Mr. Thiselton Dyer has rendered a great service, not only to botanists,
but also to physicists and mineralogists, by recalling attention to the
very interesting substance known as "tabasheer." As he truly states, very
little fresh information has been published on the subject during recent
years, a circumstance for which I can only account by the fact that
botanists may justly feel some doubt as to whether it belongs to the
vegetable kingdom, while mineralogists seem to have equal ground for
hesitation in accepting it as a member of the mineral kingdom.
It is very interesting to hear that so able a physiologist as Prof. Cohn
intends to investigate the conditions under which living plants separate
this substance from their tissues. That unicellular algae, like the
Diatomaceae, living in a medium which may contain only one part in 10,000
by weight of dissolved silica, or even less than that amount, should be
able to separate this substance to form their exquisitely ornamented
frustules is one of the most striking facts in natural history, whether
we regard it in its physiological or its chemical aspects.
Sir David Brewster long ago pointed out the remarkable physical
characters presented by the curious product of the vegetable world known
as "tabasheer," though so far as I can find out it has not in recent
years received that attention from physicists which the experiments and
observations of the great Scotch philosopher show it to be worthy of.
Tabasheer seems to stand in the same relation to the mineral kingdom as
do ambers and pearls. It is in fact an _opal_ formed under somewhat
remarkable and anomalous conditions which we are able to study; and in
this aspect I have for some time past been devoting a considerable amount
of attention to the minute structure of the substance by making thin
sections and examining them under the microscope. It may be as well,
perhaps, to give a short sketch of the information upon the subject which
I have up to the present time been able to obtain, and in this way to
call attention to points upon which further research seems to be
necessary.
From time immemorial tabasheer has enjoyed a very high reputation in
Eastern countries as a drug. Its supposed medicinal virtues, like those
of the fossil teeth of China and the belemnites ("thunderbolts") of this
country, seem to have been suggested by the peculiarity of its mode of
occurrence. A knowledge of the substance was introduced into Western
Europe by the Arabian physicians, and the name by which the substance is
generally known is said to be of Arabic origin. Much of the material
which under the name of "tabasheer" finds its way to Syria and Turkey is
said, however, to be fictitious or adulterated.
In 1788 Dr. Patrick Russell, F.R.S., then resident at Vizagapatam, wrote
a letter to Sir Joseph Banks in which he gave an account of all the facts
which he had been able to collect with respect to this curious substance
and its mode of occurrence, and his interesting letter was published in
the Philosophical Transactions for 1790 (vol. lxxx., p. 273).
Tabasheer is said to be sometimes found among the ashes of bamboos that
have been set on fire (by mutual friction?). Ordinarily, however, it is
sought for by splitting open those bamboo stems which give a rattling
sound when shaken. Such rattling sounds do not, however, afford
infallible criteria as to the presence or absence of tabasheer in a
bamboo, for where the quantity is small it is often found to be closely
adherent to the bottom and sides of the cavity. Tabasheer is by no means
found in all stems or in all joints of the same stem of the bamboos.
Whether certain species produce it in greater abundance than others, and
what is the influence of soil, situation, and season upon the production
of the substance, are questions which do not seem as yet to have been
accurately investigated.
Dr. Russell found that the bamboos which produce tabasheer often contain
a fluid, usually clear, transparent, and colorless or of greenish tint,
but sometimes thicker and of a white color, and at other times darker and
of the consistency of honey. Occasionally the thicker varieties were
found passing into a solid state, and forming tabasheer.
Dr. Russell performed the interesting experiment of drawing off the
liquid from the bamboo stem and allowing it to stand in stoppered
bottles. A "whitish, cottony sediment" was formed at the bottom, with a
thin film of the same kind at the top. When the whole was well shaken
together and allowed to evaporate, it left a residue of a whitish brown
color resembling the inferior kinds of tabasheer. By splitting up
different joints of bamboo Dr. Russell was also able to satisfy himself
of the gradual deposition within them of the solid tabasheer by the
evaporation of the liquid solvent.
In 1791, Mr. James Louis Macie, F.R.S. (who afterward took the name of
Smithson), gave an account of his examination of the properties of the
specimens of tabasheer sent home by Dr. Russell (Phil. Trans., vol.
lxxxi., 1791, p. 368). These specimens came from Vellore, Hyderabad,
Masulipatam, and other localities in India. They were submitted to a
number of tests which induced Mr. Macie to believe that they consisted
principally of silica, but that before calcination some vegetable matter
must have been present. A determination of the specific gravity of the
substance by Mr. Macie gave 2.188 as the result. Another determination by
Mr. Cavendish gave 2.169.
In this same paper it is stated that a bamboo grown in a hot-house at
Islington gave a rattling noise, and on being split open by Sir Joseph
Banks yielded, not an ordinary tabasheer, but a small pebble about the
size of half a pea, externally of a dark brown or black color, and
within of a reddish brown tint. This stone is said to have been so hard
as to cut glass, and to have been in parts of a crystalline structure.
Its behavior with reagents was found to be different in many respects
from that of the ordinary tabasheer; and it was proved to contain silica
and iron. The specimen is referred to in a letter to Berthollet published
in the _Annales de Chimie_ for the same year (October, 1791). There may
be some doubt as to whether this specimen was really of the nature of
tabasheer. If such were the case, it would seem to have been a tabasheer
in which a crystalline structure had begun to be set up.
In the year 1806, MM. Foureroy and Vauquelin gave an account of a
specimen of tabasheer brought from South America in 1804 by Humboldt and
Bonpland (_Mem. de l'Inst_., vol. vi., p. 382). It was procured from a
species of bamboo growing on the west of Pichincha, and is described as
being of a milk white color, in part apparently crystalline in structure,
and in part semi-transparent and gelatinous. It was seen to contain
traces of the vegetable structure of the plant from which it had been
extracted. On ignition it became black, and emitted pungent fumes.
An analysis of this tabasheer from the Andes showed that it contained 70
per cent. of silica and 30 per cent. of potash, lime, and water, with
some organic matter. It would, perhaps, be rash to conclude from this
single observation that the American bamboo produced tabasheer of
different composition from that of the Old World; but the subject is
evidently one worthy of careful investigation.
It was in the year 1819 that Sir David Brewster published the first
account of his long and important series of observations upon the
physical peculiarities of tabasheer (Phil. Trans., vol. cix., 1819, p.
283). The specimens which he first examined were obtained from India by
Dr. Kennedy, by whom they were given to Brewster.
Brewster found the specimens which he examined to be perfectly
_isotropic_, exercising no influence in depolarizing light. When heated,
however, it proved to be remarkably _phosphorescent_. The translucent
varieties were found to transmit a yellowish and to reflect a bluish
white light--or, in other words, to exhibit the phenomenon of
_opalescence_. When tabasheer is slightly wetted, it becomes white and
opaque; but when thoroughly saturated with water, perfectly transparent.
By preparing prisms of different varieties of tabasheer, Brewster
proceeded to determine its refractive index, arriving at the remarkable
result that tabasheer "has a lower index of refraction than any other
known solid or liquid, and that it actually holds an intermediate place
between water and gaseous bodies!" This excessively low refractive power
Brewster believes to afford a complete explanation of the extraordinary
behavior exhibited by tabasheer when wholly or partially saturated with
fluids. A number of interesting experiments were performed by saturating
the tabasheer with oils of different refractive powers, and by heating it
in various ways and under different conditions, and also by introducing
carbonaceous matter into the minute pores of the substance by setting
fire to paper in which fragments were wrapped.
The mean of experiments undertaken by Mr. James Jardine, on behalf of
Brewster, for determining the specific gravity of tabasheer, gave as a
result 2.235. From these experiments Brewster concluded that the space
occupied by the pores of the tabasheer is about two and a half times as
great as that of the colloid silica itself!
From this time forward Brewster seems to have manifested the keenest
interest in all questions connected with the origin and history of a
substance possessing such singular physical properties. By the aid of Mr.
Swinton, secretary to the government at Calcutta, he formed a large and
interesting collection of all the different varieties of tabasheer from
various parts of India. He also obtained specimens of the bamboo with the
tabasheer _in situ_. In 1828 he published an interesting paper on "The
Natural History and Properties of Tabasheer" (_Edinburgh Journal of
Science_, vol. viii., 1828, p. 288), in which he discussed many of the
important problems connected with the origin of the substance. From his
inquiries and observations, Brewster was led to conclude that tabasheer
was only produced in those joints of bamboos which are in an injured,
unhealthy, or malformed condition, and that the siliceous fluid only
finds its way into the hollow spaces between the joints of the stem when
the membrane lining the cavities is destroyed or rent by disease.
Prof. Edward Turner, of the University of London, undertook an analysis
of tabasheer, the specimens being supplied from Brewster's collection
(_Edinburgh Journal of Science_, vol. viii., 1828, p. 335). His
determinations of the specific gravities of different varieties were as
follows:
Chalky tabasheer. 2.189
Translucent tabasheer. 2.167
Transparent tabasheer. 2.160
All the varieties lose air and hygroscopic water at 100° C., and a larger
quantity of water and organic matter (indicated by faint smoke and an
empyreumatic odor) at a red heat. The results obtained were as follows:
Loss at 100° C. Loss at red heat.
Chalky tabasheer. 0.838 per cent. 1.277 per cent.
Translucent tabasheer. 1.620 " " 3.840 " "
Transparent tabasheer. 2.411 " " 4.518 " "
Dr. Turner found the ignited Indian tabasheer to consist almost entirely
of pure silica with a minute quantity of lime and vegetable matter. He
failed to find any trace of alkalies in it.
In 1855, Guibourt (_Journ. de Pharm_. [3], xxvii., 81, 161, 252; _Phil.
Mag_, [4], x., 229) analyzed a specimen of tabasheer having a specific
gravity of 2.148. It gave the following result:
Silica. = 96.94
Potash and lime. = 0.13
Water. = 2.93
Organic matter. = trace
Guibourt criticised some of the conclusions arrived at by Brewster, and
sought to explain the source of the silica by studying the composition of
different parts of the bamboo. While the ashes of the wood contained
0.0612 of the whole weight of the wood, the pith was found to contain
0.448 per cent., the inner wood much less, and the greatest proportion
occurred in the external wood. On these determinations Guibourt founded a
theory of the mode of formation of tabasheer based on the suggestion that
at certain periods of its growth the bamboo needed less silica than at
other times, and that when not needed, the silica was carried inward and
deposited in the interior.
In the year 1857, D.W. Host van Tonningen, of Buitenzorg, undertook an
investigation of the tabasheer of Java, which is known to the natives of
that island under the name of "singkara" (_Naturkundig Tijdschrift voor
Nederlandsch Indie_, vol. xiii., 1857, p. 391). The specimens examined
were obtained from the _Bambusa apus_, growing in the Residency of
Bantam. It is described as resembling in appearance the Indian
tabasheers. Its analysis gave the following result:
Silica. = 86.387
Iron oxide. = 0.424
Lime. = 0.244
Potash. = 4.806
Organic matter. = 0.507
Water. = 7.632
------
Total. 100.000
Apart from the question of its singular mode of origin, however, and its
remarkable and anomalous physical properties, tabasheer is of much
interest to mineralogists and geologists. All the varieties hitherto
examined, with the exception of the peculiar one from the Andes, are in
composition and physical characters true opals. This is the case with all
the Indian and Java varieties. They consist essentially of silica in its
colloidal form, the water, lime, potash, and organic matter being as
small and variable in amount as in the mineral opals; and, as in them,
these substances must be regarded merely as mechanical impurities.
The tabasheers must be studied in their relations on the one hand with
certain varieties of the natural semi-opals, hydrophanes, beekites, and
floatstones, some of which they closely resemble in their physical
characters, and on the other hand with specimens of artificially
deposited colloid silica formed under different conditions. Prof. Church,
who has so successfully studied the beekites, informs me that some of
those remarkable bodies present singular points of analogy with
tabasheer.
By the study of thin sections I have, during several years, been
endeavoring to trace the minute structure of some of these substances. In
no class of materials is it more necessary to guard one's self against
errors of observation arising from changes induced in the substance
during the operations which are necessary to the preparation of
transparent sections of hard substances. Unfortunately, too, it is the
custom of the natives to prepare the substance for the market by an
imperfect calcination, and hitherto I have only been able to study
specimens procured in the markets which have been subjected to this
process. It is obviously desirable, before attempting to interpret the
structures exhibited, under the microscope, to compare the fresh and
uncalcined materials with those that have been more or less altered by
heat.
Tabasheer would seem, from Brewster's experiments, to be a very intimate
admixture of two and a half parts of air with one part of colloidal
silica. The interspaces filled with air appear, at all events, in most
cases, to be so minute that they cannot be detected by the highest powers
of the microscope which I have been able to employ. It is this intimate
admixture of a solid with a gas which probably gives rise to the curious
and anomalous properties exhibited by this singular substance.
The ultra-microscopical vesicles filled with air in all probability give
rise to the opalescence which is so marked a property of the substance.
Their size is such as to scatter and throw back the rays at the blue end
of the spectrum and to transmit those at the red end.
When the vesicles of the substance are filled with Canada balsam, and a
thin slice is cut from it, this opalescence comes out in the most
striking manner. Very thin sections are of a rich orange yellow by
transmitted light, and a delicate blue tint by reflected light. I do not
know of any substance which in such thin films displays such striking
opalescence.
That the excessively low refractive power of tabasheer is connected with
the mechanical admixture of the colloidal silica with air seems to be
proved by the experiments of Brewster, showing that with increase of
density there was an increase in the refractive index from 1.111 in
specimens of the lowest specific gravity to 1.182 in those of the highest
specific gravity. Where the surface was hard and dense, Brewster found
the refractive index to approach that of semi opal. The wonderful thing
is that a substance so full of cavities containing gas should
nevertheless be transparent.
By the kindness of Mr. F. Rutley, F.G.S., I am able to supply a drawing
taken from one of my sections of tabasheer.
The accompanying woodcut gives some idea of the interesting structures
exhibited in some sections of tabasheer, though much of the delicacy and
fidelity of the original drawing has been lost in transferring it to the
wood.
In this particular case, the faint punctation of the surface may possibly
indicate the presence of air vesicles of a size sufficiently great to be
visible under the microscope. But in many other instances I have failed
to detect any such indication, even with much higher powers. The small
ramifying tubules might at first sight be taken for some traces of a
vegetable tissue, but my colleague, Dr. Scott, assures me that they do
not in the least resemble any tissue found in the bamboo. I have myself
no doubt that it is an inorganic structure. It is not improbably
analogous to the peculiar ramifying tubules formed in a solution of water
glass when a crystal of copper sulphate is suspended in it, as shown by
Dr. Heaton (Proc. Brit. Assoc., 1869, p. 127). Similar forms also occur
on a larger scale in some agates, and the artificial cells of Traube may
probably be regarded as analogous phenomena.
The aggregates of globular bodies seen in the section so greatly resemble
the globulites of slags and natural glasses, and in their arrangement so
forcibly recall the structures seen in the well known pitchstone of
Corriegills in Arran, that one is tempted to regard them as indicating
the beginnings of the development of crystalline structure in the
tabasheer. But I have good grounds for believing the structure to have a
totally different origin. They seem in fact to be the portions of the
mass which the fluid Canada balsam has not succeeded in penetrating. By
heating they may be made to grow outward, and as more balsam is imbibed
they gradually diminish, and finally disappear.
I must postpone till a future occasion a discussion of all the structures
of this remarkable substance and of the resemblances and differences
which they present to the mineral opals on the one hand, and to those of
the opals of animal origin found in sponge spicules, radiolarians, and
the rocks formed from them, some of which have recently been admirably
investigated by Dr. G.J. Hinde (Phil. Trans., 1885, pp. 425-83).
I cannot, however, but think that it would be of the greatest service to
botanists, physicists, and mineralogists alike, if some resident in India
would resume the investigations so admirably commenced by Dr. Patrick
Russell nearly a century ago; and it is in the hope of inducing some one
to undertake this task that I have put together these notes. There are
certain problems with regard to the mode of occurrence of this singular
substance which could only be solved by an investigator in the country
where it is found.
[Illustration: SECTION OF INDIAN TABASHEER, SEEN WITH A MAGNIFYING POWER OF
250 DIAMETERS.]
Most parcels of the commercial tabasheer appear to contain different
varieties, from the white, opaque, chalk like forms through the
translucent kinds to those that are perfectly transparent. It would be of
much interest if the exact relation and modes of origin of these
different varieties could be traced. It would also be important to
determine if Brewster was right in his conclusion that the particular
internodes of a bamboo which contain tabasheer always have their inner
lining tissue rent or injured. The repetition of Dr. Russell's experiment
of drawing off the liquids from the joints of bamboos and allowing them
to evaporate is also greatly to be desired. My colleague, Prof. Rucker,
F.R.S., has kindly undertaken to re-examine the results arrived at by
Brewster in the light of more recent physical investigations, and I doubt
not that some of the curious problems suggested by this very remarkable
substance may ere long find a solution.