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 hsematoporphyrin 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 
hsematoporphyrin (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 hsematoporphyrin, 
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 hsematoporphyrin 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 oxyhemoglobin (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 hsematoporphyrin, 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. 
