On the Humite of Monte Somma. 175 
Art. XX.— Abstract of a Paper on the Humite of Monte Som- 
ma; by Arcaneeto Scaccui,* of Naples, with observations, by 
James D. Dana. ¢ 
5 
M. Scaccui in his elaborate paper points out the fact that there 
are three types of crystals of Humite. These several forms he 
illustrates by excellent figures both of simple aud compound. 
crystals, and with full crystallographic descriptions mathematical- 
ly deduced. 
The crystals of the different types agree in general similarity 
of form, even to the relative positions of the series of planes, as 
shown in the following figures I, II, III, yet they differ in these 
series, and in the inclinations of homologous faces, This differ- 
ence corresponds to a difference in the lengths of the vertical 
axis, the ratio of the two lateral axes being nearly identical. The 
form is trimetric and the ratios of the axes as deduced by M. 
Scacchi, (nm? in I and II, and n‘ in III, being taken as a face of 
the fundamental form) are as follows :-— 
-s b : c e. 26422 
I 1: 0-245315 : 0227101 [which equals 40764 : 1 : 0-92575] 
I. 1: 0343769 : 0318435 [ “ « 2-9090 : 1 : 092631] 
Il. 1: 0-190730 : 0-176465[ “ 52430: 1: 0:92520] 
In the latter expression of the ratios, which we have here added, 
the Similarity in the several ratios, b : ¢, is apparent; the mean 
of the three is, 1 : 0-92575, The ratio between the vertical axis 
for the three types, as M. Scacchi states, is 7:5: 9, that is, 
VIISaTE! vazhrey! oreh7as = 40764: 29090: 52430 =7:5: 9, 
This ratio according to M. Scacchi, represents the true relation 
of the types, and the three forms are therefore spoken of as pro 
able examples of a kind of dimorphism, or pleomorphism : for 
Where the same chemical compound crystallizes in forms so 
Widely different in normal dimensions, they are properly regarded 
as distinct forms, though all three may belong to the same crys- 
lographic system, 
ymbols are given by the author, both in accordance with the 
or of three types, and based on the axes above mentioned ; 
* Poggendorff’s Annalen, 1851, Ergiinzung, iii, 161. 
