Geology and Mineralogy. 331 



escent image and a photograph can be taken as far as wave-length 

 A = 950. Photographs were taken both by the prism method and 

 by Rowland's concave grating. — Ann. der Physik unci Ohemie, 

 No. 8, 1890, p. 681. j. t. 



5. Photography of Oscillating Electric Sparks. — This has 

 been accomplished hitherto by a revolving mirror or a revolving 

 lens which spreads out the image of the spark. Professor Boys 

 employs six lenses placed on a brass disc, on different radii. The 

 disc revolves at a high rate of speed. The spark is formed on 

 one side of the disc and a photographic plate is placed upon the 

 other side. — Phil. May., Sept. 1870, pp. 248-260. j. t. 



6. Electrical Discharges in Magnetic Fields. — The experi- 

 ments of M. A. Witz upon the action of magnets on electrical 

 discharges in Geissler tubes leads him to believe that this action 

 is due to a variation in the capacity of the tubes. They become 

 true condensers and their illumination is the result of an oscilla- 

 tory discharge of the same order as that of a Ley den Jar, of 

 which the period T is a function of the capacity C of the jar, and 

 of the coefficient L of self-induction of the conductor of small 

 resistance, Tr=7r<\/CL. A variation of the capacity C would thus 

 modify the vibratory state of the gas and would be the cause of 

 the differences observed in the luminous phenomena in intense 

 magnetic fields. — Nature, Aug. 14, 1890, p. 384. j. t. 



7. Molecular Theory of Induced Magnetism, — Professor Ew- 

 tsg states in a summary to an article on this subject his convic- 

 tion of the truth of the molecular theory of induced magnetism, 

 which is Weber's theory in a modified form. Perhaps the most 

 important conclusion drawn is this, "That magnetic hysteresis 

 and the dissipation of energy which hysteresis involves are due to 

 molecular instability resulting from the intermolecular magnetic 

 actions, and are not due to anything in the nature of lrictional 

 resistance to the rotation of the molecular magnets." — Phil. Mag., 

 Sept. 1890, pp. 205-222. j. x. 



8. Note. — Error in Maxwell, vol. ii, § 544. Corrected by a 

 second fault so that final result is correct. — Nature, 35, p. 172, 

 1886. 



9. The Elements of Laboratory Work. A course of natural 

 science; by A. G. Earl, M.A., F.C.S. 177 pp. London, 1890 

 (Longmans, Green & Co.). — This is an introduction to work in 

 the physical laboratory, dealing with the fundamental phe- 

 nomena and laws. These are taken up in such a manner and 

 order of sequence as to lead the student to a knowledge of the 

 principles involved rather than the details of manipulation. 

 Some of the chapters consider the measurement of quantity of 

 matter, observations of change of position, of changes of tempera- 

 ture, of natural changes exhibited by all kinds of matter and by 

 certain kinds of matter — the former bringing in the idea of 

 gravitation, etc., the latter of electric and magnetic stress. Fur- 

 ther, an investigation of various kinds of matter, observations 

 leading to the theory of the ether, etc. 



