some difference of opinion exists between Raspail and Mohl's description; the former asserting that they are six-sided prisms, terminated at each end by a pyramid with the same base; whilst the latter describes them as right-angled four-sided prisms, vanishing into points. It is a difficult matter to decide between these two opinions, if an entire crystal be the subject of examination; for, even if magnified 1000 times, the figure is not clearly defined: but, by having recourse to some delicate manipulation, the proper shape can be then ascertained, when it can be shown that neither of the two opinions is correct in all points.

"Raspail's reason for considering the acicular crystals to be hexagonal prisms arises from the appearance they present with transmitted light, when some (but not all) are seen to exhibit two dark margins and a streak of light between them, which extends the whole length of the crystal: from this he reasons that its figure is six-sided, the lateral planes reflecting the light which impinges upon them, and consequently are seen as darkened margins, whilst the surfaces which are superior and inferior, being in a position favourable for the transmission of the rays, are transparent. This is the argument brought forward in Raspail's Organic Chemistry, in favour of the hexagonal figure, but there are no attempts to prove if any other form or position of an acicular crystal could not present a similar aspect, which is the case, as will be shown hereafter.

"As respects the summit being a six-sided pyramid, its existence does not seem discoverable, for the crystal from about its centre gradually vanishes to a point, having no angular interruption, such as is observed in the large crystals in the root of Iris florentina or the wood of Quillaja, where a regular right-angled prism is surmounted by a pyramid. Mohl's idea of the shape certainly approximates the truth more than Raspail's; but it can be shown, though the acicular crystals are four-sided, they are not always right-angled prisms, as he asserts. To witness these facts the crystals, must be obtained, by lengthened maceration, free of cellular tissue, and then crushed into fragments, when many will present an obliquely transverse fracture, which exhibits tour sides, some having the angles right angles, and others acute and obtuse angles; in fact, the transverse section of such would resemble frequently a rhombus To examine more satisfactorily the fractured ends of these minute crystals, which scarcely measure the To, part of an inch, it is most convenient to place their fragments in a watch glass, with a small portion of Canada balsam, and to heat the whole

till the balsam has acquired a viscid consistence, then let it be removed from the source of heat, and be stirred whilst the mass is cooling till sufficiently hard, when the broken Raphides will be sustained in it in all directions; and frequently a few can be observed erect, which cannot fail to give a true outline of their real form, which by this method appears to be a four-sided prism, the angles being frequently oblique, and in other instances to put on the rectangular condition. Occasionally some of the crystals are observed to exhibit a triangular section of the isosceles shape, which seems to be an anomaly: but, to account for this, it is to be remembered that it is not opposed to the laws of crystallisation for certain bodies to crystallise in halves, consequently the half of a four-sided prism, taken from one angle to the opposite one, would be triangular; and it is conceived that when a crystal exhibits this triangular section it has belonged to the compact bundle of crystals in which it has been constrained to assume this form, as being fitted to fill up certain spaces better than one of the oblique or rectangular shape.

"This being the observed figure, it is not difficult to account for the black margins observed in certain crystals, which may be occasioned by a rectangular prism resting obliquely, or by an oblique or triangular prism, as well as by the hexagonal; for, as the sides of either are not in the plane of direction of transmitted light, the rays are reflected, and that portion which reflects the light appears dark.

"Besides these methods of determining the form of acicular raphides, if a little alcohol be added to water containing them, and examined immediately after by the microscope, the crystals can be often seen to roll over and over, and some of them will, whilst revolving, present alternately a pointed and then a flat extremity, just as if a wedge were revolving on its axis; showing that such crystals are four-sided, but two of the sides only attenuated to form the pointed appearance, the other two suffering no diminution of their breadth at any point.

"That the four-sided prism is the ultimate figure of these minute crystals is rendered more probable by the occurrence of rhombohedral and rhombic prisms without pyramids, of the same composition, in the same plant, but of much greater widths; and there can be no doubt that these latter bodies and the acicular are two modifications of crystal of the same substance. The most decided proof of their being four-sided is obtained by pressing lightly on the piece of glass which covers them, whilst examined under the

microscope, when those which appear six-sided instantly appear four-sided, owing to the square crystal resting obliquely this can be seen in the minute crystals of Squill, or in the large square ones of Quillaja saponaria.

“The rounded masses, which may be termed Conglomerate Raphides in opposition to the acicular variety, seldom present more than the pyramid of each little crystal composing them; but in a few cases, where an opportunity is afforded of examining the prism, it can be seen to be rectangular and terminated by a foursided pyramid.

"Classes of Plants in which they are found.-No division of the vegetable kingdom seems without more or less quantities of these crystalline formations, which are found in a great number of Exogens and Endogens, and likewise in Acrogens, being visible in Ferns and Mosses, and, according to Unger, in the lowest of the Algaceæ, as Nostoc Muscorum, and Conferva crystallifera.

"The frequency of occurrence of these bodies is such, that it appears that, instead of those plants containing them being exceptions, those are to be considered such which have none in their tissues.

"It does not appear, from numerous observations, that the acicular and conglomerate Raphides are equally common in the several classes of Plants; but that Exogens contain perhaps the one kind as often as the other, while Endogens undoubtedly contain most often the acicular variety.

"Situation.—The position of these bodies has been a subject of controversy; Raspail asserting that they are always in the intercellular passages, whilst Turpin, Meyen, and Unger maintain that they are universally in the interior of the cells, which latter opinion is easily proved to be nearly correct by a little careful dissection. of any plant containing them.

"Raspail's advice to see these bodies is to tear a piece of the Hyacinth stem in a drop of water placed on the stage of the microscope, when numbers of acicular crystals will be visible (this method is not likely to show them in the interior of the cells); and from measuring he finds the length of the crystal longer than the ordinary cells of the tissue; and therefore he decides from this, that they cannot be contained in the interior of the cells, while he overlooks the fact that the cell in which they are contained may be often dilated to five or six times the size of those composing the ordinary tissue of the plant. The square crystals in Quillaja saponaria appear as if loose in the plant, but they are really in a

cell, which cell is applied closely to the surface, the crystal completely filling it: when muriatic acid is added the crystal is dissolved, and the cell is left visible.

"The most ready method of determining that the acicular crystals are contained within the cells is, to take a piece of the bulb of Scilla maritima and macerate it until it becomes decomposed, or to take some of the rotten portions which are frequently on its exterior; and, by examining either of these with the microscope, it will be seen that there are numbers of isolated cells which contain crystals, which cells are five or six times larger than those of the tissue which have none within them; and, what appears remarkable, the crystals seldom occupy more than a small portion of the cell though it be so dilated, and in the Squill are usually collected at one end, probably by gravitation; but in the biforines they generally completely fill a small portion of the cell, about its middle, the ends containing none whatever.

"To prove the same fact as regards the conglomerate kind, let a piece of the root of Rhubarb, or a part of the frond of Zamia pungens, be boiled till the cohesion of the tissue be destroyed, when some of the separated cells will exhibit one cluster generally in each; but the containing cell is not larger than others of the same plant, and at times very little larger than the mass within it.

"There are some exceptions to Raphides being found constantly in cells, notwithstanding Unger's assertion that they are exclusively found in their interior, and that the vascular bundles have none within them: for they can be observed in the interior of the vessels in the stem of the Grape vine; and loose in the anthers, mixed with the pollen, in Hemerocallis purpurea, Anigozanthus flavidus, and many other plants; and they can be observed in the air cavities of and many aquatics.

"The interior of the Stem is the most common situation in the herbaceous plants for Raphides, and it used to be considered the only locality; but the epidermis of the stem of many plants displays thousands, as that of the Tradescantias, Opuntia crassa,

others.

"The Bark of many trees also contains them; they are readily observed in the layers of the Lime tree bark, of two kinds: also in the barks of Araucaria imbricata, Cascarilla, Cinchona, and various other plants.

"Even the Pith is not destitute of crystals; for the Grape vine exhibits them in that situation, as does the Lime tree in the medullary rays, which are processes connected with it.

"The Leaves of multitudes of plants contain the various kinds in great abundance; Pisonia, Hemerocallis, and Calla æthiopica furnishing the acicular, whilst Rheum palmatum and undulatum are common examples yielding the conglomerate.

"The Stipules are not without Raphides, for those of the Grape vine show them in situ, but very small in size.

"The Sepals of many Orchidacea abound with crystals, as those of Bolbophyllum fuscum and others, and especially the horny labellum of Catasetum.

"The Petals of many plants, like the sepals, contain more or less crystalline matter, which is particularly evident in the small corolla of the Grape vine.

"The Fruit does not so often contain them, yet the common Grape furnishes a sufficient evidence of their existence in that organ.

"In the Root their presence can be easily discovered, especially in all Rhubarbs, varying a little in number from the locality of the specimen; most in Turkish, less in East-Indian, and least in British-grown specimens.

Of

"Number in a Cell. The number of Raphides in any cell is subject to much variation. It is seldom that a single crystal is met with; but in the Squill, Calla æthiopica, and other examples, besides the multitude of acicular crystals, some cells which are not dilated occasionally exhibit only one minute rhombohedron, as has been observed by Unger in Papyrus antiquorum. the conglomerate kind, one cluster is the usual number in the respective cells, though in Zamia pungens two such can be at times observed within the same cell. The acicular Raphides are in the greatest numbers, and vary somewhat in quantity in different cells and in different plants; some containing but very few, whilst others contain hundreds.

"Proportion to the Weight of Tissue. The mass of crystalline matter that is formed in the tissue of some plants is prodigious, whilst in others the quantity is very thinly diffused. In several species of Cactaceæ the crystals equal if not exceed the weight of dried tissue; this is especially the case in Cereus senilis. In Turkey rhubarb root, one hundred grains yielded between thirtyfive and forty grains of Raphides; and the fact of various rhubarbs giving different feelings of grittiness to the teeth when chewed, is said by Guibourt to be employed as the test of their goodness. In the bulb of Scilla maritima, not more than ten grains are contained in the same weight of dried tissue.