of Crystallisation of Epidote and of Glauber-Salt. SI 5 
angles have not yet been ascertained, sometimes variously mo- 
dified, and cleavable with great facility parallel to AC'XC, a 
plane passing through the axis and the short diagonal of the 
base. The specific gravity of this salt is — 2.462 ; its hardness 
= 2.5, between gypsum and calcareous spar ; in both these 
characters superior to the common hydrous Glauber-salt, whose 
specific gravity is = 1.481, and its hardness = 1.5... 2.0, some- 
what lower than gypsum. The forms of the two salts are in 
the same relation to each other as those of hydrous and anhy- 
drous sulphate of lime (prismatoidal and prismatic gypsum- 
haloide) ; and from the circumstance, that the forms of the dry 
sulphate of soda are prismatic, like those of the dry sulphate of 
lime, it appears that Glauberite, the combination of both, on 
account of its hemiprismatic forms, cannot be considered as a 
mere mechanical mixture of these two substances, as has been 
supposed by some mineralogists. 
The analogy between the forms of Glauber-salt and of Epi- 
dote is so striking, that after having compared the crystals of the 
two species with each other, it would be superfluous to add any 
farther remarks on the subject. But it will be necessary to say 
a few words on the Inclination qf the Axis in hemiprismatic 
forms, which we have found in one of them to be — 0° 33', and 
in the other — 1 4° 40', and on the consequences of this import- 
ant observation. 
It is impossible to derive the secondary forms of any of the 
two preceding species according to any regular process, from 
scalene four-sided pyramid, in which, like Fig. 11., the three 
lines AM, BM, CM, or a, h and c, are perpendicular to each*, 
other. But the derivation will be eftected with the greatest faci-. 
lity, if we suppose the axis AX, to be inclined to a perpendi- 
cular AM upon the base CBC'B', Fig. 10., at a certain angle,, 
variable according to the species to which the forms refer. 
This inclination of the axis is implied in some of Haiiy’s earlier 
determinations, as, for instance, in those of Epidote, of Gypsum, 
of Felspar, &c. ; but he never established it as a general prin- 
ciple, and his later labours proved that he preferred considering 
hemiprismatic forms, under such dimensions as would render it 
possible that equally inclined faces appear on both sides of the 
axis. Professor Weiss has endeavoured, by slight corrections 
