RECENT PROGRESS IN CRYSTALLOGRAPHY 95 



investigation, as the radicle NH 4 was known to be capable of 

 replacing the alkali metal without apparently altering the 

 crystallographical form, and the result has proved highly 

 interesting. The particular salts chosen were the rhombic 

 normal sulphates and selenates, R 2 S0 4 and R 2 Se0 4 , in which R 

 may be K, Rb, Cs, or NH 4 ; and the monoclinic double sulphates 

 and selenates which the salts just mentioned form with the 

 sulphates and selenates of magnesium, zinc, iron, nickel, cobalt, 

 manganese, copper, and cadmium, and whose crystals contain 

 also six molecules of water of crystallisation, the molecular 



S 

 generic formula being R 2 M(c e 4 ) 2 . 6H 2 0. The inclusion of these 



double salts has proved a very happy one, for the alkali metal 

 has been found to exert such a preponderating influence in 

 determining the characters of the crystals, that the salts furnish 

 numerous independent confirmations of the crystallographical 

 deformation accompanying the displacement of any one alkali 

 metal by any other or by ammonium. 



A very large number of crops of the crystals of each of 

 the forty salts so far investigated have been prepared in order 

 to study every possible variety of habit. They were all of the 

 highest attainable chemical purity, and were grown under con- 

 ditions involving quite unusual precautions against mechanical 

 or thermal disturbance, in order to avoid distortion. Special 

 care has also been taken from the beginning to exclude crystals 

 showing any traces of the vicinal-face phenomenon recently 

 worked out by Prof. Miers, and which has long been observed 

 by the writer. At least ten perfect specimens were most care- 

 fully selected from different crops, and their interfacial angles 

 measured on a goniometer of the highest accuracy, over 20,000 

 different angles having been measured in all. The specific 

 gravity of the crystals of each salt has been determined both 

 by the pyknometer method and by the liquid immersion 

 method of Retgers, in order to ascertain the volume relation- 

 ships, internal structural dimensions, and molecular optical 

 constants. The position and dimensions of the optical ellipsoid 

 of the crystals of every salt have been ascertained by stauro- 

 scopic measurements, determinations of the refractive index for 

 six wave-lengths of light, and measurements of the optic axial 

 angles for similar spectrum intervals. The optical determina- 

 tions have, moreover, been repeated for temperatures higher 



