1100 



of the yt/ritunsali in their saturated raother-liqiior, showed witliont 

 any doubt, that such a horizoulal [)Uine of s} mmeti-j' is really 

 present. Thus all these salts belong to the he.vagonal-hipyramldnl 

 (hemiëdric) class of the hexagonal system. 



A stereographical projection of the pattern, produced by the 

 RöNTGEN-rays {erhiuumi\\X), is reproduced in fig. 3 ; the differences 

 in intensity of the dark spots on the photographic plate, are 

 indicated here by means of larger and smaller dots. ^) 



Commonly the crystals are tabular parallel to two Oj)posite faces 

 of the vertical prism; they make thus the impression of hexagonal 

 plates. The most interesting data ai-e summarized in the subjoined 

 table. 



§ 6. In the tirst instance this review of the obtained dates shows 

 clearly, that all these salts of the metals of the rare earths belong 

 to a series of perfecthj isomorphous substances. Moreover it has been 

 proved by a great number, — i.e. several thousands, — of exact 

 measurements, tJiat the deviations of the angles from the main value 

 lüith the diferent individuals of a same crystalspecies, must he con- 

 sidered to be of the same order as the deviations of the main values 

 of corresponding am/les with the successive terms of the whole series. 

 Thus we can consider all those salts to have practically the same crystal- 

 form, whose j)arameters oscillale about the most probable main value: 



a: r==: 1:0,5062 ±0,0012 



with deviations, which cannot be considered as typical for each kind 

 of crystals separately. Therefore the differences of the molecular 

 distances in the corresponding space-lattices of these crystals, can 

 be only of the same order, as the differences in molecular volumes 

 with the successive terms of the series. It must be considered a 

 remarkable fact in this respect, that, although the specific weights 

 of these salts are, generally speaking, gradually increasing with the 

 increase of atomic weight of the rare earth-metals — (with the 

 exception of the erlnum-s-alt, which was prepared from not highly 

 pure oxide, and of the cerium-ssih, whose isolated position in this 

 respect seems to be a real fact) — these molecular volumes, and 

 just so the calculated topic parameters, show a very evident perio- 

 dicity. '^) This fact, which may be graphically represented in fig. 4 



1) The stereographical projection is made upon a circular base, with a radius 

 of twice the distam-.e between crystalplate and RöNTGEN-tube. 



" ) Tliese topic parameters are calculated for the regular triangular prism 



