Chemistry and Physics. 435 



10. Penetration of thin metallic screens by Phosphorescent rays. 

 — Lenard, at a recent meeting of the Berlin Academy, described 

 some experiments upon this subject. Rays from an aluminum 

 cathode disc were projected on a thin aluminum window , 003 mm 

 thick. The cathode rays passed through the window and made 

 the air faintly luminous. There was a strong smell of ozone. 

 Other gases, besides air were tried. . Oxygen and carbonic 

 acid were less penetrable than air. " One may say that hy- 

 drogen molecules cause less turbidity in the ether than those of 

 oxygen and the latter less than those of carbonic acid." — Nature, 

 March 30, 1893. J. T. 



11. Representation of Equipotential lines due to a current flow- 

 ing through a metallic plate. — E. Lommel, in a preliminary 

 communication, has described a method of showing these equipo- 

 tential lines. He now publishes photographs of these lines and 

 gives an explanation of Hall's phenomenon by means of these 

 photographs. A current of twenty amperes is passed through a 

 thin copper plate and magnetic filings are distributed on the plate. 

 These filings distribute themselves in the direction of the equi- 

 potential lines. If 2s represents the potential difference between 

 two points on the copper plate, M the strength of the magnetic 

 field perpendicular to the direction of the current in the plate, 

 (Hall's method), we have 2s =KJM, where K is a species of 

 friction coefficient which depends upon the material of the plate 

 employed. J is the strength of current flowing through the 

 plate. The resistance of the plate K is proportional to its section 

 and also to its thickness d. Calling K a constant and r the resist- 

 tance of the galvanometer, we have 



TT IM 



ro 

 which expresses the law, Lommel believes, of the Hall effect. — 

 Ann. der Phyisk und Chemie, No. 3, 1893, pp. 462-466. J. t. 



12. Alternating Currents: An Analytical and Graphical 

 Treatment for Students and Engineers • by Frederick Bedell 

 and Albert Gushing Crehore. 325 pp. 8vo. New York, 1893. 

 (The W. J. Johnston Co.). — The subject of alternating currents is 

 one which has come into great prominence in electrical engineer- 

 ing in the present time, and this fact gives especial value to a 

 work like the one in hand, in which the whole matter is perhaps 

 for the first time presented systematically and fully. The volume 

 is divided into two parts ; the first. of which gives after some in- 

 troductory statements, the solution of problems for the various 

 cases supposed. These include circuits with resistance and self- 

 induction, with resistance and capacity, with resistance, self- 

 induction, and capacity ; the general solution of this last case is 

 followed by a discussion of the various cases, (l) of discharge, 

 (2) of charge, (3) for harmonic electromotive force, and (4) for 

 any periodic electromotive force. The latter half of the work 

 contains the graphic treatment of a great variety of cases, which 

 the fulness of illustrations makes very clear. The work is a very 



