Chemistry and Physics. 319 



conductor and the charge escapes through it. The leakage through 

 non-conductors is due to a kind of electrolysis, the molecules 

 being split up by the rays. Professor Thomson thinks that the 

 rays are not all of the same kind. Crookes tubes generally improve 

 for a time after they are sealed off from the air pump and then 

 begin to deteriorate. Mr. A. A. C. Swlnton describes a simple 

 apparatus for determining whether a Crookes-tube is efficient. 

 It consists of an opaque pasteboard box with a simple aperture 

 at one end to which the eye is applied. The other end is provided 

 with an opaque diaphragm of double black paper, upon the inner 

 side of which is laid a piece of blotting paper impregnated with 

 platino-cyanide of barium in a crystalline state. Shadows of 

 objects held against the diaphragm are clearly seen. Mr. S win- 

 ton has thus seen the bones in the thick portion of his hand. 

 This apparatus is similar to that described by Professor Salvioni 

 of Perugia. — Nature, Feb. 27, 1896. j. t. 



13. JEhperiments with the Montgen rays. — Doelter has re- 

 cently called attention to the fact that the X-rays of Rontgen 

 may be used as a means of distinguishing between different gems, 

 as also between them and their imitations. A true diamond may 

 thus be readily told from a false one. While rock crystal, topaz, 

 ^trass and spinel are opaque to the rays, the varieties of corun- 

 dum (ruby, sapphire) transmit them to some extent. The usually 

 accepted relation between the opacity of the substance to the 

 X-rays and its density finds some exceptions, accordiug to 

 Doelter, among minerals. He finds sulphur (G. = 2) and realgar 

 (G. = 3*4), quartz (G. = 2 -6), fluorite (G. = 3*1), quite opaque; 

 corundum (G. = 4*1), as stated above, transmits the rays somewhat, 

 while diamond (G. = 3'5) and graphite (G. = 2) transmit much 

 better. Most silicates are opaque ; this is true of mica in layers 

 of l mm in thickness. In sections of £ mm it is possible to dis- 

 tinguish between the transmissive powers of amphibole, garnet, 

 quartz, mica, feldspar. — Private publication, Gratz, Feb., 1896. 



14. The Elements of Physics ; A College Text-book, by E. L. 

 Nichols and W. S. Franklin, vol. i., 228 pp., 8vo. New York, 

 1896 (Macmillan & Co.). — This volume forms the first of a series 

 of three in which the authors propose to develop the fundamental 

 principle of Physics ; it embraces the subjects of Mechanics and 

 Heat, while the other two volumes are to be devoted to Electric- 

 ity and Magnetism, and Sound and Light respectively. The 

 scope of the work and the method of treatment differ from those 

 of many existing text-books inasmuch, as the authors write for a 

 somewhat more advanced grade of students than those which 

 form the mass of many college classes. It has thus been possible 

 lor them to assume a knowledge of the Calculus and in general to 

 treat the topics more concisely and from the mathematical side; 

 long descriptions of the applications of the principles are thus 

 avoided. For those students for whom the work is prepared, 

 who desire to make a serious study of Physics, this excellent new 

 text-book cannot fail to be of much value. 



