NATURE 



{Nov, 14, 1889 



book before us takes an entirely new departure. The 

 idea of representing the various coloured reactions by 

 tinted imitations is, so far as we know, quite new. Apart 

 from this, the usual well-worn paths are followed. The 

 tables are of the simplest character, and are only sufficient 

 for the detection of common bases in salts or oxides, 

 no attempt being made to separate the members of the 

 various groups. The second part is devoted to reactions 

 for the detection of a few acids and organic substances. 



The book is apparently primarily intended for the use 

 of students preparing for the preliminary examination of 

 the Conjoint Board of the Royal College of Physicians 

 and Surgeons, but it will no doubt have a much wider 

 field of usefulness if it survives the test of experience. 

 The new method of representation seems excellently 

 adapted for young students, and certainly no harm can 

 be done by giving it a fair trial. 



The reactions illustrated include precipitates, charcoal 

 reactions, borax beads, and flame colorations, most of 

 which are fairly well represented. 



The Story of a Tinder Box. By Charles M. Tidy, M. B. M. S., 

 F.C.S., &c. (London : Society for Promoting Chris- 

 tian Knowledge, 1889.) 

 Popular lecturers have discovered for some time that the 

 history of the methods that have been used for obtain- 

 ing a light is an excellent subject wherewith to please 

 the public mind, and this book contains the reports of 

 three such lectures delivered to a juvenile auditory last 

 Christmas. An attempt has also been made to describe 

 the experimental portion of the lectures, and the author 

 has not committed the common error of giving a mul- 

 tiplicity of pretty but irrelevant experiments conveying 

 a paucity of information. In fact, in some parts the reverse 

 seems the case, for we must confess our inability to 

 discover why a consideration of the allotropic modifica- 

 tions of carbon should necessitate a detailed description 

 of the manufacture of black lead pencils. This digres- 

 sion, however, does not detract from the interest and 

 general merit of the work, which certainly contains the 

 explanation in simple language of some elementary 

 physical and chemical phenomena. 



Magnetism and Electricity. Part I. Magnetism. By 



Andrew Jamieson, M.I.C.E. (London; Griffin and 



Co., 1889.) 

 Although elementary text-books of physics continue to 

 increase in number, there is still room for one of such 

 general excellence as Prof. Jamieson's elementary manual. 

 The book is specially arranged for the use of first year 

 Science and Art Department and other electrical students. 

 Numerous questions and specimen answers are distributed 

 throughout the book, and though this may be rather 

 suggestive of cram, there is nothing in the text to justify 

 such a suggestion. It is unnecessary to go into details, 

 but it may be stated that the arrangement of subjects is 

 as good as it well can be, and on the whole the descrip- 

 tions are very clear. The numerous diagrams are also 

 excellent, those of the mariner's compass being especially 

 good ; indeed, the whole chapter on terrestrial magnetism 

 is the best elementary account of the subject which has 

 come under our notice. 



The subject is throughout considered as an essentially 

 practical one, and very clear instructions are given for 

 the making of magnets, and compass and dipping needles. 



If the succeeding parts of the book confirm the good 

 opinion created by the first, teachers of the subject are to 

 be congratulated on having such a thoroughly trustworthy 

 text-book at their disposal. 



Time and Tide : A Romance of the Moon. By Sir Robert 



S. Ball, LL.D., F.R.S. (London: Society for Promoting 



Christian Knowledge, 1889.) 



The ability of the author of this work to give a lucid 



exposition of nn abstruse subject is a matter of common 



knowledge ; and hence the fact that the book contains two 

 of his lectures delivered at the London Institution last 

 November is in itself sufficient commendation. However, 

 be this as it may, we have no hesitation in saying there 

 could hardly be a clearer explanation of Prof. George 

 Darwin's theory of tidal evolution than that contained in 

 the work before us. The hypothesis being accepted, every 

 feature of the past and future condition of our satellite 

 is described in a most comprehensive manner. It is first 

 shown how, when the earth was rotating on its axis with 

 an enormous velocity, the tidal action set up by the 

 sun caused a portion to become detached and form our 

 satellite. The employment of the term " conservation 

 of spin " facilitates considerably the demonstration of 

 the fact that as by tidal action the spin of the earth 

 decreases — as our day lengthens — so must the dimensions 

 of the moon's orbit be increased, and the length of the 

 month therefore become proportionally greater. The ap- 

 plication of Prof. Darwin's theory to other members of 

 our system is also inquired into ; and although the author 

 does not attempt to go back to the first stage in the 

 evolution of celestial species, he shows that tidal evolution 

 is an extension of the hypothesis that does so. Indeed, 

 the book is replete with information, and by the general 

 scientific reader will be found exceedingly interesting. 



LETTERS TO THE EDITOR. 



[ Tht Editor does not hold himself responsible for opinions ex - 

 pressed by his correspondents . Neither can he undertaki 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part 0/ NATURE, 

 No notice is taken of anonymous communications.^ 



Specific Inductive Capacity. 



Perhaps a better mode of performing the experiment quoted 

 by Mr. Rudge (p. 10) is to have two insulated parallel metal 

 plates, one connected with an electroscope, the other with a 

 slightly-charged Leyden-jar. On now interposing a thick slab 

 of paraffin or ebonite (recently passed through a flame) between 

 the plates, a very decided increase of divergence will be per- 

 ceived. Unless, indeed, the electroscope should happen to 

 have overflowed to earth during the charging of the jar, in which 

 case it will be opoositely charged and a decreased divergence 

 will be caused. To interpose the slab is, in fact, virtually to 

 diminish the distance between the plates, and its effect is there- 

 fore the same as that of pushing the plates closer together. 



The advantage of the Leyden-jar is that it keeps the potential 

 practically constant. If an isolated plate or sphere is used as 

 the charged body, the circumstances are not so simple, for the 

 insertion of the slab reduces the potential and slightly increases 

 the charge on the near face of the plate, so that, whether the 

 divergence of the leaves is increased or diminished depends on 

 several unimportant considerations, of which the size of the slab 

 may be one. A slab of area comparable to that of the plates 

 between which it is put would in this ca>e be the most suitable ; 

 and in any case it should be supported by a long insulator, so 

 that the operator's arm, as it approaches, shall not complicate 

 and mask the effect. Oliver J. Lodge. 



University College, Liverpool, November 9. 



" La Pietra Papale." 



Above Stresa, on the western bank of Lago Maggiore, there is 

 an enormous granite boulder, which deserves the attention of 

 geologists. It lies on the left slope of an old moraine, near the 

 little village of Gignese, and not far from the Hotel Alpino, at 

 an elevation of about 2500 feet above the sea-level. It is roughly 

 oblong in shape, and measures some 75 feet in length, and 

 perhaps half as much in breadth and thickness. The projected 

 mountain railway from Stresa to the summit of Monte Motterone 

 will pass close to the spot where it lies, and the masons are 

 already engaged in converting the smaller boulders into building- 

 stones. It is to be hoped, however, that la pietra papale^ 

 ns this splendid example of the carryinc; powers of ice i<> 



