MICROSCOPICAL DIAGNOSIS. 53 



excess of urea is present. Nitrate of urea crystals are colorless, 

 flat, rhombic or hexagonal plates, closely united to one another. 



To test for a deficiency of urea, take some of the urine, of. nor- 

 mal quantity, and reduce it to one-half its bulk by slow evaporation; 

 when cool add nitric acid as given above, and set in cool water, If 

 no crystals of nitrate of urea form in five minutes then the normal 

 amount is not present. This is a very simple method, is easily ap- 

 plied, and approximates the true results. 



Oxalate of Lime. Urine containing oxalate of lime has' 

 usually an acid reaction and a high color. The deposit is 

 scanty and generally conjoined with uric acid and the urates. 

 After the urine has been allowed to stand for a short time a 

 drop of the colorless, mucous-like deposit is placed on a slide 

 and examined with a high power of the microscope. The drop 

 of urine to be examined should be taken from a little above 

 the bottom of the vessel, for the mucous deposit at the bot- 

 tom appears to hold these crystals in its upper part ; they are 

 in the greatest abundance just in the upper part of the mucous 

 deposit. The crystals of oxalate of* lime are very character- 

 istic and cannot be mistaken for anything else found in the 

 urine. It is to be borne in mind that some of these crystals 

 are very minute indeed, appearing only as angular points. This 

 salt usually crystallizes in well defined octahedra, but sometimes 

 it is found in the dumb-bell form. The dumb-bells of oxalate 

 of lime are readily told from those of uric acid both by mi- 

 croscopic and chemical methods. The uric acid dumb-bell 

 is dissolved at once in dilute potash solution while the 

 oxalate of lime dumb-bell is insoluble even in boiling potash 

 solutions. Again, after the uric acid dumb-bell has been dis- 

 solved by the potash, if an excess of acetic acid be added 

 the characteristic rhombic crystals will appear. 



Oxalate of lime is insoluble in water, alcohol, alkalies, and 

 the vegetable acids, hence it can be readily distinguished from 

 the phosphates which are. soluble in acetic acid. 



This salt is soluble in the mineral acids and in the acid 

 phosphate of soda. The octahedra of chloride of sodium can 

 be distinguished easily from the oxalates by the fact that the 

 former are readily soluble in water, and that the urine must 

 be evaporated to show them. 



