Chemistry and Physics. 281 



As the vapors producing the absorption appear to occur in the 

 more elevated regions of the solar atmosphere they must be 

 regarded as subject to a relatively low pressure. M. Perot 

 accordingly used as his comparison source a carbon arc under a 

 pressure between 22.5 and 30 millimeters of mercury. As the 

 result of his investigation he states that he was able to determine 

 that the difference sought lay between 2.2 and 1.6 parts in a mil- 

 lion, an interval which contains the Einstein number. — Bull. 8oc. 

 Fr. de Phys., 147, Dec, 1920. 



7. The Imagiyiary in Geometry; by J. L. S. Hatton. Pp. YI, 

 215. Cambridge, 1920 (Cambridge University Press).— The 

 author's purpose is to develop a generalized conception of geom- 

 etry and of space in which each of the three coordinates is 

 regarded as a complex quantity of the form x + i^', V + iy'^ 

 z + iz'. By adding to the axioms of real geometry, but employ- 

 ing its principles and methods, the well known theorems of plane 

 and projective geometry, of trigonometry, and of the conic and 

 conicoid are extended and generalized. It is a book for geometri- 

 cians only. F. E. B. 



8. The Principles of the Phase Theory; by Douglas A. Clib- 

 BENS. Pp. XX, 382, 198 figures. London, 1920 (Macmillan & 

 Co.). — This book on heterogeneous equilibria does not attempt to 

 give a general survey of the whole field. It deals exclusively 

 with systems which contain no vapor phase, the so-called "con- 

 densed systems," with the further limitations that only one 

 liquid phase may be present and no solid solutions. Moreover, the 

 practical illustrations considered are all cases of equilibrium 

 between water and salts, though the theoretical principles 

 involved are, of course, applicable to all condensed systems. 



Even with these limitations, the field to be covered is a large 

 one, but they make it possible for the author to deal quite briefly 

 with binary systems, and to devote the major part of the book to 

 the more complicated and less familiar systems of three, four and 

 five components. A final chapter deals with graphical methods 

 for determining, from the compositions of the phases present at 

 an invariant point, what reactions may occur there. 



The book is good both in plan and execution. There is no 

 conspicuous originality in treatment, but in a field so well devel- 

 oped it could hardly be expected. The author's style is clear, and 

 the systematic and exceptionally thorough way in which the 

 behavior of each different type of system of a given group is 

 discussed before taking up concrete cases, is worthy of special 

 mention. As the treatment is non-mathematical the book should 

 be suitable for the use of readers who are unfamiliar with the 

 subject. R. G. V. N. 



9. Lessons in Mechanics ; by William S. Franklin and 

 Barry MacNutt. Pp. XI, 221. Bethlehem, Pa., 1919 (Frank- 

 lin and Charles). — This and the two companion volumes to be 

 mentioned later have been prepared to meet the needs of the two 

 year schedule in elementary physics which has recently been 



