January i8, 1900] 



NA TURE 



269 



account of the various positions in which the resonator 

 was placed ; of the results obtained with an ordinary 

 Hertzian field between two wires, and round a single 

 wire. The author next considers the interference field, 

 which is obtained between two wires whose ends are con- 

 nected to plates placed on opposite sides of the same 

 plate of an oscillator. The effects on an ordinary 2-wire 

 field of bending one of the wires so as to lengthen it by 

 [, \ and a whole wave-length are next investigated. The 

 author shows that all the effects obtained may be deduced 

 from the results obtained with a single-wire field. An 

 account of some experiments with 3, 4 and 6 wires 

 concludes this chapter. 



Chapter iv. deals with the action of the resonator. The 

 effects of varying the position and direction of the micro- 

 meter-gap, the disturbance due to the presence of the 

 resonator in the field, and the effect of varying the length 

 of the resonator are studied in detail. The form of 

 resonator with a gap bridged over by a cell and telephone 

 receives careful attention, the effect of altering the posi- 

 tion of the gap relatively to the micrometer spark-gap 

 being fully investigated. 



Chapter v. is concerned with the important problem of 

 the propagation of waves in dielectrics other than air. 

 Oil and water were the two dielectrics studied by the 

 author, and the effects obtained clear up some rather 

 obscure and apparently contradictory results obtained 

 by other experimenters in this field. 



Chapter vi. contains a useful resum^ of the more 

 important results obtained by the author. 



In Chapter vii. the author describes a system of multi- 

 plex Hertzian wave telegraphy {^iiot wireless), regarding 

 whose practical value we may well be pardoned for feeling 

 somewhat sceptical. 



The book forms a valuable storehouse of facts, and 

 the author is to be congratulated on the extremely lucid 

 and well-arranged account of his important researches. 

 They were all carried out on a large scale (in the experi- 

 ments on oil and water, 230 to 260 litres of the liquid 

 were used), and must have required an unusual amount 

 of skill, care and patience. 



A striking feature of the work is the entire absence of 

 mathematical reasoning, not a single symbol of differentia- 

 tion or integration occurring throughout the whole of the 

 book. The author has carefully avoided all theoretical 

 discussions, and confined himself to an accurate descrip- 

 tion of experimental facts. The clearness and elegance 

 of the language in which this description is given render 

 it a pleasure to read the book, which will prove a source 

 of delight to every true experimentalist. 



OUR BOOK SHELF. 



Indicators and Test Papers. By Alfred I. Cohn, Ph.G. 



Pp. IX -f 249. (New York: John Wiley and Sons. 



London : Chapman and Hall, Ltd., 1899.) 

 This book contains an account of the source, preparation, 

 application and tests for some scores of indicators and 

 test papers which have been proposed for use chiefly in 

 determining the end-point in volumetric chemical 

 analyses. The book opens with a general discussion of 

 the action, use, and theory of indicators, and ends with 

 four useful tables and a good index. The first table is 

 NO. 1577, VOL. 61] 



TrommsdorfFs showing the sensitiveness of indicators to 

 acids and alkalis, the second is R. T. Thomson's (hitherto 

 the chief English guide), the third is Dieterich's table 

 showing the sensitiveness of various test-papers, and the 

 fourth is a tabular summary of the principal indicators 

 by the author. 



The compilation of this book must have demanded 

 much patient labour, and acknowledgments are due to 

 the author for the care and pains he has bestowed upon 

 the work. It will prove a useful addition to analytical 

 literature. Whilst saying this, some points of criticism 

 cannot be withheld. In the first place it must be said 

 that the author has not dealt in a very clear way with the 

 theory of indicators. The subject is not an easy one, and 

 the average operator has not hitherto troubled himself 

 much about it. Litmus has been to him a substance 

 provided by Nature for the discrimination between acids 

 and alkalis rather than the means of furnishing blue 

 alkaline salts from which a weakly acidic substance of 

 red tint is " displaced " by the action of nearly all other 

 acids. Again, the reasons why methyl orange is good for 

 the titration of bases and not of acids is not usually in- 

 quired into. Such considerations make it the more 

 desirable that the principles underlying the use of indi- 

 cators should be stated very clearly. Mr. Cohn has given 

 explanations, including the application of the ionic theory, 

 and of the speculative mechanical theory (somewhat 

 antiquated and unfruitful) of F. Mohr, but he has not 

 set forth the matter with the desirable clearness and 

 coherence. 



Next with regard to the substance of the book, it is 

 worth considering whether, in any future edition, type of 

 two sizes might not be employed. Many of the indicators 

 described are of extremely doubtful value, and the worker 

 really wants to know definitely which indicators have 

 been found meritorious by other people than those who 

 have suggested their use. In this connection also a pro- 

 test must be raised against naming indicators after their 

 inventors. It is useful to know the composition and nine 

 synonyms of TropKolin GO, but there is surely no call 

 to add to these the term " Von Miiller's Indicator." 



The book would have been improved by references to 

 original papers. For example, the reflecting galvano- 

 meter is scheduled as an indicator, but there is neither a 

 full description of its use nor a reference to Kiister's paper 

 on the subject. References would have been valuable 

 throughout the book. A. S. 



Optical Activity and Chemical Composition. By Dr. H. 



Landolt ; translated by Dr. J. McCrae. Pp. xi -h 158. 



(London : Whittaker and Co., 1899.) 

 This small book is a remarkably clear exposition of what 

 is a somewhat recondite and difficult branch of chemical 

 physics. It is well known to students of optical science 

 that there are liquids and solid substances in solution 

 which have the strange power of rotating the plane of 

 vibration of a polarised ray of light that is passing through 

 them. Familiar examples are turpentine and other es- 

 sential oils, sugars, tartaric acid, quinine and albumen. 

 But Dr. Landolt says that more than seven hundred sub- 

 stances, all carbon compounds, are known to exhibit this 

 molecular rotation. 



Of course the fruitful discoveries of Pasteur — the right 

 and left-handed tartaric acids, racemic acid, molecular 

 asymmetry, &c., are briefly described ; and the more 

 recent simultaneous discoveries of van 't Hoff and Le Bel 

 receive fuller attention. It is shown how this property is 

 met with only where one at least of the carbon atoms of 

 an organic compound is united with four different atoms 

 or radicles ; and the results flowing ^from this kind of 

 structure are explained and illustrated—results which 

 form what is now called stereochemistry. 



But the principal object of Prof Landolt's book, as ex- 

 pressed in its title, is the connection that may be found 



