OF DENISON UNIVERSITY. 
I3I 
appear desmid-like aggregates of cerium sulphate and on the margin a 
brown cloudy zone of double salt of soda. Excess of sulphuric acid 
prevents'the reaction. 
Magnesium is detected by salt of phosphorus. The drop, before 
tested for possium or aluminum, is neutralized with ammonia and in a 
drop of water placed at a distance of about i cm. is placed a grain of 
salt of phosphorus and the two drops connected as before. The re- 
sult is the production of double, forked crystalloids similar to those 
found in natural glasses or well developed hemimorphic twins of 
ammoniated magnesium phosphate. 
Aluminum is detected by touching the drop with a platinum wire 
dipped in caesium chloride. Translucent octahedrons of caesium 
alumn are formed, or more rarely i . Ti. If the solution of the 
mineral is concentrated, dendritic forms simply are formed and water 
must be added. 
Similar tests are proposed for the detection of other elements but 
the above are of most general application. 
C. Use of the Polarizing Microscope. 
Ordinary polarized light is produced by placing the analyzer above the eye- 
piece in such a position that its Nicol’s prism stands at right angles to that in the 
polarizer below the stage. The field now appears totally dark, inasmuch as the 
only rays permitted to pass through the polarizer are extinguished by the analyzer. 
All minerals are either simply or doubly refractive, and the former may be rec- 
ognized as amorphous (like glass) or belonging to the isometj'ic crystal system. lir- 
as much as the elasticity of the ether in either case will be the same in any direction 
in such minerals, they do not interfere with the rays which pass through them, hence 
between crossed Nichols any section of such minerals remains constantly dark, 
even though the section be passed through a complete revolution by rotating the 
stage — in other words, the mineral is isotropous. Double refractive minerals, in 
sections taken in some directions, become colored in certain positions between 
crossed Nicols. Such sections become perfectly dark twice in one complete rev- 
olution. These colors are due to interference of the rays brought about by the 
double refraction. 
Optical uniaxial crystals are those falling in either the tetragonal or hexagonal 
systems. In such crystals there is but one direction in which there is no double 
refraction, i. e. that parallel to the vertical axis c, which, in this case, corresponds 
to the optical axis. The elasticity of the ether contained in the crystal is differ- 
ent in directions parallel and at right angles to the main axis. a= the a,xis of the 
greatest elacticity and c— the axis of least elasticity. exponent of refraction 
in the ordinary ray (that is, the one passing parallel to the optical axis and vibrat- 
ing at right angles to the fundamental section. e= exponent of extraordinary ray 
