536 OF SECRETION. 



system ; and if the oxygenatipn could be carried still further, the latter would 

 cease to be deposited, their elements passing off in the form of Urea and Car- 

 bonic acid. These views are borne out by the results of Lehmann's experi- 

 ments upon himself; for he found that the violent exercise, which raised the 

 proportion of Urea in the urine by more than one-third ( 672), brought down 

 the amount of Uric acid from 1'18 to -642, or nearly one-half. Another 

 change is that which gives rise to the peculiar compound termed Allantoin; 

 which naturally exists in the fluid of the Allantois of the foetal calf. This 

 may be formed artificially by boiling Uric acid with peroxide of lead ; from 

 which process there result an Oxalate of the protoxide of lead, Urea, and 

 Allantoin ; the composition of which last substance is very different from that 

 of urea or uric acid, being 8 Carbon, 5 Hydrogen, 4 Nitrogen, and 5 Oxygen. 

 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 die which was obtained from a species of 

 Murex) ; this is a crystalline substance, 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 

 principles of the urine, the nature of which principles is not yet fully under- 

 stood. 



676. Although the proportion of Lactic acid in healthy urine cannot be 

 exactly specified, it has been hitherto regarded as considerable. This sub- 

 stance, may be referred to the class of Saccharine principles ; being obtainable 

 from them (as from milk, beet-root, &c.) by fermentation. Its existence in the 

 Blood cannot be clearly demonstrated; although there is reason to believe 

 that it is present in that fluid, in combination with alkaline bases. Its exist- 

 ence in the Urine, however, appears from the recent experiments of Liebig to 

 be at least doubtful; in fact, he asserts that it cannot be present there. Yet 

 it may be questioned whether this is not too dogmatic an assumption ; for its 

 presence in the Blood can scarcely be regarded as unlikely, when we consider 

 the large amount of Saccharine matters taken into the circulation, and the 

 facility with which these matters are converted into Lactic acid under the' 

 influence of a ferment ; and if it be present in the Blood, it will be likely to 

 find its way into the Urine. It is not easily distinguished, when in combina- 

 tion, from certain other Organic acids ; which renders the determination of the 

 question of its presence or absence a matter of some difficulty. The idea of 

 Dr. Prout, that the precipitation of Lithic acid in the Urine is often to be 

 attributed to the presence of an undue amount of free Lactic Acid, certainly 

 corresponds well with the fact, that such deposits are frequent results of slight 

 disorders of the digestive process, which occasion mal-assimilation of the Sac- 

 charine principles. 



677. It has been shown (671), 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 ( 434) ; 

 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 



