Treated by dilute nitric acid, the pulverulent matter is converted into rhomboidal crystals of uric acid,fig. 78. Collected on a filter and washed with rectified alcohol, the residue of the amorphous matter, treated with potassa, disengages ammonia, to be detected by the smell, test paper, and hydrochloric acid. The forms b and c cannot be confounded with any other substance ; the pulverulent form might be mistaken for phosphate of lime, and the modes of distincruishinn. the two precipitates are explained with'. the description of the latter. The forms b and e (first described by Quevenne) we have repeatedly observed in the sediinent of urine containing lithates in abundance. Though mainly composed of urate of ammonia, the acid is apparently in union also with potass, soda, lime, and magnesia. Quevenne considers that they may in some sort be regarded as pro ducts of putrefaction, as they do not appear until the urine has stood for about three clays ; but this is decidedly' erroneous, as we have re peatedly seen the simple globular, and some times the stellate globular, forms in urine which had not stood twenty-four hours.
3. The oxalate of lime calculus is generally more or less accurately spherical in shape ; though conimonly of rather small or moderate size, it may acquire very considerable bulk : a model of a mulberry calculus, now before us, (Univ. Coll. Museum) measures 71 inches in circumference, and some of the prominences on the surface reach five lines in height. Of dark brown, purplish, blackish, or olive colour, this species of calculus is remarkable for the-rough tuberculated character of its surface, which gives it the aspect of a mulberry. Its section exhibits commonly a granular, but soinetinies a laminated, arrangement; the internal colour is the same as the external ; the density and hardness (especially of the laminated variety') vary considerably. The dark colour of these calculi is generally ascribed to admixture of the colouring matter of blood, thrown out from the irritation their rough surface produces on the tissues it comes in contact with. The oxalate of lime calculus may occur of pure white colour, with sharp angular crystals on the surface ; of this rare variety the University College Col lection contains a remarkable specimen.
The rarity of oxalate of linae crystals in uri nary deposits was matter of received opinion until the inquiries of Dr. G. Bird led him to the inference that in the cases of disease oc curring in London . . . the oxalate of lime is of far more frequent occurrence in urine than the deposits of earthy phosphates.* The oxa
late deposit, when in abundance, appears to the naked eye, after the application of gentle heat, as a white glistening powder, which under a low magnifying power resolves itself into " crys tals of the oxalate in beautifully formed trans parent octohedra,with sharply defined edges and angles (,fig. 80, a), It sometimes happens that concerning oxalate of lime crystals. They (oc tohedral form) are of somewhat more frequent occurrence in females (14 of 42) than in males (11 of 42). They are most frequently present in acute affections and in ailmia ; and at that period of acute affections when anwmia is most likely to be fully developed, at the onset of convalescence. They occur in spermator rhea temporarily. Their frequency in rheuma tism has been exaggerated, our proportion being only 3 of 15 cases. They are not present in all cases of any given disease, and probably originate in some special condition of the blood. Observation continues to exhibit to us the frequency of a deposit of oxalate of lime crystals, at the period of convalescence of acute diseases ; so much so that we regard their sudden appearance in an acute disease as a sign of that fortunate change. This deposit is of temporary (say a few days) duration, and not to be confounded with the more or less permanent condition appertaining to a peculiar diathesis.
4. Of the cystin or cystic oxide calculus there are two varieties, physically considered; and the physical peculiarities are probably re ferable to chemical differences. The pure cystin calculus is of oval shape, acquires mo-. derate size ; its surface, tolerably smooth, has a crystallised aspect. Internally it appears formed of a multitude of irregularly aggregated crystals, with their edges rounded off,' and has the colour and shining look of bees'-wax fig. 81). Small portions broken off are semi transparent ; an ammoniacal solution gives thin lamellar hexagonal crystals by evaporation.
the oxalate is present in the form of exceedingly minute crystals ; it then resembles a series of minute cubes often adhering together like blood-discs : ;hese, however, are readily and distinctly resolved into octohedra under a higher magnifying power. If the cry stals be collected and ignited on platinum foil, oxalic acid is de composed and carbonate of lime left : the sub sequent addition of dilute nitric acid dissolves the residue with effervescence." The crystals are insoluble in boiling acetic acid or liquor potmsx.