NA TURE 



[November i i, 1897 



to be studied with a completeness hitherto unattainable. In 

 brief, the apparatus is a spectroscope with two collimators, the 

 slit of the first being as usual for the entrance of the rays from 

 the source of light, in the author's case the electric arc, while 

 the slit of the second is employed as a selecting slit as in Captain 

 Abney's well-known apparatus. The two optical tubes, how- 

 ever, remain fixed, while the dispersing apparatus, a single large 

 prism of high dispersion, is made to rotate so as to bring the 

 various parts of the spectrum across the selecting slit in turn. 

 The selected ray issuing from the second slit is slightly diffused 

 by a screen of finely ground glass carried in a short sliding tube 

 in front of the slit, which is all that is required in order to flood 

 the whole field of any of the author's observing instruments, 

 such as goniometers, polariscopes, and microscopes, with evenly 

 distributed monochromatic light. The accompanying illustra- 

 tion. Fig. 2, shows, by the kind permission of the Chemical 

 Society, the arrangement as actually used with the gonio-spec- 

 trometer in the determination of the refractive indices. 



Having thus described the difficulties of the investigation and 

 the measures taken to overcome them, a brief outline of the 

 results attained up to the present will now be given. 



over, as a natural corollary, there is a corresponding progression 

 in the morphological constants, the axial ratios. Still further, 

 the influence of the nature of the metallic atoms is observed to 

 exercise a curious effect upon the prevailing habits of the 

 crystals. For example, the sulphates and selenates of potassium 

 exhibit preponderating development of the brachypinacoid, the 

 crystals usually being tabular in this direction ; on the other 

 hand the csesium salts are characterised by the prominence of 

 the basal plane, while the rubidium salts are distinguished by a 

 prismatic habit due to the predominance of a brachydomal form 

 intermediate between the two planes just mentioned. 



Turning now to the optical properties, it has been found to 

 be a rule without exception that the refractive indices of any 

 rubidium salt are intermediate between those of the correspond- 

 ing potassium and caesium salts, and nearer to the former than 

 to the latter, the differences being as one to three. In accord- 

 ance with the biaxial character of the crystals, each salt has 

 three refractive indices corresponding to the different degrees of 

 facility for the transmission of light along the three rectangular 

 directions of the axes of the optical ellipsoid. In making the 

 comparison, the same result is obtained whether the same 

 direction is chosen, or the mean of all the three indices of each 

 salt is taken to represent its general refraction. Indeed, as the 

 diff"erence of refraction along different directions in the same 

 crystal is small, compared with the change brought about by the 

 replacement of one metal by another, the rule remains generally 

 true if no precaution as to similarity of conditions is observed. 



A very interesting result of this is that if the optical ellipsoids 



The use of the term " isomorphous " is not strictly correct, 

 except in the cases of such series as crystallise in the primary 

 forms of the cubic system, in which, for geometrical reasons, the 

 interfacial angles are always identical. The normal sulphates 

 and selenates of the three alkali metals under consideration 

 crystallise, anhydrous, in the rhombic system, and the double 

 -salts of the series R2M(S04)2- 61120 in the monoclinic. The same 

 planes are common to all the six simple salts, the replacement 

 of sulphur by its family analogue selenium not effecting any 

 change in this respect ; likewise the double sulphates are 

 characterised by planes common to their series. But corre- 

 sponding interfacial angles on the different members of the same 

 series are not identical, but differ by amounts varying from a 

 few minutes to a couple of degrees. They are much smaller in 

 the simple salt series than in the double salts. The result of 

 elaborate measurements has revealed the fact that, without a 

 .single exception, the values of the angles of any rubidium salt 

 are intermediate between those for the corresponding potassium 

 and caesium salts. There is consequently a progression in the 

 angles of inclination of the crystal faces following the order of 

 progression of the atomic weights of the alkali metals. More- 



NO. 1463, VOL. 57] 



Fig. 2. 



of the three rhombic sulphates or selenates are constructed, 

 frpm the refraction data, about the same origin, using rectangular 

 axial coordinates, which correspond to the mutually identical 

 axes of morphological and optical symmetry, they are found to 

 envelop each other ; supposing the ellipsoid to be the optical 

 indicatrix of Fletcher (the optical reference surface now univers- 

 ally employed), the indicatrix for the csesium salt is the outer 

 one, and that for the potassium salt the innermost, while that 

 corresponding to the rubidium salt lies between the two, without 

 touching either, but nearer to the innermost. In the double 

 sulphate series the indicatrix likewise expands as the atomic 

 weight of the alkali metal rises, but monoclinic symmetry only 

 demands that one of the axes of the indicatrix shall be identical 

 with the single symmetry axis of the system, the other two 

 rectangular axes of the indicatrix being free to move together 

 in the symmetry plane. The expansion of the indicatrix 

 when potassium is replaced by rubidium or the latter by 

 CEesium, is actually found to be accompanied by a rotation 

 for several degrees about the symmetry axis, and the amount of 

 this interesting rotation is more than twice as much for the latter 

 chemical change as it is for the former. 



