360 



EXCKETION. 



Fig. 116. 



held in solution. Its oxalic acid is in all probability gradually 

 formed, as we have said, in the urine itself; uniting, as fast as it is 

 produced, with the lime previously in solution, and thus appearing 

 as a crystalline deposit of oxalate of lime. It is much more probable 

 that this is the true explanation, since, in the cases to which we 

 allude, the crystals of oxalate of lime grow, as it were, in the cloud 

 of mucus which collects at the bottom of the vessel, while the 

 supernatant fluid remains clear. These crystals are of minute size, 



transparent, and colorless, 

 and have the form of regular 

 octohedra, or double quad- 

 rangular pyramids, united 

 base to bise. (Fig. 116.) They 

 make their appearance usu- 

 ally about the commence- 

 ment of the second day, the 

 urine at the same time con- 

 tinuing clear and retaining 

 its acid reaction. This depo- 

 sit is of frequent occurrence 

 when no substance contain- 

 ing oxalic acid or oxalates 

 has been taken with the food. 



OXALATE OF LIME ; deposited from healthy urine, AT j /? 



during the acid fermentation. At the end OI SOIHC days 



the changes above described 



come to an end, and are succeeded by a different process known as 

 the alkaline fermentation. This consists essentially in the decom- 

 position or metamorphosis of urea into carbonate of ammonia. 

 As the alteration of the mucus advances, it loses the power of pro- 

 ducing lactic and oxalic acids, and becomes a ferment capable of 

 acting by catalysis upon the urea, and of exciting its decomposition 

 as above. We have already mentioned that urea may be converted 

 into carbonate of ammonia by prolonged boiling or by contact 

 with decomposing animal substances. In this conversion, the urea 

 unites with the elements of two equivalents of water ; and conse- 

 quently it is not susceptible of the transformation when in a dry 

 state, but only when in solution or supplied with a sufficient quan- 

 tity of moisture. The presence of mucus, in a state of incipient 

 decomposition, is also necessary, to act the part of a catalytic 

 body. Consequently if the urine, when first discharged, be passed 

 through a succession of close filters, so as to separate its mucus, it 



