that the 4686 lin 

 should also have a complex structure. According t 

 Rydberg's theory, the line should be a doublet havin 

 the same frequency difference as the members of th 

 Balmer series. The recent measurements of Buisson an 

 Fabry gave 0132 A.U. as the separation of the tw 

 components of Ha, and it follows by calculation thi 

 the two components of the 4686 line should b 

 separated by 00674 A.U. From Bohr's theory, th 

 details of the structure of the line could not be antic 

 pated, but from the supposed analogy between th 

 mechanism of emission of the 4686 line and the lint 

 of the Balmer series, it was hoped that a knowledg 

 of the structure of the line would serve as a guic 

 in testing different hypotheses for explaining tf 

 doubling of the hydrogen lines. 



The origin of the "'4686" line has recently bee 

 studied by Merton (Nature, vol. xcv., p. 64; Pro 

 Roy. Soc, vol. xci., p. 382, 1915), who used a methc 

 based on Lord Rajieigh's theory of the width ' 

 speitrum lines. He concluded that either the bread;! 

 of the line is controlled by circumstances at presei 

 unknown, or that the line originates from systems 

 sub-atomic mass. Later experiments by the san 

 author (Proc. Roy. Soc, vol. xci., p. 421, 191, 

 February, igi6) show that the widths of some spf, 

 trum lines are not wholly due to the motion of t| 

 molecules. _ ' 



In our first set of experiments the line was excit' 

 by passing a condenser discharge through a helii ' 



