624 



SCIENCE 



[N. S. Vol. XXXI. No. 799 



were prepared under varying conditions and 

 tested by chemical and spectroscopic methods 

 with utmost refinement as to purity. Not less 

 than 10 crystals of each were measured for 

 crystallographic form and constants and about 

 twelve orientated sections or prisms of each 

 were measured at various temperatures and 

 for six wave-lengths of light in determining 

 the indices of refraction. Determinations of 

 the volume of each salt were also made with 

 great exactness, and the solubility of each in 

 water established. The enormous amount of 

 exacting labor represented by these researches 

 will be abundantly clear to any one who has 

 made such a study of even a single substance. 



In chapter X. are presented the results ob- 

 tained in goniometrical examination of both 

 the normal sulphates and selenates, or ortho- 

 rhombic series of crystals and of the double 

 sulphates and selenates or monoclinic series. 



Chapter XI. treats of the volume relation- 

 ships of the simple and double sulphates and 

 selenates and the conception of molecular dis- 

 tance ratios or topical axes. 



Chapter XII. presents the optical relation- 

 ships of the two series of sulphates. 



Chapter XIII. is devoted to an explanation 

 of the phenomenon of crossed-axial-plane dis- 

 persion of the optic axes which is shown to be 

 " due to very low double refraction, combined 

 with close approximation of the intermediate 

 index of refraction to one of the extreme in- 

 dices; and to the fact that change of wave- 

 length of light or change of temperature, or 

 both, cause the intermediate index to approach 

 still nearer to the extreme one in question 

 until it becomes identical with it, and eventu- 

 ally to pass it, the relative positions of the..,, 

 two thus becoming reversed." The experi- 

 mental evidence leading to this explanation is 

 beautifully clear. 



In chapter XIV. the results of the thermal 

 investigation of the sulphates is presented, and 

 in chapter XV. a summing up of the chief 

 results of the investigations. This suiomary 

 seems of sufficient interest to justify some- 

 what extensive quotation, as it appears to be 

 the clearest statement yet given of just what 

 the modern concept of isomorphism involves. 



The crystals of the dififerent members of the 

 rhombic series of isomorphous sulphates and 

 selenates of the alkalies. 



<L>.. 





and those also of the monoclinic series of double 

 sulphates aad selenates. 



E:,M 



(seoj, 



.6H,0, 



while conforming to the same symmetry— that of 

 their particular isomorphous series — and exhibit- 

 ing the same facial forms inclined at angles which 

 never differ by more than one or two degrees, 

 exhibit progressive variations in their exterior 

 geometrical configurations, interfacial angles and 

 crystallographic elements, in their internal struc- 

 tural properties and constants of which the ex- 

 ternal form is only the outward symbol, in their 

 optical characters and in their thermal behavior; 

 and these variations follow the order of progres- 

 sion of the atomic weights of the three alkali 

 metals belonging exclusively to the same family 

 group of the periodic classification, potassium, 

 rubidiiun, caesium, which by their interchange 

 give rise to the series. The variations are, there- 

 fore, functions of the atomic weight of the alkali 

 metal. . . . Similar variations attend the replace- 

 ment of sulphur by selenium in the acid radicle 

 present in the salts. 



The thallium and ammonium salts of the two 

 series exhibit properties fully entitling them to 

 inclusion in these respective series of isomorphous 

 salts, understanding by the term " isomorphous 

 series " a series, the members of which bear a 

 definite chemical analogy, crystallize with like 

 symmetry and develop forms the interfacial angles 

 of which differ only by an amount which has not 

 yet been observed to exceed 2§ degrees. The 

 more exclusive " eutropic series " within each of 

 these isomorphous series, that is to say, the series 

 in which the members exhibit the progression of 

 the whole of the crystal properties according to 

 the atomic weight of the interchangeable chemical 

 elements, comprises solely the salts of the alkali 

 metals K, Rb and Cs which belong strictly to the 

 same family group of the periodic classification, 

 the thallium and ammonium salts being excluded 

 by their essentially different chemical nature and 

 their difi'erent crystallographic properties which 

 follow therefrom. 



Finally, a third general conclusion is drawn : 



