$76 Professor Mohs’ Reply to Professor Weiss, 
on the many conversations I had with you, and our late excellent 
friend Dr Mitchell, at Ereyberg, or advert to the catalogue of 
a famous collection of minerals which I published at Vienna, 
some years afterwards. The principles I adopted in this work, 
although not then reduced to their present purity and simpli- 
city, by no means differ from those which I still acknowledge ; 
and I trust that I have at least put beyond all doubt the pos- 
sibility of a natural history method in mineralogy, by a strict 
adherence to those principles. 
After having developed and settled with myself clear ideas of 
the different single parts that constitute the natural history of 
the mineral kingdom, I now fixed my attention more particularly 
upon that which is called the Characteristic. It was obvious to 
me, that the application of the characteristic required an exami- 
nation of the properties of minerals, in order to obtain such as 
might be usefully employed as characters ; and that those which 
are derived from the regular forms could not here be neglected. 
I could not make any use of the descriptions of crystals, con- 
ducted according to the methods hitherto employed ; because 
it was not my object to unfold the variety of nature, but, on the 
contrary, to collect this variety under one and the same idea. I 
found, also, that the Abbe Haiiy’s Primitive Forms were equally 
inapplicable to my purpose, for they do not contain the image 
of that variety which I more particularly intended to represent 
in the characters. 
The four rhombohedrons of calcareous-spar ( rhombohedral 
lime haloide J, designated in Haiiy’s works by the letters m,f p 9 
g, possess the property, that, their axes being supposed equal, 
the faces of every more obtuse rhombohedron are tangent to the 
terminal edges of every more acute one ; and, consequently, if the 
horizontal projections are supposed equal, that their axes de- 
crease in the ratio of the powers of the number $. Nothing 
could be more simple than to apply to these forms the notions of 
the series of characters which I had previously made use of, 
and found to be very well adapted to many purposes. Thus, I 
derived the series of rhombohedrons between its limits , with which 
you are sufficiently acquainted. 
I had made the remark, that some of the forms which the ce- 
lebrated Werner considered as fundamental in his descriptions 
