432 Scientific Intelligence. 



very much lighter than hydrogen. — Abstract of paper communi- 

 cated to this Journal by the Author. 



6. Electricity and Magnetism ; a Mathematical Treatise for 

 advanced undergraduate students; by Francis E. Nipiier, A.M. 

 2d ed., revised with additions, 8vo, pp. xii, 430. St. Louis, 1898. 

 (J. L. Boland Book and Stationery Co.) — Professor Nipher has 

 succeeded in producing a text-book admirably adapted for higher 

 class room work and we are not surprised that a new edition 

 should so soon be called for. lie tells us that he has made an 

 attempt in it " to avoid wasting the time of the reader over 

 puzzles and obscurities which are made difficult and called easy." 

 In this edition, he gives in the preface reasons for the treatment 

 he has adopted and says: "It is for such reasons that the author 

 has determined not to be guided by those of his reviewers who 

 seem to think that the time of the student should be spent in 

 rapidly acquiring a set of rules by means of which electrical 

 machinery may be designed. The great advances in engineering 

 have been made by those who applied their brains to useless 

 things and made them useful. This is a sufficient reason why 

 the engineer should be first of all a student." g. f. b. 



7. Electrical currents excited by JRontgen rays. — It has been 

 shown by various observers that these rays can dissipate electri- 

 cal charges. A. Winkelmann shows that they can also charge 

 bodies. He states that J. Perrin (Comptes Rendus, cxxiv, p. 

 496, 1897) has also proved this independently and has arrived at 

 practically the same result. Winkelmann, however, goes on more 

 exhaustively to show that the air between the source of the 

 X-rays and the charged body is broken up into ions ; and he 

 measures the ohmic resistance of different layers of air thus ion- 

 ized. The method of studying the resulting charges consisted in 

 charging a condenser by the rays and in discharging this through 

 a ballistic galvanometer. The sensitiveness of the galvanometer 

 was such that the discharge of one microfarad charged with 

 0*032 volt gave a deflection of 2 cm . An electrometer was also 

 used. It was shown that the electrical charges on plates of dif- 

 ferent metals were not produced by the direct effect of the X-rays 

 but were the result of the ionizing of the intervening air ; for the 

 plates could not be charged when they were coated with a layer 

 of varnish which was permeable to the X-rays. If we adopt the 

 hypothesis that the charging is due to the ions conveying charges, 

 we find that the changes in resistance of the intervening air is 

 explained : for this resistance depends on the number of ions in 

 the unit of volume. This resistance was found to depend on the 

 intensity of the Rontgen rays ; on the number of breaks of the 

 induction apparatus per second, and between plates near together, 

 on the resistance in the current circuit. The specific resistance of 

 air can thus vary greatly under the influence of the rays. The 

 proportion of ionized molecules to the whole number of molecules 

 in the unit of volume was found to be 4*6 X 10' 1 3 . This is the 

 same order of magnitude as the result obtained for hydrogen by- 

 Prof. J. J. Thomson. — Wied, Ann., No. 9, pp. 1-28, 1898. j. t. 



