MB Professor Mobs' Reply to Professor Weiss, 
properties, but especially in their form, some of which even are 
not hemi- but tetarto-prismatic. Thus, in the greater part 
of the varieties of the Common Felspar of Werner, and in 
some of his Adularia, which varieties I consider to belong, with 
the Albite of Berzelius, to one and the same species, the incli- 
nation of the two most apparent faces of cleavage is = 93° 20' 
and 86° 40', instead of 90°, as in prismatic felspar. A similar 
difference in the angles has also been noticed by Professor Fuchs 
of Landshut, with respect to what he calls Porcelain-Spar *. 
In Epidote (prismatoidal augite-spar) M. Weiss says, that I 
had given the system of crystallization as unknown. Yet both 
the editions state the system of crystallization to be prismatic, 
and the combination hemiprismatic ; only I would not give the 
dimensions of the fundamental form, because I suspected the 
correctness of Haiiy’s data, just like Professor Weiss, who did 
not, however, improve them by immediate measurements. You 
will find in the Treatise a new determination founded upon an 
accurate measurement of* the angles. In this I have given the 
fundamental forms another position, different from that of 
Weiss, who had already rejected Hauy’s ; and if you will com- 
pare the relations of the derived forms, mentioned in both the 
volumes of the Treatise, with those of Professor Weiss, the 
greater degree of simplicity, which distinguishes the former, will 
not escape your attention. 
The series of crystallizations of Cross-stone (paratomous 
kouphone-spar) certainly belongs to the prismatic, not to the 
pyramidal system, as Professor Weiss says. The prismatic sys- 
tem is indicated by the horizontal prisms that appear in the com- 
bination, by the striae of the faces of the pyramid, and by clea- 
vage, which is different in the direction of the different faces of 
the four-sided prism, and proves this to be a compound form, 
even if the termination is not taken into account. The dimensions 
of Haiiy are certainly incorrect ; but as yet we have no better. 
It is more than probable, that accurate measurement will exhibit 
a difference in the terminal edges of the fundamental form, how- 
ever small this should be found. A transition from one system 
* We think the application of the optical character to the felspar would lead 
to curious and important results, and therefore recommend this investigation to 
the attention of optical ipinerplogists.— E jd. 
