Chemistry and Physics. 71 



colder and heavier air, the same conditions arise as in the case 

 of the wind blowing over a surface of water. Waves are formed 

 on the surface of the under layer of air, which are perpendicular 

 in direction to the wind and which follow each other at regular 

 distances. These air waves can have very great dimensions. 

 The same wind which would produce waves on water a meter long, 

 could produce in layers of air which differ in temperature 10°, 

 waves two to five kilometers in length. Sea waves five to ten 

 meters in length would be represented by air waves fifteen to 

 thirty kilometers in length. R. Emden describes a journey in a 

 balloon from Munich — in November, 1896 — and in the course of 

 it he saw below him stretched over Munich great layers of clouds, 

 which looked like giant sausages. Fiiteen of these rolls or layers 

 were counted, and they ran at right angles to the direction of the 

 wind. The author discusses Helmholtz's paper, "On Atmos- 

 pheric Movements" (Gesammette Abhandlungen, iii, p. 309), and 

 believes that the phenomenon he witnessed is exactly such as 

 Avould result from Helmholtz's theoretical deductions. — Wied. 

 Ann., No. 10, 1897, pp. 374-378. j. t. 



10. Light, visible and invisible: A series of lectures delivered 

 at the Royal Institution of Great Britain at Christmas, 1896; by 

 Silvanus P. Thompson. 294 pp. New York and London, 1897 

 (The Macmillan Company). — The special conditions under which 

 the lectures here presented to the general public were delivered 

 has determined the form and method of this volume by Professor 

 Thompson. The general reader is the gainer, because of the sim- 

 plicity and directness of the style adopted and the liberality in 

 the introduction of illustrations. The volume occupies a some- 

 what new field, since it not only reviews the phenomena of light 

 in the narrow sense, with a discussion of the invisible spectrum, 

 but treats also of the long electric waves discovered by Hertz 

 and further of the discoveries of Rontgen. An interesting chap- 

 ter is devoted to the work of Hertz and his followers and an 

 appendix follows on the electrical theory of light. The last lec- 

 ture is devoted to the production and phenomena of the X-rays of 

 Rontgen, which the author ventures to call " Rontgen light." 

 This subject is clearly presented with numerous illustrations of 

 a familiar kind. The appendix contains a summary of other 

 kinds of "invisible light." 



In the strictly optical part of the work the author develops the 

 subjects of reflection, refraction, etc. from the wave theory, and 

 in the appendix following shows how the essential formulas for 

 lenses can be deduced by this method (see Phil. Mag., Oct., 1889). 



11. Bibliography of X-ray Literature and 7?esearcA(1896-1897). 

 Being a ready reference index to the literature on the subject of 

 Rontgen or X-rays. Edited by Charles E. S. Phillips. With 

 an Historical Retrospect and a chapter, " Practical Hints," by the 

 editor. Pp. xxxii and 68. New York, 1897. (The Electrician 

 Company, London ; D. Van Nostrand, New York.) — The litera- 

 ture of the X-rays has increased so rapidly since the publication 



