308 Mr Haidinger's Comparative View ofilie Series 
would have been able to render the description and figures of 
Epidote as clear and intelligible as those of Pyroxene or Amphi- 
bole. As they are, they shew more that their author was inti- 
mately acquainted with the subject he treats, than they are 
adapted to the use of the student. By a decrement of one 
series of molecules along the acute vertical edge of the pri- 
mitive form, this prism is changed into an irregular six-sided 
one, whose transverse section contains two angles of 114° 37', 
two of 116° 40', and two of 128° 43'. The difference between 
the two first of these angles is expressed in the ratio of the sides of 
the primitive form, and has suggested to tlaliy the name of Epi- 
dote. This observation is the mere remarkable, as the instrument 
which Haiiy employed requires the crystals to be very perfect, in 
order to perceive the small difference of about 2°. Epidote has 
been the subject of a particular and very elaborate memoir by 
Professor Weiss in which he re-establishes that analogous posi- 
tion of forms, in which Home de ITsle had endeavoured to com- 
pare them with the forms of Augite,with this difference, that Borne 
de ITsle considered the crystals, of both substances, in that posi- 
tion which Haiiy has chosen for epidote, while Weiss has placed 
them in that which has been given by Plaliy to the crystals 
of pyroxene. This was a considerable step farther in their 
study, in order to find out analogies with the forms of other 
substances. Weiss, however, has not been fortunate in fixing up- 
on that situation of the crystals which makes the faces r. Fig. 1. 
Plate X., vertical, or parallel to the principal axis, because in this 
situation the relations between the simple forms become un- 
necessarily complicated, Not having himself measured with 
care the crystals of this substance, he was forced to rely on the 
data of Haiiy, But he referred theoretically the ratios among 
the different forms occurring in the species to that of three 
lines perpendicular to each other, and thus it happened that, 
although more intelligible and easy to be compared with other 
species, his mode of considering the forms of Epidote is less cor- 
rect and applicable to nature than that of Haiiy. 
In the position in which Professor Mohs considers the crys- 
tals of Epidote or prismatoidal Augite-spar *1*, the faces M cor-, 
* Abhandl. d. Akad. d. Wissensch. zu Berlin.^ for 1818 and 1819, p. 242, 
^1* Grundriss der Mineralogies Th. i, p. 561. 
