1358 
d. The herahydrates of nickelsulphate and nickelselenate, which 
both belong to the tetragonal-trapezohedrical class. 
e. The heptahydrates of the sulphates of zine, magnesium and 
nickel, which belong to the rhombie-bisphenoïdical class. 
Some of these compounds were already studied by us formerly, 
so that only the stereographical projections of the Röntgen-patterns 
will be reproduced in this paper for comparison, only of the new 
objects have we also reproduced the original photogrammes, as far 
as possible. 
It may be remarked here, that also in these experiments the 
hardness of the Röntgen-radiation used, was kept constant almost 
completely by means of an osmotic palladium-regulator with combined 
micro-gasburner, and by the use of a milli-ampere-meter, which 
allowed us to keep the current-strength of the discharges in the 
tube within very narrow limits. The differences of intensity and 
hardness were thus reduced to a minimal amount. In these cases 
also a rotating gas-interruptor (improved “Cox”- manufacture) was 
used, as well as an intensifying sereen behind the photographie 
film. 
§ 3. Arragonite; Strontianite; Witherite; and Cerussite. 
A review of the parameters, specifie gravities and calculated 
topical axes of these rhombic carbonates may first be given here: 
Chemical 
Mineral: | Compe: | Axial ratio: | Een Topical Axes: 
Arragonite CaCO; la b:c = 0.6228: 1:0.7204 2.93 (4:4:9=2.64:4.23:3.05 
Strontianite | SrCO3 ee :¢ = 0.6090: 1 : 0.7237 3.70 lem = 2.73: 4.49:3.25 
Witherite BaCO, a:b:c=0.5949:1:0.7413 4.30 even = 2.84:4.70:3.44 
Cerussite PbCOz ja:b:c=0.6102:1:0.7237| 6.57 pto = 2.75:4.51:3.26 
Although these four minerals doubtless must be considered to be 
isomorphous ones, as is e.g. confirmed by their ability to form solid 
solutions and by the complete analogy of their twin-formation, the 
above mentioned review teaches however quite clearly, that only 
in the case of the three first named terms of the series the corre- 
sponding distances of the erystal-structure increase in a regular way 
with an increase of the atomic weight, and in all three axial directions. 
In the case of cerussite, however, these distances again decrease to 
about the values characteristic of the strontium-salt. Indeed, also the 
