Notices of Memoirs — Professor H. A. Miers on Cri/stals. 267 



between crystal and solution which prevail there. It is believed 

 by the author that a study of the vicinal planes and of the liquid 

 in contact with them, may lead to some understanding of these 

 conditions. 



In order to ascertain the composition of the liquid, attempts were 

 made to determine its refractive index by means of total reflection 

 within the crystal. This appears, indeed, to be the only method 

 which can give direct information concerning the ultimate layer in 

 contact with the acrowing: face, and it is somewhat remarkable that it 

 has not been applied before. Considerable difficulty was experienced 

 in making this measurement, but ultimately good readings were 

 obtained which gave the value 1-34:428 as the refractive index in 

 sodium light, at 19° C, of the liquid in contact with a growing 

 crystal of alum. The refractive indices of a series of solutions of 

 known strength, ranging from dilute to supersaturated, having been 

 previously measured, the above index was found to correspond to 

 a liquid containing about 10*80 grammes of alum in 100 grammes 

 of solution. A saturated solution at 19° C. was found to have the 

 refractive index 1-34:232, and to contain about 9*01 grammes of 

 alum in 100 grammes of solution. 



Sodium chlorate was examined in the same way : it was found 

 that the liquid in contact with a growing crystal has at 19° C. 

 the index 1-38734, and contains about 47-8 grammes of salt in 

 100 grammes of solution ; a saturated solution of sodium chlorate 

 at 19° C. has the index 1-38649, and contains about 47-2 grammes 

 of salt in 100 grammes of solution. 



The liquid in contact with a growing crystal of sodium nitrate 

 has at 19° C. the index 1-38991, and contains about 48-45 grammes 

 of salt in 100 grammes of solution ; a saturated solution at 19° C. 

 has the index 1-38905, and contains about 481 grammes of salt in 

 100 grammes of solution. 



In each case the liquid in contact with the growing crystal is 

 slightly supersaturated. It was not found to exhibit double re- 

 fraction even in the case of sodium nitrate. No experiments seem 

 to have previously been made upon the nature of this liquid. 



G. Wulff has suggested that vicinal faces are due to concentration 

 streams in the solution. In order to test this view, crystals of alum 

 were measured after growing for several hours in solution kept 

 continually agitated in order to eliminate the action of the con- 

 centration streams. Almost no effect was produced upon the angles 

 of the vicinal faces. 



In sodium chlorate and sodium nitrate the solute is about 45 times 

 more dense in the crystal than in the adjacent liquid. Now planes 

 with high indices in a space-lattice contain fewer points in unit area 

 than planes with simple indices. The author suggests that vicinal 

 faces grow upon a crystal in preference to simple forms because the 

 crystallising material descends upon the growing face in a shower 

 which, is not very dense. 



