Med.'), otherwise so correctly executed, could not have failed to urge upon him the conviction that the oxalate of lime crystallizes in the quadratic and not in the cubic system.

There is sufficient evidence to show that other authors were aware of the fact, that the system in which the oxalate of lime crystallizes, is not the regular or cubic system. In the third edition of Dr. Prout's work on Stomach and Urinary Diseases,' plate i, fig. 6, there is a diagram, to which there is the following explanation on the next page: "Fig. 6 represents a flat octahedron; the form stated by Wollaston and others to be that most frequently assumed by crystallized oxalate of lime. According to the recent observations of Mr. Brooke, however, some natural crystals of the oxalate of lime, containing one proportion of water, presented the form of an oblique rhombic prism. Whether the salts thus appearing in two different forms are the same in composition, or whether the octahedral form has been erroneously assumed, is not ascertained." See 'Lond. and Edinb. Philos. Mag., vol. xvi, p. 449 (1840). Funke, in pl. i of his 'Atlas der physiologischen Chemie,' figures some oxalate of lime. The (artificial) crystals were small. He saw some octahedra with one of their lateral angles towards the observer, similar to the four lower diagrams of Dr. Beale's fig. 185, and concluded that they were quadratic octahedraan opinion which is certainly justified by the more common appearances; and, moreover, that they were of the variety in which the vertical axis is the longest, which is wrong. It is wrong for the following reason: If the vertical axis is longest, the other two axes must be equal, which they are not. On making the two equal axes the horizontal ones, the shortest axis becomes the vertical.

Dr. G. Bird, at p. 221, quotes an experiment of the American physician already mentioned, Dr. Bacon, of Boston, by which this observer obtained four-sided prisms of pure oxalate of lime. The experiment runs thus, § 225: "Dr. Bacon examined the effects of the different acids upon artificially prepared oxalate of lime. He ascertained that when dissolved in hydrochloric acid . . . and allowed to evaporate spontaneously, a mixture of transparent rhomboidal plates, minute octahedra, and four-sided prisms, often arranged in rosettes, with zeolitic crystals, were obtained. The rhomboids and zeolite groups acted powerfully on polarized light, the prisms less strongly, and the octahedra not at all. On submitting these crystals to analysis, the rhomboids were found to contain hydrochloric acid, but the prisms and octahedra were pure oxalate of lime." Now the occurrence of

these prisms is perhaps the strongest possible reason that could be advanced why the oxalate of lime cannot crystallize in the cubic system, leaving dimorphism out of the question for the present. The cubic system, by the nature of its axes, three axes of equal length, and at right angles to each other, precludes the possibility of a prism or column ever occurring in it. Besides, even had it been possible for the prisms to be in the regular system, they could not have polarized.

It is a matter of great surprise that Dr. G. Bird should have overlooked the incompatibility of the prism with the regular or cubic octahedron. Whether his fig. 40, representing the "square prisms with a four-sided pyramid at each end, forming a dodecahedron" (a term quite unusual in crystallography for this combination, which certainly has twelve planes, but not equivalent ones, a requisite for a dodecahedron), is drawn from specimens under his own observation, is not stated. Perhaps they are given after Dr. Bacon. Here we must, however, state that the crystals obtained from a solution of oxalate of lime in hydrochloric acid, do not at once permit a conclusion to be drawn upon the primary crystallizing form of the oxalate of lime, inasmuch as they may be a mixture of the above-described double-salt of chloride of calcium and oxalate of lime, plus water of crystallization, and of oxalate of lime and oxalic acid.

Prismatic crystals of oxalate of lime were first observed by Brooke as already mentioned. They were then described by C. Schmidt (Ann. d. Chem. und Pharm.,' lxi, p. 304), who says that he measured them, and found them to belong to the fifth system of crystallization, the oblique, or monoclinoedric (Naumann), or hemiorthotype (Mohs), viz., the system in which two of the axes are at right angles to each other in an horizontal plane, and the third inclined but in a vertical plane passing through either of the other two. It is, however, most improbable that these measurements were correctly made, inasmuch as nobody has ever since observed any oxalate of lime in the oblique system. In the urine, prismatic crystals of oxalate of lime were first observed by Dr. F. W. Beneke (Zur Physiologie und Pathologie des phosphorsauren und oxalsauren Kalkes,' 1850). He described them as quadratic prisms, with pyramidal terminations, and gave good drawings of all the forms of oxalate of lime on the first table appended to his publication. According to Ch. Gerhardt, ('Lehrbuch der organischen Chemie,' 1854, German edition by R. Wagner, Nürnberg, vol. i, p. 285,) oxalate of lime occurs in plants in the shape of quadratic or pyramidal octahedra, which are frequently combined

with the quadratic prism. The quadratic octahedron may be the primary one (P: P = 46° 28′), or an obtuse one P: P119° 34') (123° 19′ Rose). The measurements seem to be those of Schmidt, adapted to the pyramidal system. They can therefore not be relied upon. It is very doubtful whether the primary octahedron does ever occur.

Oxalate of lime is represented as crystallizing in quadratic octahedra by E. von Gorup-Besanez, p. 182 of the second edition of his 'Anleitung zur zoochemischen Analyse,' 1854. He says, that the long or pointed octahedra are of rare Occurrence. Robin and Verdeil, pl. vi, figs. 2 and 3, of their 'Atlas' represent pyramidal octahedra.

Preparation of objects for microscopical observation.-It is perhaps well to mention here, that all precipitates of the oxalate of lime used for any experiment were carefully washed until they yielded no more traces of their soluble components, so that no crystallization could have taken place under the influence of the evaporation of the water surrounding the object on the slide. The oxalate of lime has very little affinity for water, and any preparation of it not permanently mounted gets dry very fast; for this reason I always take a flat cell of Brunswick black for every specimen, which not only allows a larger amount of fluid to remain with the crystals, but can also be converted into a temporary mounting, simply by warming it a little, and pressing the circular thin glass cover gently against the cell, whereby it closely adheres to the latter, and precludes evaporation. The object may be mounted permanently if desired, by surrounding the cell with Brunswick black. A little preserving fluid should therefore always be put into the cell, if the observer has the intention to make permanent objects.

Crystals of the oxalate of lime are obtained from the urine of children a few hours after a meal of rhubarb. The sediment after being washed consists of numerous crystals of all sizes, the large ones being particularly favorable for examination. Mixed with these are in most cases numerous ellipsoid bodies, more rarely some dumb-bells. It is partly on specimens of this description that the following observations on the polarizing power of the oxalate of lime were made. The oxalate which occurs in the urine pathologically, be it produced in the kidney, bladder, or after emission of the urine, is generally of a smaller size, and mixed with other matter, and therefore is not so favorable an object for microscopical examination. Very large crystals occur in the urine of the horse, but here they are mixed with a large

quantity of stellar crystals of the carbonate of lime. The latter is best dissolved in dilute acetic acid, which leaves the oxalate undissolved.

Artificial oxalate of lime is best prepared by mixing a solution of an oxalate-for example, oxalate of ammonia-with a soluble and dissolved preparation of lime-for example, chloride of calcium, lime-water, or phosphate of lime (in the urine). Precipitated from concentrated solutions, the oxalate of lime appears in seemingly amorphous lumpy masses, as they are described by most writers; by taking, however, very dilute solutions, and mixing them at a higher temperature, close imitations of the urinary crystals may be obtained. The admixture of a soluble oxalate to urine gives a great variety of crystals, some of which are described in the following pages.

On submitting the crystals to the microscope, it is necessary to use a sufficiently high power. Though a half-inch object-glass is quite sufficient for recognising the characteristic crystals, yet the quarter-inch is stereoscopically better, that is to say, because under a higher power fewer points are in focus at a time, and the observer becomes more conscious of the relations of the different points, sides, edges, and angles of the crystals, because they have more relief, and appear as bodies, and not as flat markings, as they do under lower powers. The crystals obtained from plants, such as the cuticle of the onion and others, are generally much larger than those obtained from animal fluids, and therefore require lower powers, though under higher powers they are seen to great advantage.

Pure artificial oxalate of lime, from oxalate of ammonia and chloride of calcium.

Experiment 1.-To a boiling concentrated solution of oxalate of ammonia, a concentrated solution of chloride of calcium was added, whereby a copious white precipitate was produced. Under the microscope, this precipitate consisted apparently of irregular lumpy masses, but on the application of higher powers, and on taking single particles for examination, it was seen that these were composed of four smaller roundish particles each, so as to represent a cross with rounded-off angles. Mixed with these were small squares, and crosses lying on squares. When the test had cooled by standing over-night, prisms with pyramidal ends could be discovered here and there. The lumpy masses polarized white

light; the prisms red, blue, and violet. See the small crossed and large coloured prisms on the right side of fig. 4, Pl. V.

Experiment 2.-To a concentrated solution of oxalate of ammonia of ordinary temperature, a solution of chloride of calcium was added. The precipitate obtained consisted of the same particles as in the first experiment, but they were rather larger and better defined. There were, however, no prisms in this precipitate, which polarized light between two Nichol prisms without variegation of colour.

Artificial oxalate of lime from water.

Experiment 3.-The water of the New River Water Company, with which my house is supplied, contains in solution, like most water, a certain proportion of carbonate of lime. I therefore thought it suitable for an experiment with the oxalate of ammonia. As I wanted crystallization to go on as slowly as possible, I did not take a solution of the oxalate of ammonia for precipitating the lime, but dropped a crystal of about two grains in weight in about an ounce of water contained in a test-tube, and allowed it to stand over-night. I obtained a white thin deposit, consisting of innumerable crosses, being prisms with pyramidal ends crossed at right angles.

The appearances are represented in fig. 4, Pl. V, on the left of the coloured prisms. Some prisms were single; in other cases a great many prisms were crossed at various angles. But the crosses with right angles were the ground form. All the crystals polarized light without much colour.

This experiment gave me the key to the explanation of the nature of the bodies found in the first two experiments. The lumpy masses with four indentures are in fact nothing but crossed prisms, whose ends, however, are irregular, round, sometimes thickened; the angles are frequently so much filled up, that a round body is produced, which it would be difficult to give an account of, but for the other crystals. Here we have then in the first instance crossed prisms, the angles and edges of which become rounded off, and the corners filled up, and that the more, the more the formation of the crystals is hastened, and the more concentrated the solutions are out of which they form. The roundest body from an oxalate of lime precipitate will, on careful examination, yet exhibit four points corresponding to the places where under more favorable circumstances there would have