518 Notices of Memoirs — H. A. Miers — On Measuring Crystals. 



Seat are much thicker the nearer they are to that old volcano, as if 

 the ground had slowly sunk while they were being deposited. 



JSear Ldn the author was shown a small gia, said to have been 

 formed during an earthquake in February, 1885. The crack was 

 of a freshness corresponding to such a date, and was only a few 

 inches wide, and so short that it could not be determined whether it 

 extended beyond one bed of lava. It certainly was not an example 

 of the escape of liquid lava from below a crust, nor of a subsidence 

 over a steam cavity, and its chief interest in this connection is as 

 showing that at least three separate sets of causes are at work in 

 producing the gias of Iceland. 



YII. — On a New Method of Measuring Crystals, and its Appli- 

 cation TO THE Measurement of the Octahedron Angle of 

 Potash Alum and Ammonia Alum. By H. A. Miers, M.A., 

 F.G.S. 



THE two fundamental laws of Crystallography — namely (1) the 

 constancy of the angle in crystals of the same substance, and 

 (2) the law of simple rational indices — seem to be violated by those 

 crystals which are liable to irregular variations in their angles, or 

 those which have the simple faces replaced by complicated "vicinal" 

 planes. 



Both these anomalies are exhibited by potash and ammonia alum. 

 Brilliant and apparently perfect octahedra of these salts show large 

 variations in the octahedron angle ; other crystals show low vicinal 

 planes in place of the octahedron faces. 



If it be true, as is supposed, that the octahedron angle varies in 

 different crystals, it would be interesting to ascertain whether pro- 

 gressive variations can be traced during the growth of a single 

 crystal, and whether some or all of the octahedron faces change their 

 direction in space if the crystal be heild fixed during growth. 



In order to solve this problem a new goniometer has been con- 

 structed, in which the crystal is fixed at the lower end of a vertical 

 axis, so that it can be immersed in a liquid during measurement. 



This device is in reality an inversion of the ordinary goniometer 

 with horizontal disc ; the liquid is contained in a rectangular glass 

 trough with parallel-plate sides ; one side is placed rigidly per- 

 pendicular to the fixed collimator, and the other is perpendicular to 

 the telescope, which is set at 90° to the collimator. The trough is 

 supported on a table which can be raised and lowered, so that the 

 crystal can be placed at any required depth in the liquid. If the 

 liquid used be its own concentrated solution the crystal can be 

 measured during growth, and the changes of angle, if any, can be 

 observed at different stages. 



In order that it may be held rigidly, the crystal is mounted, when 

 small, in a platinum clip, which it envelops as it grows larger. 



The results derived from the measurement of a large number of 

 alum crystals are as follow : — 



(1) The faces of the regular octahedron are never developed upon 

 alum growing from aqueous solution. 



