6 1 S THE CHEMISTR \ ' 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 
(if 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 thuse 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 l»een overlooked, that we are 
compelled to attach no importance, beyond what accrues from historic 
interest, to much of the wort 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 maybe safely asserted that the four 
substances now to be described form the basis of urinary chromatology. 
These are urochrome, urobilin, uroerythrin, and hcematoporphyrin. 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- 
