$8t Professor Mohs’s General Reflections on 
other; and we should avoid such signs as, though shorter in 
themselves, and of equal distinctness in regard to the mathema- 
tical department, do not convey this idea of series. The theory 
of forms, founded upon the series, is confirmed in a remarkable 
manner by the physical quality of the faces which limit the 
forms, and of the cleavage-planes corresponding to them, and 
which is expressed in the former by the intensity and kind of 
lustre, the smoothness or roughness, the existence of striae in 
certain determinate directions, — and in the latter, by their higher 
or lower degree of perfection, and the different facility with 
which they may be obtained. Although the phaenomena of 
crystallisation are not alone sufficient to form the sole foundation 
of the Natural History of the mineral kingdom, as we have al- 
ready observed, they yet form one of the most important de- 
partments of the properties of minerals, since, even in respect to 
cleavage, they are so very closely allied to the other physical 
qualities of natural bodies. 
This connection is apparently contradicted by certain obser- 
vations, which, however, will, in reality, be found rather to 
countenance it when viewed in a proper light. Several sub- 
stances have been found frequently to assume the same form, 
while one and the same substance often appears under forms of 
two different classes not compatible with each other. The in- 
ferences generally drawn from this circumstance, were they well 
grounded, would indeed serve to depreciate the value of crystal- 
lography as a means of distinguishing mineral species, accord- 
ing to the principles of Natural History. The first of these ob- 
servations, which was confined by Hauy to the forms of the tes- 
sular system, we may admit as taking place to its greatest ex- 
tent : it is indifferent, whether, in this respect, we mean by 
substance the composition of the mineral, or the natural-his~ 
torical species * As to the latter, the determination of the spe- 
cies does not solely depend upon the forms and other relations 
connected with it, for different species may assume one and the 
same form , although it has not yet been sufficiently demon- 
strated that this takes place in nature in any other species than 
such as possess forms belonging to the tessular system. But 
that one and the same substance may assume two different in- 
compatible forms, is true only if we consider the chemical com- 
