77 /E COLOUR OF URINE. 617 
An effort has been made to refer the varying degree of pigmentation 
to a standard colour scale, so that the condition of a given specimen, as 
regards colour, might be quantitatively expressed. Hut much difficulty 
intervenes, in that variations may be due, not alone to differing amounts 
of a single colouring matter, hut to independent and quite irregular 
variations in at least three or four. The endeavour to attain to quanti- 
tative precision lias on this account proved unsuccessful in practice. 1 
We may content ourselves with speaking of physiological urine as pale, 
normal, or high-coloured respectively, and assist the description by 
comparison with other substances of familiar appearance (" straw- 
coloured," "sherry-coloured," etc.). 
Pale urine is usually of low density, and contains a small proportion 
of solid matter. It results from all causes which promote a copious 
tlow of fluid from the kidneys, such as free ingestion of liquids, a check 
to the cutaneous transpiration (as from the effect of cold), and emotional 
excitement. 
High-coloured urine is generally of high density, and is excreted 
when the transpiration from the skin is more than usually free, or 
under conditions of high metabolic activity. After a full meal the 
urine is often at once copious and of full colour. 
In general the amount of pigment rises with an increase in the con- 
stituents excreted by the renal epithelium, and not with the glomerular 
excretives. The depth of colour may be affected by the reaction of the 
urine ; other things being equal, an acid urine will show a darker tint 
than one that is alkaline. 
Examined directly by means of the spectroscope, fresh normal urine 
is found nearly always to show no definite absorption-band ; a diffuse 
absorption of the more refrangible rays being alone evident. 
But by the aid of the spectrophotometer ,2 we may measure the amount of 
light absorption in any region of the spectrum apart from the presence of 
actual bands. When light passes through urine, the amount of absorption 
increases progressively from the mid-red to the violet. 
Suppose the absolute absorption at any two points in this region of 
spectrum be measured ; say in the neighbourhood of the Fraunhofer lines, E 
and F respectively. If in any one specimen of normal urine the absorption 
near F is found to lie twice as great as that near F, then if the urine contained 
but one pigment, this same ratio Avoukl be found in any other specimen. The 
absorption at F would in all cases be double that at E. For, clearly, the 
dilution of an individual pigment would decrease the absolute absorption 
throughout the spectrum, but would leave the relative absorption at any two 
points unaffected ; similarly, concentration would increase the absolute, but 
would nowhere affect the relative, absorption. But different specimens of 
normal urine do not agree in this way. One urine may show more relative 
absorption (say) in the mid-green, another more in the blue. This can only be 
due to the fact that more than one pigment is concerned. 3 Although it yields 
no definite bands, the spectroscopic properties of fresh normal yellow urine 
thus indicate some complexity in its pigmentation ; but the same experimental 
evidence indicates nevertheless that no more than one pigment is usually 
present in a relatively large amount. 
1 By the use, however, of Lovibond's tintometer, the colour of urine under varying 
circumstances may be very exactly imitated, and expressed in terms of a scale. 
2 See this textbook, article "Haemoglobin," p. 213. 
3 This argument only holds for colouring matters which do not undergo dissociation in 
solution. 
