of Crystallisation cfEpidoie and of OlcLidjer-S alt. 507 
There exists a very remarkable analogy of this kind between 
the crystalline forms of Epidote (prismatoidal Aiigite-spar) and 
of Hydrous Sulphate of Soda (prismatic Glauber-salt), as ob- 
tained from a recent and more accurate examination of these 
two substances, in the Treatise on Mineralogy by Professor 
Mohs. The relations of the different simple forms occurring in 
the two species are almost identical, though the absolute measure 
of the angles of the one is considerably distant from that of the 
other. 
The crystals of Glauber-salt have been hitherto but imperfect- 
ly described. Haiiy ^ ascribed to this species, on the authority 
of Rome de ITsle, the form of a square prism, terminated by 
an isosceles four-sided pyramid, in which he has been followed 
by a great number of mineralogists. Count Bournon states 
the primitive form of the substance to be an oblique-angular 
four-sided prism of 108® and 72® or nearly so. He came nearer 
the truth than Haiiy, but did not yet establish the hemiprisma- 
tic character of the crystalline forms of the species. 
The history of the forms of Epidote is not so simple as that 
bf Glauber-salt. Several varieties of the species itself had been 
formerly comprised under the general name of Schorl, and its 
forms were compared by Rome de ITsle to those of Augite, a 
species which he considered to be nearly allied to Epidote, (para^ 
tomous Augite-spar), with this difference only, that the crystals 
of one of these substances were commonly elongated, and im- 
planted in another direction than those of the other. Support- 
ed by his mathematical considerations of forms, Haiiy, who first 
established Epidote into a particular species, gave another posi- 
tion to the crystals of this substance J than that which he assign- 
ed to the forms of Augite §. He supposed the elongation of 
those crystals to take place in the direction of their principal 
axis, and gave for their primitive form a four-sided prism, of 
which the transverse section is a rhomboid of 114® 37' and 65® 
23' ; the sides of this prism are to each other in the ratio 
= 110 : 96 ; and its base is perpendicular to the lateral faces. 
The analogous position of both substances indicated by Rome de 
ITsle, was no doubt preferable, and had Hauy adopted it, he 
* Tahl. Comp, p, 19. 
t Traite, U iii. p. 10^, 
rj- Catalogue^ p. 183. 
§ Traite, t. iii. p. 80® 
