] 
bs 
TRANSACTIONS OF THE SECTIONS. 51 
to represent the protaxial form, and designating it by the symbol X, to avoid em- 
ploying any particular system of notation, we obtain the axial ratios of the three 
forms as given in the following Table :— 
Considered | Considered 
as a diaxial | as a triaxial 
pyramid. pyramid. 
x 1: 1:661 | 1: 1-175 
4X 1:0°830 | 1: 0°587 
1X 1: 0°554 | 1: 0.992 
The idocrase is exceedingly rich in pyramidal planes, but any one may be selected 
for the basis of the notation of the variable forms. Taking for this purpose a plane 
of common occurrence, and which makes with the basal planes of the crystals an 
angle of 151° 61), we obtain the axial proportions— 
1: 0°5351, or 1: 0°3783 (ériax.). 
These, compared to the axial proportions exhibited by the Hausmannite, show that 
4X in that substance is equivalent to the present form of the idocrase, the slight dif- 
ference being due to the isomorphic dissimilarity in the composition of the minerals, 
or to unavoidable errors in the measurement of the angles. 
Having thus demonstrated the apparent isomorphism of silica and alumina, the 
author points out certain objections to the theory. In the first place, the natural 
forms of these compounds, quartz and corundum, are not exactly alike. Both cry- 
stallize in the same system, but differ in their axial proportions in the ratio of 4 silica 
to 5 alumina*, or according to the number of atoms present in each substance. 
The second objection arises from the fact, that, by the adoption of the hypothesis, 
a peculiar and scarcely probable theoretic composition would result to many of the 
silicates ; the proportions of oxygen in the base, for instance, would in several cases 
be 15, 24, or even 38 times less than in the acid. 
The quéstion therefore remains entirely undecided; and the author, in calling 
attention to the subject, and showing its important bearings on the general philosophy 
of the science, points out the serious want that must continue to be felt, from this 
state of uncertainty, in numerous mineralogical investigations. No satisfactory 
classification of the silicates can well be framed, neither can we account for the anoma- 
lous composition presented by certain of these bodies, by the staurolite for example, 
which consists of silica and alumina in inconstant proportions, whilst its external cha- 
facters and crystallization remain unchanged. 
On the Incrustation which forms in the Boilers of Steam-Engines, in a Letter 
addressed to Dr. George Wilson, F.R.S.E. By Joun Davy, M.D., F.R.S., 
Inspector-General of Army Hospitals. 
On entering upon this inquiry, which I did after my return from the West Indies in 
December 1848, and after communicating a short paper’to the Royal Society on 
Carbonate of Lime in Sea-water, it appeared to me desirable to collect as many spe- 
cimens as possible of incrustation from the boilers of steam-vessels, now so widely 
employed in home and distant navigation. By application to companies and to 
friends in our seaports, as Dundee, Hull, Southampton, Hayle, Liverpool, Whitehaven, 
Ihave succeeded in procuring specimens of incrustation, formed by deposition, in 
yoyages from port to port in the British and Irish Channels and the North Sea; be- 
tween Southampton and Gibraltar; in the Mediterranean and the Black Sea; and in 
the Atlantic Ocean between Liverpool and North America, and between Southampton 
* In the quartz thombohedron the axes are as 1: 1095, in corundum as 1: 1°362. One 
of these forms might however be considered, in the language of the French crystallo-~ 
aa to be the “inverse” of the other, as in some of the cale-spar rhombohedrons ; 
the calculated plane angles of the corundum rhombohedron do not differ more 
than two minutes from the measured dihedral angles of the quartz form. 
E2 
