PETROLOGICAL ABSTRACTS AND REVIEWS 583 



After an instructive introduction to the study of isomorphous series tlie 

 author proceeds to describe improved apparatus that he found it necessary 

 to devise before his measurements could be made with the required precision. 

 Perhaps the most important of these is a cutting and grinding goniometer. 

 By means of this instrument it is possible to cut truly plane surfaces, 

 oriented with almost mathematical accuracy. Either parallel-faced sec- 

 tions or 60° prisms may be cut by changing appliances. The instrument 

 and its operation are described in detail. 



A spectroscopic monochromatic illuminator for use in the study of the 

 optical properties is substituted for the usual Na-light, K-light, Li-light, 

 etc., with very satisfactory results. It involves little that is new in principle, 

 though considerable ingenuity is expressed in the mechanical details. In a 

 dilatometer for measuring the expansion of variously oriented sections, and 

 in an elasmometer for the study of the elastic properties the well-known 

 interferometer method of measuring minute distances is employed wifh 

 gratifying results. 



After describing his apparatus the author takes up a discussion of his 



S 

 own studies of two isomorphous series, namely the orthorhombic Rj^^ O4 



series and the monoclinic R2M1 O4J • 6H2O series, in which R may be 



potassium, caesium, rubidium, ammonium, or thallium, and M may be 

 any one of a number of bivalent metals, though discussion is limited chiefly 

 to the zinc and magnesium compounds. 



The members of each series were submitted to a comparative study in 

 regard to their external form and various vector properties, as refractive 

 index, birefringence, and expansion coefficients. A number of these 

 properties were shown to vary progressively when a heavier metal is sub- 

 stituted for a lighter, but apparently these variations do not bear any 

 simple mathematical relation to the atomic weights. A marked divergence 

 of the ammonium and thallium salts from the rest of the series is explained 

 on the basis of their chemical differences from the true alkali metals. 

 Similar progression, with a divergence in the cases of thallium and ammo- 

 nium, is noted in the solubilities, but unfortunately the absolute solubility, 

 in grams per liter, is discussed instead of the more significant molar solu- 

 bility. 



In the orthorhombic series the axes of the ellipsoid of thermal expansion 

 were foilnd to be coincident with the axes of the optic ellipsoid and in the 

 monoclinic series one of the axes of the thermal ellipsoid coincides with 

 the axis of symmetry, as might have been anticipated. It is unfortunate 



