If the amorphous urate be decomposed by a little hydrochloric acid, it will yield uric acid, easily recognised by its characteristic form under the microscope, or when treated with nitric acid and ammonia, will answer to the murexed test.
It sometimes happens that in testing an acid urine suspected to contain albumen, the urine may contain so large an amount of uric acid in solution that, upon adding a drop of nitric acid to it, a bulky precipitate of uric acid, exactly resembling albumen, is thrown down, and it may be erroneously regarded as this substance if examined under the microscope immediately upon its formation. Upon being allowed, however, to stand some time, and then placed under the microscope, the well-known crystals of the acid will reveal themselves.
In such urine no precipitate takes place when the liquid is heated, another essential feature in which it diverges from albumen.
Phosphates.—The urinary earthy phosphates occur under two varieties, viz. the phosphate of ammonia and magnesia, known as the triple phosphate, and the phosphate of lime.
In the engravings below, the principal crystalline forms of the triple phosphate are shown.
Fig. 1.—Crystals of triple phosphate with spherules of urate of soda. (Beale.)
Fig. 2.—Crystals of triple phosphate with triangular prisms, with truncated extremities. (Beale.)
Of these the triangular prismatic, with the truncated extremities, is the most common. In some cases the prisms are so much reduced in length as to resemble the octahedral crystals of oxalate of lime, for which they are sometimes mistaken by the inexperienced. When any doubt exists on this point it must be set at rest by having recourse to the chemical tests given further on. The triple phosphate is rarely met with alone, urate of ammonia, and sometimes uric acid and oxalate of lime, being present, although generally occurring in neutral or alkaline urine. The triple acid is sometimes found in that which is acid.