TRANSACTIONS OF THE SECTIONS. 
31 
liicngdesirethatwe should possess strictly comparable observations of atmospheric 
dectricitv, magnetism, and the aurora made at Alten or Hammerfest (in Finmnrk), 
aBombay, aod in this country; for, he said, it may be safely affirmed, that not- 
titiitaoding all that has been written about the relationship of the aurora with 
dattie and magnetic pluenoraena, the regiona, or npjmrcnt regions of the aurora 
ke nerer been subjected to electric observation in a manner at all approaching to 
maev, or comparability with obsen-ations made hero or elsewhere. Neither hw 
BfiKtrometer at Bombay ever yet been compared with one in England (or even in 
£• 1 ^ probably). We have surely been deducing conclusions on this very curious 
Mject vithout safficient mattrifl. 
('nder the auspices of Col. Sabine, Dr. Lee and Col. Sykes, something must be 
aaopiished. In fact, an electrical apparatus is now almost on its way to Bombay 
Ktls cod of the Honourable East India Company. 
fti AeSefecU of aiul Danger arising from the Use of Corrective Magnets for 
hiad Attraction on the Compasses of Iron-huill Vessels^ Dy the Rev. W. 
ScORESBY, D.D., F.li.S. _ 
i^AtComposition and Optical Properties of a vai'ietyof\\^tA\i('from Mexico. 
James Apjohn, M.D.* Professor of Mineralogy in tlu} University of 
lha mineral occurs in detached mammillary masses, sometimes larger th^ a 
Jdant, and pellucid in a high degree. It is harder than glass, but is scratched, 
with difficulty, by steel, and, according to a single experiment very carefully 
its specific gravity is 2’10l6. As the mean of four experiments scarcely dif- 
‘^gfrom each other, it yielded 
Silex.97-48 
Water.2-52 
iOO- 
which correspond accurately with the formula 
I5S 155-05, 2IIO, Og. 2HO. 
Mexican hyalite therefore is quite peculiar as respects the amount of its con 
**Wil water. fi.i« ,w.. njofc two-fifths of what has been found m ttic 
water, Uus being not r 
« hydrated variety of the mi 
tUWSV SWiMA a, TV .. 
vvu variety of the mineral whose analvsls 1ms been published, 
iiemost •- • • — - 
^einost remarkable circumstance connected w'ilh this mineral remains to he 
®on«d. The hyalites are, in all treatises on mineralogy, described 
.... i.^diiiva aic, in an vrcaiisea uu juum 
but upon trausmitting through tlie mlnerul whose composition liaa been 
JJ»bovc. a ray of plane polarized light, and ciamining this iu the usual manner 
;^«nie atmlysing eye-picce, it was found to i>os8 C6 .t in a very marked deg^® 
of depolarizing the ray. The action, however, exerted by it appeared 
rywM kind; far while in all doubly-refracting minerals, 
/hitherto described, lliere are two positions of the crystal 
;l i between two tourmalins whose axes arc croseed, the *’2 . , 
-L„ 1 T'P'® oetwren two tourmalins whose axes are — “e-- 
W *B no poHition into which a lamina of this hyalite can be g 
‘tv plane in which the light will not be restored. 
‘wartif.n .. , . ... v „.i ia nf the o 
■•-5 imi jis own plane m which the light wm noi o«-- jvo.w.v... 
therefore of this mineral on polarized light is not of the S 
"‘y knowledge of this difficult subject extends. 
that, like rock.cr?stal in one particular direction, and essentuU 
‘i* solutions of sugar, dextrine. &c., it possesses the power “f ® 
|oa of the plane of polarization of a plane polarized ray, or of , 
^ UitQogh a greater or )e.«is angle. Tliis view would wem to l )0 established by 
which the aullior dctailcil. , . , ,• «. timr this 
tBliu'r . “wrved I>r. Apjohn thinks he is jusunea m u -^rt» too 
kp r^j . power denominated rotatory polarization. This ^ I 
I in direction it ia traversed by the plane polarized ray, 
' ckcum?' ‘Erection it ia traversed by the plane poia^eu xaj, 
^“»staiice which would seem to identify it with that exerted by liquids, and 
