Chemistry and Physics. 141 



12. The duration oj Electrical Shadoios with solid and fluid 

 znsidators. — Electrical shadows produced by the brush discharge 

 of the conductor of a Holtz machine were first noticed by Professor 

 A. W. Wright, who used as the shadow-giving bodies the 

 finger, wire gratings, paper and cork. Holtz has also worked 

 upon this subject, and used for one electrode a metallic point, 

 and for the other a circular metal plate which was covered with 

 a thin layer of silk. The point can be made positive or negative. 

 One can consider the space between both electrodes as a field of 

 electric force, the force lines of which are made visible by the 

 brush discharge. The electric force in this field is not of the 

 same strength, being greatest along the shorter line between the 

 point and the plate. When one brings a dielectric into this field, 

 an electric polarization occurs between the smallest particles, or 

 according to Maxwell's theory, a displacement current arises. 

 The work expended in causing these polarization or displacement 

 efiects produces the shadows. The duration of the electric 

 shadows measures the time of the setting up of the polarization 

 effects or the duration of the displacement currents in the 

 dielectric. Quincke has measured this element of time with a 

 large number of dielectrics. There does not seem to be a simple 

 relation between the duration of the electric shadows and the 

 dielectric constants. — Sitzungsberichte der Academie der Wissen- 

 schaften, Berlin, May 30, 1895. j. t. 



13. A Laboratory Course of Experimental Physics ; by W. J. 

 Loudon and J. C. McLennan. 8vo, 302 pp. New York and 

 London, 1895 (Macmillan and Co.). — The present volume is the 

 result of the experience of the authors at the University of 

 Toronto in their efforts to provide a suitable course in experi- 

 mental physics for a large college class. The difficulties that they 

 have met with and try here to overcome are such as many teachers 

 encounter elsewhere and the latter cannot fail to be benefited 

 by their work. The volume is divided into two parts. The first 

 is elementary and gives such experiments as can be performed by 

 beginners, and which require only a knowledge of elementary 

 mathematics. The second part is for advanced students and in- 

 volves a ready familiarity with the use of the Calculus. In the 

 first portion are given the experiments dealing with the depart- 

 ments of hydrostatics, gases and light, while those belonging to 

 acoustics, heat and electricity are introduced in the second part. 

 Some teachers may find it advantageous to deviate from this rather 

 arbitrary order of subjects. The work as a whole has been very 

 well carried through and is a valuable addition to the available 

 text-books in this department. 



14. Terrestrial Magnetism. An international quarterly journal 

 published under the auspices of the Ryerson Physical Laboratory, 

 A. A. Michelson, Director. Edited by L. A. Bauer. Yol. I, No. 

 1. Chicago (The University of Chicago Press. Two dollars a year). 

 — This new journal is to be devoted exclusively to terrestrial 

 magnetism and its allied subjects, as earth currents, auroras, 



