Chemistry and Physics. 381 



strongly cooled plate is placed just above the liquid each mole- 

 cule as it reaches the plate will be condensed to a solid before 

 it has the opportunity of meeting the other molecules and be 

 returned to the liquid. A separation will thus be effected 

 between the slower and the faster moving molecules. Another 

 method proposed was to allow the molecular flow to take place 

 through a small opening. In this case the lighter and more 

 rapidly moving molecule should hit the opening more often than 

 the heavier one and if it is prevented from returning by trapping- 

 it on a cooled plate a separation between the components of the 

 vapor should be secured in this way. 



Both the evaporation method and the effusion method were 

 successfully tried by these authors who found that the quantities 

 separated were inversely proportional to the square roots of the 

 molecular weights of the isotopes as determined by Aston. — Phil. 

 Mag. 43, 31, 1922. p. e. b. 



6. Neivcomb-Eiigelmann Populare Astronomie; Sixth Edi- 

 tion edited by H. Ludexdorff. Pp. XII, 889. Leipzig, 1921 

 ( AYilhehn Engelmann) . — The favor with which this work by real 

 authorities has been received may be judged by the increasing 

 rapidity with which the successive editions have been exhausted. 

 First appearing in 1881 as a translation of the Popular Astron- 

 omy of Simon Xewcomb, the later issues have preserved little 

 resemblance to the original work, so numerous and extensive have 

 been the changes. The editors have however always sought to 

 preserve the historical treatment which characterized the 

 author's treatise. 



Our knowledge of the structure of the universe and stellar 

 activity has so widened since the appearance of the last edition 

 in 1914 that the three chapters on Stellar Astronomy forming 

 Part IV have been entirely rewritten, and two new sections on 

 the Development of Mechanics since Newton and on the Principle 

 of Relativity have been added to Part I. In addition all the 

 material has been revised in the light of the newest research and 

 the numerical data have been corrected from the most trust- 

 worthy sources. Many sections are the work of specialists as, for 

 example, those on Meteors, the Physical Activity of the Stars, and 

 Star Clusters and Xebulae, by Eberharcl; Stellar Parallax, 

 Proper Motion, Double Stars, and Variables, by Luclendorff ; The 

 Fundamental Laws of Mechanics, the Three-Body Problem, The 

 Sun, and Xovae. by Freundlich; the Planets, the Structure of 

 the Universe and Cosmogony, by Kohlschiitter. The unavoidable 

 increase in the size of the book has tempted the editor to omit the 

 appendix devoted to biographical sketches but in response to the 

 desire of his colleagues they have been retained. 



The title "popular' 7 connotes little more than "non-mathe- 

 matical. ' ' Outside of that it is a serious treatise for the student, 



