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As soon as the powerful hooks of the fifth segment appear, the entire area from the alimentary canal to the body wall-is occupied by their muscular apparatus. This consists of a dense series of fibres, which slant from the matrix of the bristles superiorly upward and outward to decussate with the fibres at the upper and outer angle of the body-wall. A still stronger series of fibres occur in the inferior division; the inner are nearly vertical, the rest incline downward and outward. It would appear, therefore, that this powerful muscular mass chiefly acts on the hooks, so as to bring their curved points against the wall of the tube or tunnel. The strong inferior fibres likewise gain additional purchase by passing through the ventral longitudinal layer to be attached to the basement-tissue of the hypoderm. In this region the nervecords form two almond-shaped bodies in transverse section in the ventral hypoderm, and they are separated by a distinct interval.

Posteriorly the nerve-cords still remain separate, and a large neural canal lies between them. A well-marked pennate process of the ventral longitudinal muscle occurs at its inner (median) edge. The foregoing forms, in conclusion, were compared with the structure of Maa mirabilis, Johnst., an aberrant member of the Spionidæ.

2. Note on Circular Crystals. By E. W. Dallas.

At long intervals notices of circular crystals have appeared before this Society. In 1853 Sir David Brewster read a paper on the subject, which followed one by Mr Fox Talbot in 1836, and which again had been preceded by one from Sir David Brewster about twenty years before. It is not easy to account for these long intervals, unless they may be attributed to difficulty and uncertainty in manipulation, for except in very few instances the crystals observed by Sir David Brewster are of microscopic size, and, he remarks, require the perfection of optical appliances for their observation, and naturally so when crystals of the 200th of an inch in diameter are looked upon as of respectable size.

Some time ago, being occupied with the subject, I found that by impeding crystallisation by means of gum arabic, circular crystals were formed of a greater size.* This took place with certain salts, * Perfect crystals were exhibited up to two inches in diameter.

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but not with all that were tried. Among the successful instances may be mentioned sulphate of copper, binacetate of lead, muriate of morphia, and other similar salts, which afford beautiful crystals, and are very easily manipulated. The method of proceeding is similar in all; for instance, to a solution of sulphate of copper, let gum arabic or common dextrine be slowly added until it pours oily, then tried, and more gum or more salt added until the result is satisfactory. No precise general rule can be given, as each salt will be found different in the quantity of gum required. The solution is then to be spread thinly over a plate of glass, and dried rapidly before a clear fire, or, if the plate is small, it may be dried over a gas-burner. Upon cooling, the plate, if left to itself, soon shows numerous small specks, which will gradually develope themselves into circular crystals. The process is hastened and a better result obtained by breathing on the plate, when, after a short time, they may be observed to start out very beautifully. These crystals while growing are extremely sensitive, any variation in the moisture applied for their formation resulting in the production of rings.

The growth of the crystals once begun is extremely regular. When the centre is of inappreciable size they are circular, and they proceed onwards in that form until stopped by other crystals, or until the whole vacant space is occupied by them. Should the origin have a definite shape, then that is carried on by the crystals arranging themselves always perpendicularly to the outline of that origin, while, should it be a straight line, they form beautiful fringes perpendicular to the line, and terminate at each extremity in semicircles.

The centres round which the crystals arrange themselves may be either some foreign body in the film, or be determined by some molecular arrangement of the salt at a particular point in the film. They seem to originate spontaneously, and subject to no apparent rule, for foreign particles, and even minute crystals, will not always determine centres; in fact a film may be full of little crystals of the salt, and to very few of them can the circular arrangement be traced.

The crystals present themselves under two aspects in all the salts that I have examined,-a true and an abnormal form. I designate the true form as that in which the crystallisation proceeds by the formation of spicular crystals radiating from a centre and

in optical contact, while in the other many different forms may be observed, in which the component crystals are more or less of a laminate structure, often presenting most beautiful appearances. I have made this distinction, having observed that in some cases at least the true crystals are permanent, while the laminate are not so under like conditions, but change into the true form by time, or may become altogether disintegrated in a damp atmosphere into a confused mass, having few optical properties. The crystals also possess distinctive optical properties.

The production of either of these classes of crystals appears to depend on two conditions, namely, on the thickness of the film, and on the amount of moisture applied in their production,-a thin film and a moderate supply of watery vapour inducing the true form, while a thick film and an increased quantity so alter the structure that at last, although the crystallisation may proceed from a centre, the circular character is entirely lost.

Spherical crystallisation, to which the circular is to be referred, is very frequent in mineral substances. It may be seen also in the wellknown experiment of the rapid crystallisation of a supersaturated solution of acetate of soda, by the introduction of a centre round which the salt forms a spherical mass, and the surface, when the action has ended, presents all the appearance of a circular crystal.

However difficult it may be to account for the origin of crystals, their growth in a circular form, when once the centre is determined in a film, is very obvious in those cases where they are produced. In the preparation of the film, not only is the superfluous solvent rapidly evaporated by heat, but a considerable part of the water of crystallisation is also driven off, and there is left on the glass plate a film of an amorphous substance, which, either by attracting moisture spontaneously from the atmosphere, or by having it added, allows the salt, whatever it may be, to resume its crystalline form. That this is the case may be seen from the fact that the crystallisation will take place, and that in a circular form, if the drying of a plate is stopped just at that point when there is sufficient water left to enable the crystals to form when the plate is cooled. In this case their formation is a repetition of the acetate of soda experiment already alluded to. A plate may sometimes also be dried and crystallised, and on being again exposed to heat the crystals

will disappear, and the plate may be re-crystallised, but not so well as at first, and not from the same centres. This is the case with

the binacetate of lead.

There is one remarkable form of these crystals which is of frequent occurrence, and which Sir D. Brewster seems to have observed only in mannite. The form looks very extraordinary, a properly prepared plate presenting the appearance of being covered with paraboloids. These are simply circular crystals which have been formed in the film while the general crystallisation has proceeded from one side, and is caused by the crystals overtaking one another in their onward progress in one direction. Taking this case in its simplest form by supposing equal rates of crystallisation proceeding from the edge of a plate and from a centre near the edge, their line of contact will necessarily be a parabola, for it is evident the edge is a directrix and the centre the focus of such a figure, but this will very seldom occur, since the growths are not only generally unequal, but also vary in themselves.

The question may be put whether gum arabic is the substance best suited for these experiments. I have not tried many, and with those that I have experimented upon the results were not so satisfactory. That other agents may be employed, according to the salts or other substances to be treated, is certain; for example, collodion, as may be seen in a coated photographic plate that has been allowed to dry after excitation in the nitrate of silver bath, when circular crystals of iodo-nitrate of silver are often produced. This salt, be it observed, although produced through the agency of water, being at once decomposed by that element.

Having confined my observations mainly to a very small number of salts, it would be premature to offer any general conclusion on the structure of their circular crystals or on their optical properties; besides, from the great interest Professor Tait has shown in the subject, I am in great hopes that he may be induced to make some investigation in it. I shall only mention one point that I have observed in the effect of some crystals on the black cross, something in their structure producing a more or less spiral arrangement of the arms in a horizontal direction, while on one occasion a vertical arrangement was observed, in which the arms seemed to be raised one above the other like four quadrant steps. This effect I have only

seen once in a crystal of sulphate of magnesia, and have not been able to reproduce anything of the kind, but the crystallisation of that salt is so varied and irregular that many things may pass unobserved.

The only other point I shall lay before the Society is, that I have succeeded in producing a crystallisation very similar to that of frostpictures on a window pane, and I hope to be able to make the imitation more perfect. For this purpose I have employed the sulphate of copper and magnesia,-a salt that crystallises under the rhombohedral system, the same with that of ice. This salt crystallises in the films from centres in a most remarkable manner in four different modes, viz., the true circular, the laminate, a branched or dendritic form, and another that I hardly know how to designate, unless it may be called the ostrich plume form. All these different forms may be observed on the plates, either simply or in combination, and produce most varied and singularly beautiful effects.

3. Preliminary Note on the Flame produced by putting Common Salt into a fire. By C. M. Smith, Esq. (Communicated by Professor Tait).

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