
TRANSACTIONS OF THE SECTIONS. 41 
GEOLOGY AND PHYSICAL GEOGRAPHY. 
Ona New Variety of Magnetic Iron Ore ; with Remarks upon the Application 
of Bicarbontte of Baryta to Quantitative Analyses. By Vrofessor T. 
Anprews, W.D., F.R.S., M_R.LA. 
Tuts mineral occurs in the schist rocks of the Mourne Mountains, near their junction 
with the granite. In external characters it resembles somewhat the common mag- 
netic oxide; but its lustre is inferior. It occurs both in the amorphous state and in 
imperfectly-formed octahedrons. Its composition was found to be,— 
Sesquioxide Of irOn .....scescssecesecseeseceseens 71°41 
Protoxide of iron ..........06005 eebeda ceevdesase 21°59 
IMaonesiat i itigs. ates Ae ESB. soo So 
The formula of this mineral is evidently Fe, O, + (FeO, MgO), a part of the protoxide 
of iron being replaced by magnesia. Although not mentioned in any of the published 
analyses of magnetic oxide of iron, magnesia appears to be a constant constituent of 
this mineral. The author gave the results of analyses of magnetic oxide in which 
2°00, 0°71, and 0:09 per cent. of magnesia had respectively replaced an equivalent 
amount of the protoxide of iron. It is remarkable that not a trace of lime could ever 
be detected in any specimen of magnetic oxide. Oxide of manganese is usually also 
present, but in minute quantity. In this analysis a solution of the bicarbonate of 
baryta was employed to separate the sesquioxide of iron from the magnesia. A 
solution of this compound, which is readily prepared by passing a current of carbonic 
acid into water containing recently-precipitated carbonate of baryta in suspension, the 
author finds to effect a very complete separation of the sesquioxide of iron from the 
oxide of manganese and from magnesia, and considers that it may be very usefully 
employed in quantitative analyses for effecting the separation of the bases just men- 
tioned, presenting many advantages over the insoluble carbonate of baryta, as well as 
over the other reagents usually employed for the same purpose. 
On the Sources of Common Salt. By W. BoLuaErt. 
Observations on the Diamond. By Sir Davip Brewster, K.H., F.R.S. 
In the course of last spring I was requested by H.R.H. Prince Albert to give my 
opinion respecting different forms into which it was proposed to reduce the Koh-i-Noor 
diamond, in order to make it an ornamental gem. In the state in which it then was, 
it exhibited an inferior display of colours to its glass model, and it was only by sur- 
rounding it with a number of vivid lights that its coloured refractions could be de- 
veloped. Having had occasion to observe some remarkable phenomena in small 
portions of diamond, an account of which was published in the Transactions of the 
Geological Society for 1836, I was desirous of examining so large a mass of diamond 
as the Koh-i-Noor before it was reduced in size, and covered with facets which would 
not permit it to be examined. His Royal Highness readily granted my request, and 
I had thus an opportunity of submitting it to the scrutiny of polarized light. In 
place of producing no action upon this species of light, as might have been expected 
from its octahedral structure, it exhibited streaks of polarized tints, generally parallel 
to one another, but in some places of an irregular form, and rising to the yellow of 
the jirst order of colours. © These tints and portions of polarized light were exactly 
the same as those which I had long ago found in many other diamonds, and figured 
in the Edinburgh Transactions for 1815 and 1816. In placing the Koh-i-Noor under 
a microscope of considerable power, I observed in it, and also in each of the two 
small diamonds which accompanied it, several minute and irregular cavities, sur- 
rounded with sectors of polarized light, which could only have been produced by the 
expansive action of a compressed gas or fluid that had existed in the cavities when 
the diamond was in a soft state. In an external cavity, shown in the model, and 
which had been used for fixing the gold setting, I observed, with common light, a 
portion of yellow light, indicating a yellow substance. Mr. Garrard and others con- 
sidered it as gold rubbed off the gold setting; but as gold is never yellow by trans- 
mitted light, I considered the colour as produced by a yellow solid substance of 
unknown origin. Sir Henry De la Beche having suggested to me that it would be 
