460 PHYSIOLOGICAL CHEMISTRY 



fact that they dissolve in acetic acid with the evolution of carbon dioxide 

 gas. 



Calcium Phosphate (Stellar Phosphate). Calcium phosphate may 

 occur in the urine in three forms, i.e., amorphous, granular, or crystal- 

 line. The crystals of calcium phosphate are ordinarily pointed, wedge- 

 shaped formations which may occur as individual crystals or grouped 

 together in more or less regularly formed rosettes (Fig. 105, page 322). 

 Acid sodium urate crystals (Fig. 137, page 462) are often mistaken for 

 crystals of calcium phosphate. We may differentiate between these 

 two crystalline forms by the fact that acetic acid will readily dissolve 

 the phosphate, whereas the urate is much less soluble and when finally 

 brought into solution and recrystallized one is frequently enabled to 

 identify uric acid crystals which have been formed from the acid urate 

 solution. The clinical significance of the occurrence of calcium phos- 

 phate crystals in the urinary sediment is similar to that of " triple 

 phosphate" (see page 458). 



Calcium Sulphate. Crystals of calcium sulphate are of quite rare 

 occurrence in the sediment of urine. Their presence seems to be 

 limited in general to urines which are of a decided acid reaction. 

 Ordinarily it crystallizes in the form of long, thin, colorless needles or 

 prisms (Fig. 128, page 405) which may be mistaken for calcium phos- 

 phate crystals. There need be no confusion in this respect, however, 

 since the sulphate crystals are insoluble in acetic acid, which reagent 

 readily dissolves the phosphate. As far as is known their occurrence 

 as a constituent of urinary sediment is of very little clinical significance. 



Uric Acid. Uric acid forms a very common constituent of the sedi- 

 ment of urines which are acid in reaction. It occurs in more varied 

 forms than any of the other crystalline sediments (Plate V, opposite 

 page 380, and Fig. 136), some of the more common varieties of crystals 

 being rhombic prisms, wedges, dumb-bells, whetstones, prismatic 

 rosettes, irregular or hexagonal plates, etc. Crystals of pure uric acid 

 are always colorless (Fig. 122, page 380), but the form occurring in 

 urinary sediments is impure and under the microscope appears pig- 

 mented, the depth of color varying from yellow to a dark reddish- 

 brown according to the size and form of the crystal. 



The presence of a considerable uric acid sediment does not, of neces- 

 sity, indicate a pathological condition or a urine of increased uric acid 

 content, since this substance very often occurs as a sediment in urines 

 whose uric acid content is diminished from the normal merely as a re- 

 sult of changes in reaction, etc. Pathologically, uric acid sediments oc- 

 cur in gout, acute febrile conditions, chronic interstitial nephritis, etc. 

 If the microscopical examination is not conclusive, uric acid may be 



