The evening times., 1899-12-26
AN EXHIBIT OF MINERALS.
The DispfSS In the National Museum
ralkNasr G,0mnIefe

rsN0w Complete.
The National Museum has completed a
series OF mineral specimens, models. and
labels to illustrate and define the several
properties or characters OF all the min
erals at present known to science. The
series has been arranged in wall cases on
the west side OF Mineral Hall.
The catalogue has just been issued, also.
which explains the arrangement under the
two general heads OF chemical mineralogy
and physical mineralogy. the former con-
sidering mainly the elements in combina-
ton. and the latter treating Of such physi-
cal properties as form on molecular struc-
ture.s cohesion. and elasticity. mass or
volume. heat. magnetism and electricity.
light, touch, taste and odor. and charac-
ters depending upon the resistance to
chemical action.
To the casual visitor, the characters de-
pending upon the action Of Hight are the
mCst interesting, and valuable dis
cussioD Of these characters is offered for
their instruction paper on the subject.
issued by the Museum, says:
Light Is the sensation produced upon
the eye resulting from the excitation of a
NiuidtVy muliOu ny iuHlluOuS DOuy III
the particles of highly elastic impondera
BILE medium called ether, which is assum-
ed to pertsde'all space, including the most
minute pores Of all matter, whether solid.
liquid, or gaseous.
These vibrations are propagated liz
straight lines and in all directions FROM
the luminous point. The smallest portion
OF light which can be separated Is called
ray. and II may be considered as COM
bination Of Two vibrating motions. one OF
which, for the sake OF convenience. may be
regarded as vertical and the other hori-
zontal.
"A ray OF light will pass through any
medium Of the same density In a perfectly
straight line. but if it pass out Of that
medium into another of different density
it may be disposed of in different ways,
being either transmitted, absorbed. re-
fected. refracted. or polarized.
"All bodies transmit light to grea. er
or less degree. Among minerals the amount
of light transmitted. or the degrees of
transparency, are classed as transpar ent
substransparent, translucent. and opaque.
The property of diaphanety occurs in
every degree among minerals. and is he-e
represented in its several shades by five
specimens Of quartz.
The color Of a mineral depends upon its
power of absorbing certain portions OF
light; that is absorbing certain rays OF the
spectrum that pass through or fail upon
Its surface. A yellow mineral. for exam
ple will absorb all the rays Of the spec
trum except the yellow; a green mineral
will reflect chiefly green rays, while
white. mineral will reflect all. and a black
mineral will absorb all the rays. The colo.
then. is the result OF the mixture Of tkos-
rays OF light which are not absorbed The
color Of a mineral is OF two kinds-essen-
tial and non-essential
THE essential color is that Of the mine
raj itself in its purest state. and belongs I
not only to the mass. but to the finest par
ticle that can be mechanically divided The |
essential color is determined by the color |
of the fine powder Of the mineral. or by |
rubbing it on surface Of ung.azed por- |
celain. The color of this powder or mark /
is known by the streak.
This character is illustrated by a Sc |
ries of twenty-five minerals. To the LETT OF |
each specimen is a small vial containing |
the powder of the mineral. II will be oo- |
served that although in many cases the es- |
sential color and that of the mineral are
the same. in others the color Of the tieak 4
differs from that of the mass.
The non-essenttal color is in general 4
that of the impurities contained in The .
mineral and the color of the mass VII .
differ from that of the streak: tina, Is, n
same mineral species may display sever
different colOrs, all of which disappear IA
powder.
This non u-essential character OF the
,olor le seen iinT specimens OF quartz AID
fuorite. in which the several distinct c1i-
,rs Of the Individual masses all disappear
in the powder.
The variations In color are classed. FIRST.
rs metallic and non metallic, and ali shades
are referred to eight fundamental colors:
White grey, black, blue. green, yellow.
red. and brown. Second. according TO Pc
,uliarities in the arrangement Of co,ors.
Is play of color. opalescence, iridescence,
.arnish. and asterism.
Third. as 10 the difernece in color
shown for light transmitted in different di-
-ections through the crystal This case OF
.olor absorption is called pleochroism, and
$ peculiar TO doubly -refracting minerals
Further. certain minerals when viewei
Inder gisen conditions present blush ap-
learance resulting from the absorption Of
,ertair rays Of light. This property Is
allied nuorescence-
Among minerals lustre Is that character
depending upon the power and manner OF
.efeeting light and is dependent upon the
nature Of the rejecting surface and the
luantiiy or intensity Of the light reflected.
The kinds of lustre are: Metallic. subne-
allic adamantine vitreous, resinous, pear
V, and silky. The degrees OF intensity are:
;plendent, shining. glistening, gimme ing.
md dull