584 PETROLOGICAL ABSTRACTS AND REVIEWS 



that the elastic properties of oriented sections are not discussed, as they 

 would doubtless form an interesting addition to the subject. 



The author's studies of crossed axial dispersion are presented in an 

 extremely interesting chapter. A plate shows the crossing of the optic 

 axes due to change in wave-length, the series of figures for two tempera- 

 tures being given. Although it is not pointed out by the author, these 

 figures show that for these salts the change in position of the optic axes 

 for monochromatic light, with increasing temperature, is in the same direc- 

 tion as the change in position due to decrease in wave-length at constant 

 temperature. 



The author's conclusions are concisely summarized in the next to the 

 last chapter. The new term eu tropic series is defined as a series "in 

 which the small angular differences and also the physical properties of the 

 crystals obey the law of progression according to the atomic weight of the 

 interchangeable elements which give rise to the series and belong to the 

 same family group of the periodic classification." Thus the thallium and 

 ammonium salts belong to the above isomorphous series, but are excluded 

 from the eutropic series to which the potassium, caesium, and rubidium 

 salts belong. 



Because of the close similarity between the ammonium and rubidium 

 salts the author concludes that there is unoccupied space within the lattice- 

 work of the crystal to allow for the eight additional atoms when the 

 ammonium radical is substituted for rubidium. This may not be con- 

 sidered as proven, as it rests on the tacit assumption that the dimensions 

 of the atoms, or their spheres of influence are the same, an assumption 

 which seems hardly warranted, especially in view of Pope and Barlow's 

 interesting explanations of the same phenomenon. 



In the concluding chapter a comparison is drawn of the present status 

 of crystallography with that of twenty years ago when the author began 

 his work as a? pioneer in the field of the more refined measurements of 

 crystallographic characters. He points out that the measurements recorded 

 are now on a plane of accuracy with atomic-weight determinations, whereas 

 formerly gross errors entered into a large part of the work. Bare mention 

 is made of the work of Pope and Barlow and their valence theory. 



The book is well worth the attention of everyone interested in crystal- 

 lography whether from the chemical or physical side, and leads one to 

 look forward to the author's more general treatise which is to appear 

 shortly. 



Albert D. Brokaw 



