6i8 THE CHEMISTRY OF THE URINE. 



The pigments of the urine have long received attention and have 

 been the subject of many laborious researches ; but, owing to the great 

 difficulties they present to the investigator, our knowledge of the 

 chemistry of most of them has remained indefinite. These difficulties 

 arise from various causes. Pigment metabolism appears to be always 

 of a highly conservative nature. The colouring matters found in the 

 epidermal structures of animals, serving for ornament, protection, or 

 other purposes, are almost always present in strikingly small quantity ; 

 and those which are purely excretory leave the body in equally small 

 proportionate amount. 



The highly developed optical activity of these substances, which has 

 led us to group them together in a special class as " pigments," at the 

 same time gives to them a prominence in various phenomena, dispro- 

 portionate to the actual quantity in which they are present. The 

 urinary pigments are (at least, under normal conditions) quite minute 

 in amount, and this fact is the primary difficulty in the path of chemical 

 investigation. As Bunge has written, many endeavours have resulted 

 merely in applying Greek and Latin names to substances which have 

 been obtained in quantity too small for proper investigation. 



The extremely delicate indications of the spectroscope have been of 

 the greatest assistance in overcoming this fundamental difficulty, and 

 our knowledge of pigments has been much extended by its use. But 

 evidence so gained has to be checked and assisted by other methods. A 

 complex spectrum may indicate a mixture of substances ; but it may, 

 with equal probability, be due to one alone. A mixture, on the con- 

 trary, may show but a single absorption-band, for the reason that of the 

 pigments present one alone extinguishes light in a specific region. 



It is therefore easy, by a mere qualitative use of the spectroscope, 

 to mistake a mixture for a chemical individual. On the other hand, 

 very slight variations in the physical condition of a pigment, or a 

 minute change in its molecular constitution, may produce a great 

 effect upon its spectrum, and, unless we are aware of these conditions, 

 we may be led to see wide differences where chemically there is little or 

 none. 



When, again, endeavours are made to isolate pigments by chemical 

 means, the great instability which they exhibit as a class is apt to lead 

 to error. So often has this danger been overlooked, that we are 

 compelled to attach no importance, beyond what accrues from historic 

 interest, to much of the work which has been done on this problem. 



It is of prime importance, when we endeavour to obtain these 

 unstable substances in their integrity, that the use of highly active 

 reagents should be avoided. 



We shall deal only with the pigments of which we have comparat- 

 ively accurate knowledge ; but it may be safely asserted that the four 

 substances now to be described form the basis of urinary chromatology. 

 These are urochrome, urobilin, uroerythrin, and hwmatoporpJiyrin. Other 

 pigments exist, and some have doubtless yet to be recognised, but they 

 are exceptional, or take but very small share in the coloration of the 

 urine. 



Preformed pigments of normal urine (a) The essential yellow 

 pigment, urochrome. In 1864,Thudichum gave the name of urochrome 

 to preparations obtained from normal urine by complicated processes of 

 extraction. Thudichum's products undoubtedly contained a large pro- 



