EXCREMENTITIOUS SUBSTANCES. 85 



the latter being found to replace the former, when more exercise is taken by 

 the subjects of them, whilst a still greater amount of exercise favors the meta- 

 morphosis of the uric acid into urea, by the higher oxygenation which the aug- 

 mented respiration will tend to produce. 1 When Uric acid is dissolved in dilute 

 nitric acid, and the fluid is evaporated until it assumes a reddish tint, the solu- 

 tion, being allowed to cool to 150, and being then saturated with ammonia, 

 deposits a substance which crystallizes in short four-sided prisms, that present 

 a garnet-red hue by transmitted light, but have a cantharides-green lustre by 

 reflected light. This substance has been named Murexide, on account of its 

 reddish-purple color, resembling that of the Tyrian dye which was obtained 

 from a species of Murex ; but it was long since maintained by Dr. Prout to be 

 in reality a Purpurate of Ammonia ; and this view of its composition, although 

 contested for some time, is now generally admitted the Purpuric acid, or 

 Murexan, being obtained in a separate form, and entering into combinations of 

 other bases. Murexide is one source of the colors of the pink and lateritious 

 sediments, which so frequently present themselves in the urine; these hues 

 partly depend, however, upon the peculiar coloring principles of that secretion 

 ( 64). It is by the formation of murexide that the presence of Uric acid is 

 best determined chemically; for the purplish-red residue which its solution in 

 nitric acid leaves on evaporation, suffices to distinguish it from every other 

 organic substance, except perhaps caffeine ; and whatever doubt may remain is 

 dissipated by the development of a splendid violet tint on the addition of caustic 

 potash, this being the result of the action of that substance on murexide. 



56. Owing to the almost complete insolubility of Uric Acid in water, so long 

 as it remains uncombined, it could not be a constituent of the urinary secretion, 

 unless held in solution by some other substance. This substance appears from 

 the researches of Liebig, which have been confirmed by other Chemists, to be 

 phosphate of soda, which yields up a part of its base to form an acid urate of 

 soda, and is itself converted into a superphosphate. This urate of soda is not 

 possessed of any great solubility, especially in cold water ; and thus it may be 

 precipitated, if present in excess, when the temperature of the urine is lowered, 

 although it was in a state of perfect solution at the time that the urine was void- 

 ed. An augmentation of the normal amount of uric acid in the urine almost 

 always takes place when there is any febrile disturbance in the system ; and it 

 may be so great as to prevent the whole of the acid from being united with a 

 base, so that free uric acid presents itself in the urine. It is very seldom, how- 

 ever, that such is the case; for the lateritious sediment deposited in diseases 

 attended with fever, consists, according to Lehmann, not of amorphous uric acid 

 as was long believed, nor of urate of ammonia as maintained by Dr. G-olding 

 Bird, 3 but chiefly of urate of soda, mixed with very small quantities of urate of 

 lime and urate of ammonia. After urine has been discharged, however, for an 

 hour or more, crystals of free uric acid may frequently be found in it ; this being 

 consequent upon a change in the constituents of the urine itself on exposure to 

 the atmosphere, whereby lactic acid is developed in it, as has been demonstrated 

 by Scherer. So, again, a deposit of urate of ammonia may present itself in urine 

 that has been exposed to the air for a still longer period, and has undergone 

 the alkaline fermentation ; but it is very rare to find crystals of this salt in the 

 urine of paraplegic patients which has become alkaline within the bladder, and 



1 See Prof. Liebig's "Chemistry in its applications to Physiology and Pathology," 2d 

 edit. p. 137. 



2 By Dr. Golding Bird, however, it is still maintained that uric acid normally exists in 

 the urine in combination with ammonia, derived from the phosphate of soda and ammonia 

 which he believes to be one of its ordinary constituents ; and that in the deposits in question 

 the urate of ammonia predominates, though he allows that the urates of soda and lime are 

 also to be found. See his " Urinary Deposits," 2d Am. Ed., pp. 69 and 113. 



