MINERALOGY. 6G7 



increase in volume which it causes when introduced into a liquid con- 

 tained in a calibrated cylinder. The author suggests an improvement 

 in the latter part of the process, and gives the results of numerous 

 determinations of the specific gravity of quartz fragments. The numbers 

 obtained vary rather widely in the second decimal place, which the 

 author concludes is due to impurities and inclosures in the specimens 

 taken; which, however, would hardly seem possible if the material was 

 selected with any care. For the mechanical separation of mineral mixt- 

 ures the author regards the solution of D. Klein (see report for 1882), 

 the borotungstate of cadmium, as being the best means, preferable to the 

 Sonstadt solution — that is, potassium-mercury iodide. 



Kohrbach ( Wiedemami's Annalen, xx, 169) has suggested as a substi- 

 tute for the Sonstadt solution a solution of barium-mercury iodide. 

 It is obtained by mixing together thoroughly in a flask 100 parts barium 

 iodide and 130 parts of mercury iodide, then adding 20 cc. of distilled 

 water, and heating to 150° or 200° over an oil-bath. The solution is 

 agitated constantly, and when complete it is evaporated down in a 

 water-bath, and then on cooling a solution in which topaz floats is 

 obtained. The specific gravity of the solution is from 3.575 to 3.588. 

 It is less easy to use than the Sonstadt solution, being very hygroscopic, 

 and besides it is decomposed on the addition of water, crystals of red 

 mercury iodide separating out. It can only be diluted consequently by 

 the addition of a dilute solution specially prepared. Biittgeubach has 

 proposed a method by which different minerals may be separated from 

 their mixtures on a large scale by taking advantage of their different 

 degrees of cohesion. If, for example, two minerals of different brittle- 

 ness are thrown against a solid surface, one is broken up into smaller 

 fragments than the other. Thus when sphalerite and pyrite were thrown 

 several times against a cylinder and then sifted, it was found that the 

 larger sieves collected the fragments of pyrite, while the particles of 

 sphalerite, being finer, passed through and were collected on others. 

 This is only useful as a metallurgical process. 



In the subject of heat a memoir by Fletcher (Phil. Mag., October, 

 November, December, 1S83) must be mentioned, which discusses from 

 a mathematical standpoint the expansion of crystals upon change of tem- 

 perature. This memoir forms a continuation of an early discussion of 

 the subject by the same author, in which a series of propositions are 

 established by mathematical reasoning, which result from the assumption 

 that the geometrical and physical character of a crystal are the same 

 along all lines having the same direction. Since the publication of the 

 first paper an extensive series of measurements has been made by Beck- 

 enkamp with a view to determining the degree of permanency of the 

 thermic axes (as defined by Neumann), and this second paper is largely 

 devoted to a discussion of the bearing of Beckenkamp's measurements 

 and their mathematical explanation. M. Dufet (Bull. *S'oc. Min. France, 

 VI, 75) has given a discussion of the variation in the indices of refrac- 



