118 



KNOWLEDGE. 



March, 1913. 



Buckingham, Hertford, Essex, Bedford, Huntingdon, Cam- 

 bridge, and Norfolk, and only few reports from Kent, Hants, 

 Surrey, Middlesex, Berks, Suffolk and Lincoln. 



Of the autumnal movements (1910) there are particularly 

 good accounts of the Golden-crested Wren, and of the 

 unusual irruption of the Mealy Redpoll (Linota linaria), 

 which, during October, visited in numbers the whole line of the 

 east coast from the Shetlands to Kent and many inland places. 

 The first record of the Magpie as a migrant to our shores is 

 remarked upon, twenty birds having been seen arriving at 

 Thanet (E. Kent) from the north-east on 28th September, and 

 fifteen from the north, during an easterly gale, on 14th 

 October. 



The Scottish records, which are so fully published elsewhere, 

 are only partially utilised by this Report, and it might prevent 

 misleading conclusions being arrived at, if Scotland, like 

 Ireland, was excluded from the purview. 



H. B. W. 



PHYSICS. 



Experimental Researches on the Specific Gravity and 



the Displacement of some Saline Solutions. — By 



J Y. Buchanan, M.A., F.R.S. 227 pages. 12-in.X9i-in. 



(Neill & Co. Price 7/6 net.) 



The author of this book, who acted as chemist and physicist 

 on the memorable " Challenger " expedition, has made a close 

 study of the subject of specific gravity for the last forty years ; 

 and the present volume is devoted to researches on saline 

 solutions carried out during recent years by the aid of 

 hydrometers. As usually constructed, the accuracy of the 

 common hydrometer depends upon the correct calibration of 

 the scale, which is marked off by reference to other standards ; 

 and hence most workers, when conducting precise work on 

 specific gravities, employ the pyknometer in preference. Mr. 

 Buchanan shows, however, that by standardising a hydrometer 

 by reference to its own displacement, it is possible to secure 

 results of a much greater degree of accuracy than is attainable 

 by any method involving a series of weighings. Two forms of 

 hydrometer are described : — the closed type, which may be 

 made to sink in the liquid under trial by placing weights on 

 the top, and which is used for the less dense solutions ; and 

 the open type, which may be weighted internally, and will, 

 therefore, maintain its stability in the very dense solutions for 

 which it is used. Full details of the method of standardisa- 

 tion are given. 



The book contains the records of some thousands of observa- 

 tions, extending over some years, of the specific gravities of 

 water solutions of a number of salts, ranging in strength from 

 saturation to iAi of a gram-molecule per litre. Amongst the 

 salts used are chlorides, bromides, iodides, iodates and nitrates 

 of sodium, potassium, caesium, rubidium, lithium, barium, 

 calcium and lead ; and also various mixtures of salts in some 

 definite proportion of their molecular weights. Many in- 

 teresting relations between the specific gravities and displace- 

 ments and the molecular weights are revealed as the result of 

 accurate observations, and are shown in the form of diagrams 

 and graphs. Of special interest is the graph on page 154, 

 which represents the fluctuations in the increment of displace- 

 ment in solutions of common salt ranging in strength from 

 J to lis of a gram-molecule per litre, the results indicating a 

 series of interactions between the water and the salt at these 

 low concentrations. It would be interesting to compare a 

 water solution of a non-electrolyte, such as sugar, with those 

 of salts in this connection. 



An observation made incidentally with a supersaturated 

 solution of calcium chloride is at once remarkable and sug- 

 gestive. It was noted that this solution, prior to crystallisation, 

 was in a state of unrest, undergoing a rhythmic series of 

 isothermal expansions and contractions, which were detected 

 by the delicate hydrometer used. Further investigation on 

 these lines with other supersaturated solutions, and with 

 suffused or overcooled liquids, is highly desirable ; and might 

 assist in diagnosing the cause of these abnormal states. 



The effect of temperature upon the accuracy of the observa- 

 tions is discussed, and details given of the methods used to 

 secure a constant temperature during the readings. A favourite 

 working temperature was 19 ^"C, which could most easily be 

 attained and kept constant in the room. Many hints for 

 accurate working may be gathered from the book, which 

 may be recommended to all interested in the determination 

 of specific gravities, and also to those engaged in the study 

 of solutions, who may find in it suggestions for attacking the 

 various problems from another standpoint. 



Chas. R. Darling. 



Elementary Physical Optics.— By W. E. Cross, M.A. 

 311 pages. With many diagrams. 7J-in X5-in. 



(The Clarendon Press. Price 5/- net.) 



When some years ago some Royal Institution lectures were 

 delivered on Waves and Ripples in the Air and in the Ether, the 

 lecturer, while describing and illustrating by many examples 

 the wave motion of light, was obliged to consign his treat- 

 ment of optical problems of deflection, refraction and disper- 

 sion of light on this basis, to an appendix to his lectures, 

 subsequently published. Mr. W. E. Cross, who is the Head 

 Master of King's School, Peterborough, has moreboldlygrappled 

 with the difficulty. It is comparatively simple in illustrating 

 geometrical optics to convince the pupil of their validity by 

 using the " ray," or the line, as the unit ; but having been taught 

 in such a way it will be long afterwards before the boy will 

 have arrived at sufficient proficiency in mathematics to draw any 

 general conclusions from the experiments presented to him. 

 If however the idea of a wave front, or advancing trains of 

 ripples of light be presented to him, he will understand it, but 

 he may find a difficulty in reconciling theory with experimental 

 effects. After all a boy can see a ray, but cannot discern a 

 light wave. 



Mr. Cross compromises. He uses the ray in demonstrating 

 the action of lenses, mirrors and prisms, but he explains their 

 action by the change of curvature brought about in the wave 

 fronts, or by change of velocity, when, as in refraction, the 

 wave passes from one medium to another. By treating a ray 

 not as a mathematical conception, but as a narrow cone of 

 light, isolated from the wave series of which it forms a part, 

 but possessing none the less all the properties of light waves, 

 he arrives at a conception which causes no confusion of ideas, 

 and in which there is no discrepancy between experiment and 

 theory. The idea is not only sound : it is worked out so well 

 as to give a solidity and an interest to the pupil's conception 

 of the nature of light such as the older method cannot impart. 



E. S. G. 

 RADIOACTIVITY. 



Studies in Radioactivity. — By W. H. Bragg, M.A., F.R.S. 

 196 pages. 70 diagrams. 8j-in. X 5^-in. 



(Macmillan & Co. Price 5/- net.) 



This work by Prof. Bragg forms the latest volume in 

 Messrs. Macmillan's series of Science Monographs. The 

 ' volumes in this series are intended to represent " the ex- 

 pression of modern scientific work and thought in definite 

 directions," and each volume will be by a specialist and 

 mainly descriptive of his own contributions to the field of 

 scientific work dealt with. They will thus be more adapted 

 to the requirements of the advanced student than to those of 

 the beginner. Prof. Bragg's book admirably fulfils the inten- 

 tion of the series, and he has very wisely not omitted to 

 describe briefly the researches of other experimentalists, as 

 well as his own most valuable ones, which come within the 

 subject of the work. 



Prof. Bragg's researches deal mainly with the passage of 

 o, /3, 7 and X-rays through gases and solid bodies, the arrest, 

 scattering and loss of energy of the rays, and their ionising 

 powers ; and he points out many close similarities (together 

 with no less marked differences) in the behaviour of the three 

 types of rays. Prof. Bragg considers that the corpuscular 

 theory of the 7 and X-rays will prove the more useful, and 

 brings forth several interesting arguments against the ether- 



