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SCIENCE PROGRESS 



own specific interfacial angles, constant for that salt to within 

 2' of arc, and which constitute a definite property by which the 

 salt can be identified. The amount of the difference introduced 

 in these angles by replacing one element of the alkali group 

 by another varies in the two series, being smaller in the 

 rhombic simple sulphates and selenates than in the monoclinic 

 double salt series. The maximum difference in the former 

 series, between a potassium and a caesium salt, is only 41', 

 whilst in the latter series of lower symmetry it is as much as 

 2 21'. These maxima, however, are but rarely attained, and 

 the average change is only a quarter of a degree in the rhombic 

 series, and does not exceed a degree in the monoclinic series. 



Fig. 7. 



With respect to the relation between the amount of the 

 change of angle and the atomic weights of the metals inter- 

 changed, three very interesting facts have been revealed : — 

 (1) The magnitude of the angle between any pair of faces of 

 any rubidium salt is intermediate between the values of the 

 corresponding angles on the analogous potassium and caesium 

 salts. (2) In both series the average of all the angular changes, 

 and likewise the maximum amount of change, whenever potas- 

 sium is replaced by caesium is almost exactly twice as much as 

 when potassium is replaced by rubidium ; in other words, both 

 the average and maximum change of angle are directly pro- 

 portional to the change of atomic weight. (3) In the monoclinic 

 series, where the three axes of the crystals are not all fixed 

 at right angles to each other as in the rhombic series, but in 



