When ammonia is added to fresh urine the triple phosphate is precipitated, and if it be then examined by the microscope it will be found to consist of beautiful stellate crystals, and to form a most attractive object. The presence of phosphoric acid can be demonstrated by the ordinary reagents.
Phosphate of lime dissolves in strong acids without effervescence. The presence of lime, as well as of phosphoric acid, can easily be verified by the usual tests.
Oxalate of lime. The principal crystalline forms of oxalate of lime, when it occurs as a urinary deposit, are the octahedral and the dumb-bell. Of these the most common is the octahedral. These octahedra (which have one axis much shorter than the other two) vary considerably in size, but there is reason to believe that the diversity in appearance which they exhibit is due to crystals of precisely the same shape occupying different
positions as to the direction of their axes, when examined by the microscope. There are a great many diversities of the dumb-bell form of oxalate of lime, which seem to be
derived from circular and oval crystals. The subjoined cuts illustrate the varieties of crystalline oxalates the most generally met with. When the crystals of oxalate are extremely minute, they are very liable to be overlooked, since they then appear as almost opaque cubes, and may not unnaturally be taken for urate of soda, to which they bear no slight resemblance; but from which they differ by being insoluble in potash or acetic acid, and not dissolving on the application of heat. We have already alluded to their resemblance to the dumb-bells of the earthy phosphates. Another distinctive feature is that the oxalates rarely sink to the bottom of the vessel, but are diffused through the mucous cloud, which forms in urine after a short time.
Oxalate of lime. (Beale.)
Cystine. Cystine is an occasional ingredient in urine, when it occurs as a whitish precipitate crystallised in hexagonal plates. At other times, but not so frequently, it is met with dissolved in the urine. It may be separated from the urine holding it in solution by the addition of an excess of acetic acid. Under the microscope cystine bears somewhat of a resemblance to uric acid, from which, however, it differs when under treatment with ammonia. When ammonia is added to cystine the cystine dissolves, but by the spontaneous evaporation of the ammonia remains behind in its original form; whilst, if the ammonia be allowed to escape under the same circumstance from the urate of ammonia which has been formed, this remains behind as an amorphous mass. Ammonia, therefore, dissolves the cystine without entering into chemical union with it. Potash also readily dissolves
cystine, as do also oxalic acid and the strong mineral acids. It is, however, insoluble in boiling water, in weak hydrochloric acid, and, as we have seen, in acetic acid.