PHOSPHATES. 209 



chloric acid. The dumb-bell form gives rise to calculi of 

 the bladder. 



Oxalic acid and its salts are found in many fruits and 

 vegetables, like tomatoes, celery, rhubarb, etc., and 

 when these are eaten it appears as the calcium salt in 

 the urine. It is also produced in the body from certain 

 foods, as from large quantities of nitrogenous foods or 

 from the carbohydrates, where the oxidation is not com- 

 plete. A small amount, then, may be normal, and if it is 

 transitory is of no great consequence. If the excretion is 

 continual it is due probably to some constitutional weak- 

 ness. 



PHOSPHATES. 



The phosphates of the alkalies, being readily soluble 

 in water, do not appear as urinary sediments. The phos- 

 phates of calcium and magnesium are insoluble in water 

 or alkalies, although they dissolve in acids. They, conse- 

 quently, appear as sediments whenever the urine becomes 

 alkaline, but are not found in acid urine unless the acid 

 reaction is very faint. They can be distinguished from 

 other urinary sediments by dissolving in acetic acid with- 

 out effervescence. 



Triple phosphate, NH 4 MgP0 4 , is a salt of phosphoric 

 acid having two bases, ammonium and magnesium. When 

 it is made by precipitating a phosphate by ammonia and 

 magnesium sulphate the crystals are usually stellate or 

 snow-flake formed. As it is made in the urine, however, 

 they are more commonly in the form of rhombic prisms. 

 The terminations of the prisms are commonly truncated; 

 so that the crystals have a shape which approaches that 

 of the end of a coffin, and this gives rise to the common 

 appellation: "coffin-lid crystals." (Plate II, 8.) The 



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