MINERALOGY. 273 



modern chemistry. An important addition to the literature 

 of the science is Mr. G. W. Hawes's volume on the "Mineral- 

 ogy and Lithology of New Hampshire," which forms a part 

 of Professor Hitchcock's "Survey of the State." 



RESEARCHES IN PHYSICAL MINERALOGY AND CRYSTAL- 

 LOGRAPHY. 



M. Dufet has made a series of experiments in regard to the 

 indices of refraction in mixtures of isomorphous salts, and 

 has arrived at the conclusion that the differences between 

 the indices of a mixture of two isomorphous salts and those 

 of the component salts themselves is in an inverse ratio to 

 the number of equivalents of the two salts which enter into 

 the mixture. Although obtained with artificial compounds, 

 the results have an important bearing on the many series of 

 isomorphous minerals. 



The effect of increase of temperature on the indices of re- 

 fraction of the natural sulphates of barium, strontium, and 

 lead (barite, celestite and anglesite) has been investigated 

 with great accuracy by Arzrnni. He concludes that an in- 

 crease of temperature diminishes the three indices in each of 

 the species named, but to an unlike degree, though in an an- 

 alogous manner. He finds also that the directions of great- 

 est, mean, and least expansion bear no relation to the three 

 axes of optical elasticity. 



Sorby has published additional explanations of the meth- 

 od of determining the indices of refraction in thin sections, 

 mentioned in the last volume of the Annual Record. The 

 principle involved is indeed not original with him, but he has 

 given it a new and more extended application. It promises 

 to be of very considerable practical value. Another method 

 of obtaining the indices of refraction has been redescribed 

 and elaborated by Kohlrausch, which is of easy application 

 and gives quite accurate results. It is based upon the de- 

 termination of the angle of total reflection of the given sub- 

 stance when observed in a medium as carbon disnlphide 

 which has a higher index. Determinations made by him on 

 a variety of substances, both isotrope and optically uniaxial 

 and biaxial, gave most satisfactory results, 



The papers of Professor J. P. Cooke upon the haloid salts 

 of antimony contain some facts which have an important 



M2 



