SECRETION OF URINE. 643 



Oxalatc of the protoxide of lead, Urea and Allantoin ; the composition of which last substance 

 is very dillcrent from that of urea or uric acid, being 8 Carbon, 5 Hydrogen, 4 Nitrogen, and 

 5 Oxygen. 



d. By the operation of Nitric Acid upon Uric acid, several new products are generated, 

 some of which are of much practical interest. To one of these the name of Murexid has 

 been given, on account of its reddish purple colour (resembling that of the Tyrian dye which 

 was obtained from a species of Murex) ; this is a crystalline subslance, sparingly soluble in 

 cold water, but copiously soluble in warm, imparting to it its vivid colour. By Dr. Prout it 

 was long since described as consisting of a peculiar acid, the Purpuric, in combination with 

 Ammonia ; this view of its composition is not generally received by German Chemists ; but 

 it has lately been supported by Fritzche, who has shown the real existence of the acid, by 

 obtaining Purpurates of other bases. This substance is one source of the colours of the 

 pink and lateritious sediments which so often present themselves in the Urine ; these hues 

 partly depend, however, on the influence of nitric acid upon the peculiar Colouring princi- 

 ples of the urine, the nature of which principles is not yet fully understood. 



846. It was supposed until recently, that Lactic acid is a normal constitu- 

 ent of Human Urine. It appears to have been demonstrated by the experi- 

 ments of Liebig, however, that this is not the case ; and that another organic 

 substance, which forms a crystalline compound with zinc, very similar to the 

 lactate, has been mistaken for it. The composition of this substance, which 

 usually forms about one part in 200 of Urine, has been recently determined 

 to be 8 Carbon, 8 Hydrogen, 3 Nitrogen, and 3 Oxygen. It thus differs from 

 Lactic acid in containing Nitrogen ; as well as in proportion of its other com- 

 ponents. 



847. It has been shown ( 843). that the Urine contains a considerable 

 amount of Saline matter; the excretion of which from the system appears to 

 be one of the principal offices of the Kidney. Various saline compounds, 

 and the bases of others, are being continually introduced with the food ( 648) ; 

 and these, after performing their part in the organism, must be eliminated from 

 the circulating fluid, in order to prevent injurious accumulation. Of these 

 we shall now examine the chief sources. The mode in which the Muriates 

 find their way into the Urine is easily understood. Of the Common Salt in- 

 gested, a considerable part is decomposed into Muriatic Acid and Soda ; the 

 former being found uncombined in the Gastric juice ; and the latter in the Bile. 

 By the mixture of the Bile with the Chyme, a re-union of these two consti- 

 tuents takes place ; and Salt is again formed, which is received into the Cir- 

 culation that it may be eliminated (its part in the economy having been now per- 

 formed) by the Kidney. The quantity of the Sulphates present in the Urine, 

 appears to have no relation with that of the amount of Sulphuric acid ingested ; 

 for it much surpasses what could be thus accounted for, being often consi- 

 derable, when no Sulphate whatever can be detected in the food. But most 

 of the azotized compounds employed as food have Sulphur in combination 

 with them ; and there can be no doubt, that this undergoes oxidation within 

 the system, and thus generates Sulphuric acid, which unites with any free 

 or weakly-combined bases it may meet with, to form the Sulphates present in 

 the Urine. The Phosphates are probably derived in part from the Phos- 

 phates taken in with the food, and in part from the free Phosphorus, which 

 its elements contain. Of the latter, great use is made in the production of 

 Nervous matter ( 249) ; the continual waste of which must set it free again. 

 When thus set free, there is obviously no channel for its elimination, save by 

 its conversion into Phosphoric acid, and its union with an alkaline base.* 

 That this is really the case, would appear from the fact noticed by Dr. Prout, 

 and confirmed by many others, that mental or bodily labour which involves 



* This circumstance has been entirely overlooked by Liebig, in his late discussion (foe. 

 a/.) of the Constitution of the Urine ; the Phosphates being regarded by him as having their 

 sole origin in the Phosphates of the ingesta. 



