Chapter 10

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.

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.

JOHN W. JUDD.

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THE EDIBLE EARTH OF JAVA.

In 1883 Mr. Hekmeyer, pharmaceutist in chief of the Dutch Indies, exhibited at Amsterdam some specimens of Javanese edible earth, both in a natural state and in the form of various natural objects. A portion of this collection he has placed at our disposal, and has given us some information regarding its nature, use, etc.

These clays, which are eaten not only in Java, but also in Sumatra, New Caledonia, Siberia, Guiana, Terra del Fuego, etc., are essentially composed of silex, alumina, and water in variable proportions, and are colored with various metallic oxides. They are in amorphous masses, are unctuous to the touch, stick to the tongue, and form a fine, smooth paste with water. The natives of Java and Sumatra prepare them in a peculiar way. They free them of foreign substances, spread them out in thin sheets, which they cut into small pieces and parch in an iron saucepan over a coal fire.

Each of these little cakes, when shrunken up into a little roll, looks somewhat like a grayish or reddish fragment of cinnamon bark. The clay is also formed into imitations of various objects.

We have tasted this Javanese dainty, and we must very humbly confess that we have found nothing attractive in the earthy and slightly empyreumatic taste of this singular food. However, a sweet and slightly aromatic taste that follows the first impression is an extenuating circumstance.

According to the account given by Labillardiere, confirmed by the information given by Mr. Hekmeyer, the figures are often craunched by women and children, to the latter of whom they serve as dolls, toys, and even money-boxes, as shown by the slits formed in the upper part of the larger objects, which are usually hollow.

We have not sufficient documents to carry us back to the origin of that tradition that would have it that the human form has been given to certain food preparations from remote times. Savants will not be slow to see in this a vague relic of the horrible festivities that succeeded human sacrifices among primitive peoples. For want of prisoners and of designated victims, a symbolic representation would have gradually developed, and been kept up, though losing its religious character. We merely call brief attention to this obscure problem, not having the pretension to solve it.--_Revue d'Ethnographie_.

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