June 12, 1913] 



NATURE 



57' 



A First Book of Rural Science. By J. J. Green. 

 Pp. viii + 146. (London: Macmillan and Co., 

 Ltd., 1913.) Price 15. 6d. 

 The teacher who wants to give a rural bias to 

 his school work still has to depend at least as 

 much on his text-book as on his garden for help 

 in his lessons. Amidst the vast number of books J 

 on rural science that the nature-study movement | 

 has called forth, a few stand out prominently as 1 

 being eminently adapted to the purpose. Amongst 

 them we have no hesitation in placing this little i 

 book. The information is sound, and is clearly 

 and concisely set out ; while the order is both 

 logical in method and convenient in practice. 



Beginning with seeds, the author follows on with 

 plant growth, plant nutrition, and reproduction. 

 Next he passes to the subject of soils, and then to 

 the relationship between the soil and the crop. 

 Throughout the author displays a vivid knowledge 

 of rural conditions, and he seeks to connect up 

 the child's training with the things that come into 

 the scholar's daily experience. This desirable end 

 is successfully accomplished. New varieties of 

 plants, for example, are now among the common 

 incidents of rural life. The book gives a short 

 but good account of how they are formed. The 

 micro-organisms of the soil have also come in for 

 much attention from agricultural lecturers and 

 others, and here, again, sufficient information is 

 given to enable the student to form an intelligent 

 grasp of the matter. Manures are described in 

 sufficient detail for the purpose, and manurial 

 trials are illustrated. Altogether the book can be 

 cordiallv recommended both to teachers and 

 students. 



Dent's Practical Notebooks of Regional Geo- 

 graphy. By Dr. H. Piggott and R. J. Finch. 



Book ii. Asia. Pp. 64. Book iii. Africa. 



Pp. 48. (London: J. M. Dent and Sons, Ltd., 



1913.) Price 6d. net each. 

 These books, and others like them, are a welcome 

 indication that teachers in schools are beginning 

 to understand that children learn more satisfac- 

 torily by doing than by listening. The authors are 

 experienced teachers who recognise that with the 

 small amount of time available for geography in 

 ordinary classes every expedient must be tried to 

 select only practical exercises of prime importance. 

 In these little books the practical work is all worth 

 doing, and the instructions given are precise and 

 to the point. 

 Earthquakes and other Earth Movements. By 



Prof. John Milne. Sixth edition. Pp. xvi+388. 



(London : Kegan Paul, Trench, Triibner and 



Co., Ltd., 1913.) 

 The additions and alterations rendered necessary 

 by the knowledge gained since 1903, the date of 

 publication of the fifth edition of his book, are 

 collected by Prof. Milne in an additional appendix 

 of some eleven pages. The chief topics of the 

 appendix are the teleseismic observations which, 

 Prof. Milne says, have already thrown new light 

 upon the homogeneity and rigidity of our world, 

 and have led to the explanation of phenomena in 

 other departments of science. 

 NO. 2276, VOL. 91] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The lonisation of Gases in the Schumann Region. 



In July last I published a note in the Physikalische 

 Zeitschnft (July 13, p. 583) on the ionisation of gases 

 by light and the spectrum of aluminium in the 

 Schumann region. As my views therein expressed 

 have recently been misquoted in print on two occa- 

 sions, I fear I did not make myself clear. It may be 

 worth while, therefore, to add a word. 



It was my object to explain the results of Lenard's 

 volume ionisation experiments by exhibiting the spec- 

 trum of his source of light. To this end I published 

 a spectrogram of the aluminium spark in air in the 

 Schumann region. The illustration showed that, 

 though the spectrum contained some strong lines 

 between X1850 and M600, there was but one group 

 of any strength between Aiboo and M250; this group 

 occurred near A1300. Lenard's data indicated that the 

 rays which produced most of the ionisation lay on 

 the more refrangible side of A1600. I stated, there- 

 fore, that the group near X1300 was probably respon- 

 sible for most of the effect which Lenard observed, 

 because it was the only strong group existing in the 

 aluminium spectrum in the region under considera- 

 tion. This is evidently very different from the opinion 

 ascribed to me by Mr. A. LI. Hughes (Phil. Trans., 

 vol. ccxii., p. 226): "... Lyman concludes that the 

 ionisation of air by light docs not take place unless 

 the light contains wave-lengths less than about 

 M300." 



While I am on the subject, I should like to add that 

 the question as to what wave-lengths are responsible 

 for the volume ionisation observed in gases seems to 

 me to be still open. We know that the effect increases 

 with decrease in wave-length in the Schumann region, 

 but that it "sets in about M350" is not perfectly 

 obvious. Prof. Palmer has been kind enough to test 

 the volume ionisation due to the mercury arc, at my 

 suggestion. He finds a small but perfectly definite 

 effect. This confirms the recent results of Bloch, 

 obtained with an arc in quartz (C.R., vol. civ., 

 p. 1076). I have just concluded a study of the spec- 

 trum of the arc, and I have been unable to discover 

 any lines below A1400. The most refrangible line 

 which I have observed through quartz is at M775. 

 It seems fairly certain, therefore, that some volume 

 ionisation can" be produced by light of wave-length 

 longer than M700. Theodore Lyman. 



Jefferson Laboratorv, Cambridge, Mass., May 22. 



Artificial Hiss. 



Has Lord Rayleigh tried the effect of holding a 

 piece of sheet iron or of compressed charcoal in the 

 small pointed flame of an ordinary foot blowpipe 

 when the air supply is somewhat in excess of the 

 needs of the flame? By adjusting the gas supply, the 

 air pressure, and the position of the iron sheet, sounds 

 can be obtained varying from / to s or sh. The oxy- 

 hydrogen flame, supplied with a slight excess of 

 oxygen, is even better. The air entering a vacuum 

 desiccator through a narrow stopcock gives a fairly 

 good s sound. " E. R. Marle. 



Hartley University College, Southampton, 

 May 30. 



