626 THE CHEMISTRY OF THE URINE. 



Pathological urines rich in the pigment will generally yield it easily to 

 acetic ether and to amylic alcohol. 



Properties. An account of the properties of haematoporphyrin will 

 be found in the section devoted to blood pigments ; but the pigment as 

 found in the urine has certain peculiarities which must be referred to here. 



When the urine is sufficiently rich in the pigment for the absorption- 

 bands to be visible without treatment (always a pathological condition), 

 it is found that the bands observed are those of the so-called alkaline 

 hgematoporphyrin (Fig. 57, Spectrum 2). Indeed, if a solution of the 

 pigment showing the acid spectrum (but, of course, free from excess of 

 mineral acid) be added to urine, the bands are seen to change to those 

 of the alkaline form, even though the urine itself be of normal acidity. 

 Acid sodium phosphate will, in fact, yield base to the haematoporphyrin, 

 unless, indeed, the salt is in great excess, when it can, on the other 

 hand, convert the alkaline form of the pigment into the acid. These 

 facts form an interesting commentary on what we have said in the 

 section devoted to the acidity of the urine, as to the complex conditions 

 which govern the phenomena of chemical reaction in the fluid. 



Urinary hcnematoporphyrin may be in the form of unstable modi- 

 fications. Alkaline solutions of the pigment obtained from many 

 specimens exhibit a five-banded instead of a four-banded spectrum 

 (Macmunn). Occasionally, too, urate sediments may be pigmented with 

 a form of the pigment which, in alkaline or neutral solution, shows a 

 spectrum of two bands resembling that of oxyhsemoglobin (Fig. 57, 

 Spectrum 3). Dilute mineral acids, however, promptly change this spec- 

 trum to that of ordinary acid hsematoporphyrin (Fig. 57, Spectrum 1). 

 There is some evidence that a colourless chromogenic substance, related 

 to haematoporphyrin, may occur in the urine, as the pigment has been 

 observed to increase in amount after standing. 



Chromogenic substances in urine. Two, at least, of the pig- 

 ments we have now described (urobilin and hsematoporphyrin) may 

 exist, as we have seen, in the form of chromogens colourless, or less 

 coloured, precursors. But the urine contains other chromogenic sub- 

 stances, which in the original urine always, or nearly always, retain 

 their colourless form ; and, as a rule, take no share in the true 

 pigmentation of the fluid. 



We do not include, under the term of " chromogen," all substances which, 

 by the action of strong reagents, happen to be capable of yielding a coloured 

 derivative. 



We purposely exclude such bodies as the so-called " humous substances " of 

 Udransky indefinite products of wholly doubtful nature obtained by such 

 processes as fusing urinary precipitates with caustic alkali, or boiling the 

 previously concentrated urine for hours with hydrochloric acid. These are 

 probably derived from the carbohydrates and other constituents of the urine, 

 by the destructive action of the reagents. Beyond the fact that they happen 

 to be amorphous, and yellow or brown in colour, there is nothing to suggest 

 that they are related to urochrome or any other definite pigment. 



We shall deal only with those chromogenic substances which are of 

 importance, either because they may, though with great rarity, appear 

 as actual pigments, or because they yield their coloured derivatives 

 with comparative ease, and may thus lead to confusion when the urine 

 is being investigated in other connections. 



