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August 26, 1886 | 
NATURE 
401 
been revealed. This consists in examining their action on polarised 
light, that is, on light which, by reflection or refraction under 
suitable conditions, has acquired special properties, and become 
incapable of being reflected or refracted like ordinary light, 
except under certain well-defined conditions. 
To use a somewhat crude comparison, the luminous ray, after 
traversing certain media, assumes the appearance of an iron 
‘rod that has been passed through a rectangular drawing-frame. 
If on leaving the frame it meets an opening of like form and 
size, it will pass through without difficulty ; but if the opening 
be placed crosswise, it can no longer pass. 
There is this difference between the rod and the ray—that in 
all the intermediate positions a portion of the latter will pass 
through, the quantity increasing according as a more parallel 
disposition is assumed. Hence, if we take two apparatus corre- 
sponding to the frame and the opening, one of which supplies 
the polarised ray and the other intercepts it at right angles, the 
result will be complete darkness on the field of the instrument. 
But if we now place between both a crystalline plate of some 
substance which does not crystallise in the cubic form, we shall 
generally see the dark field illumined and often assuming the 
most lovely colours—an effect due to an action discovered by 
Arago and explained by Fresnel. With a homogeneous crystal, 
and when the light falls in parallel pencils on the plate, a uniform 
tint is diffused over the whole field of the instrument. If the 
crystal be not homogeneous, but formed of diverse parts jointed 
or regularly grouped together, but in positions not parallel, weshall 
get different tints for the different parts. By turning the crystal 
round, certain coloured strands will be extinguished, as we say, 
that is, witl cease to transmit the light, while others will remain 
luminous. Hence we have here an extremely delicate and 
accurate means of studying the structure of crystals in their most 
intimate details. Haiiy had already remarked that all crystals 
are doubly refracting, except those belonging to the cubic system. 
Brewster soon after thoroughly established the relation that 
exists between the optical properties and crystalline symmetry, 
stating, amongst other points, that cubic crystals alone have no 
action on polarised light. Nevertheless, observation had shown 
that certain substances affecting the cubic form had such action, 
and illumined the obscured field of the polarising apparatus. Biot 
had even suggested a term to designate, if not to explain, this 
exception, calling it ‘‘ lamellar polarisation.” 
To the researches of M. Mallard we are indebted for the true 
account of this anomaly, which in fact he has explained away. 
He shows that the cubic crystals acting on the polarised light 
are not really cubic, but formed by the regular grouping of parts 
belonging to other crystallinesystems. Boracite, for instance— 
chloroborate of magnesium usually taking the form of rhombic 
dodecahedra, that is, a solid of twelve equal rhombs belonging 
to the cubic system—is formed by the union of twelve straight 
pyramids with rhon bic bases, whose summits unite in the centre 
of the crystal, and whose bases are the rhombic facets. 
M. Mallard’s beautiful experiments with parallel rays have 
been confirmed by those of M. Emile Bertrand with convergent 
rays, showing in isolated portions of the garnet and of boracite 
all the properties belonging to regular crystals of orthorhombic 
substances, 
There can be no doubt as to the correctness of the explana- 
tion given by M. Mallard of the optical anomalies of crystals 
which had been regarded as cubic, but which have once more 
served to illustrate the trite remark, ‘* Trust not appearances.” 
The optical investigation of crystals, due mainly to the late 
M. de Senarmont, has become a familiar process which no 
mineralogist can henceforth afford to neglect. 
These same methods, employe with much greater magnifi- 
cation than in Amici and Norremberg’s primitive appliances, 
also render the greatest services to the geologist in the study of 
rocks. They enable him to determine with an otherwise unat- 
tainable accuracy the minutest elements of these formations, in 
which minerals are intermingled in diverse proportions. After 
Sorby, the pioneer in this line of investigation, Zirckel and 
Rosenbusch in Germany, Fouqué and Michel Lévy in France, 
have turned to the best account the new method, which has 
thrown much light on the origin and mode of formation of 
certain rocks, by showing what substances were first solidified 
and what parts resisted longest the cooling process. 
All these determinations are aided by the study of the optical 
sign of crystals—that is, the relative velocity with which the two 
polarised rays are propagated in certain directions—the observa- 
tion of the position of the axes wherever possible, that of 
dichroism, and even the approximate measurement of the in- 
dices of refraction. 
This last has been much facilitated by an instrument recently 
devised by M. Emile Bertrand. With a transparent or opaque 
plate of some crystallised substance, and by means of not more 
than four readings made in two positions of the crystal, we 
obtain, by the determination of the angle of total reflection, 
the two or three indices, and consequently the wave-surface of 
the crystal for all bodies not having too high an index of refrac- 
tion, And these operations, hitherto impracticable except with 
prisms or plates of great size, may now be made on extremely 
small crystals, such as those of rocks. 
But however paramount the importance of optical properties, 
others also claim attention in crystallo-physics. Although of 
less practical interest in the determination of crystals, they may 
still open up many new avenues of inquiry to the physicist. 
The curious property possessed by some hemihedral minerals 
of becoming charged with electricity with contrary signs at 
the two extremities of certain axes when heated or chilled has 
long been known. MM. J. and P. Curie have now shown that 
compression on the same crystals acts like the cooling, depres- 
sion or traction like the heating process. In both cases the 
phenomenon appears due to the greater proximity or distance of 
the molecules. It is remarkable that the phemonenon may be 
reversed, so that hemihedral crystals with inclined facets properly 
charged with electricity, positive at one and negative at the 
other extremity, will contract or expand as the case may be. 
As regards synthetic mineralogy, it is now known, thanks 
mainly to the researches of Berthier, Becquerel, Senarmont, 
H. Sainte-Claire Deville, and Daubrée, that minerals may be 
reproduced in our laboratories, and that we already possess a 
valuable means of study, enabling us to understand the condi- 
tions in which the natural minerals and their compounds may 
have been produced. We are thus advancing towards a 
chemical knowledge of certain species, whose formula analysis 
alone has failed to establish, and it may even soon be possible to 
produce useful substances under the very form from which they 
derive their properties. 
The observation of the crystalline products accidentally 
formed in the metallurgic furnaces first led to this line of study, 
the firstfruits of which Mittscherlich and Berthier obtained by 
fusion. 
By melting certain silicates, certain rocks or substances with 
the same chemical composition, and then exposing this vitreous 
mass to a temperature somewhat lower than that of fusion, MM. 
Fouqué and Michel Lévy have succeeded in reproducing the 
identical minerals found in lavas, basalts, and other eruptive 
rocks. Such are the anorthite and labrodorite feldspars, amphi- 
gene, pyroxene, peridot, magnetic iron, &c. 
The case is otherwise with the granites, the problem of whose 
origin is far more difficult to solve. Nevertheless, of their three 
constituents two have already been artificially obtained. 
Quartz had long ago been reproduced by Senarmont by heat- 
ing gelatinous silica with a solution of hydrochloric acid to about 
300° €. But Hautefeuille was the first to obtain fine crystals of 
orthoclase and albite feldspars by heating silica with alumina 
and the necessary alkalies in presence of a solvent such as 
a fused alkaline vanadate or tungstate. 
But the conditions of this beautiful experiment do not appear 
to have been realised in nature. The nearest approach to them 
was probably the series of essays made by our President jointly 
with M. Edmond Sarrasin, by heating a solution of alkaline 
silicate with a precipitated silicate of alumina to nearly 500° C, 
inastrong steel tube lined on the inside with platinum. According 
to the alkalies and proportions employed, the result is albite or 
orthoclase mixed or not with quartz, the crystals resembling those 
occurring in nature and presenting the same peculiarities of form 
and grouping. The well-ascertained presence of drops of water in 
the granitic quartz seems to show that these granites must have 
been formed in the presence of aqueous solutions. Thus the 
natural conditions have already been approached, but will not 
be entirely realised until the hitherto recalcitrant mica has been 
obtained. 
The first essays at reproducing the zeolite group of minerals 
have been made by De Schulten, who, by heating the silicate of 
soda in tubes of aluminous glass, has procured small icositetra- 
hedra of analcime, such as occur in the lavas of the Cyclops 
Islands. 
As regards precious stones, the solution of the problem from 
the scientific, if not the economic, standpoint, was long ago 
