Chemistry and Physics. 275 
Stated his conviction, after recent and careful examination of the Nor- 
wegian Chromic Iron Ore, that Aridium, which Ullgren first announced 
as a new metal obtained by him from that mineral, is nothing more than 
oxyd of iron admixed with traces of phosphoric acid and oxyd of 
ote to J. D. Dana’s Contributions to Chemical Mineralogy, 
iti ages 218 to 
220, fractions are used as preferable often to whole numbers. ‘Thus, 
Si, al Bit, #l si* #18, are obviously more immediately appreciated 
sis affords. The actual number of molecules may be for the first, 
for example, Al4o Si30 or A124 Si1s, etc., and Mr. T. 8. Hunt has 
given good reason for believing that the molecules are in some suc 
multiples. Hence 414 Si3, taken as an expression of the absolute 
number of molecules is beyond doubt false ; taken as a ratio simply, 
the fact is better exhibited by the fraction. sa i 
In the formulas, such as (¢8?-+34) Si or (285+28) Si, the sum of the 
fractions is a unit; and in the first of these two formulas, they show 
that Rs is to # as 1:1; in the second, as 3:2. These expressions 
or formulas of course come under the more general formula (R3,#) 
ma 
Tourmaline that the planes of it are less common on crystals than 
f -4R (the R of authors). But it is true also In Calcite that 
-$R is one of the most common forms, much more common than h. 
- On the Production of Crystalline Structure in Crystallized Pow- 
ders, by Compression and Traction ; by Sir Davin BrewsTER, Kika 
ssess it, has been long known; but with this class of phenomena, 
those which I am about to describe have no connexion whatever 
es, wh h 
Obtained a transparent film, exhibiting the phenomena of double re- 
flexion and polarization from its surface as perfectly as if I had been 
a large crystal. Ge 
