THE COL UR OF URINE. 6 1 7 



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. But much difficulty 

 intervenes, in that variations may be due, not alone to differing amounts 

 of a single colouring matter, but to independent and quite irregular 

 variations in at least three or four. The endeavour to attain to quanti- 

 tative precision has 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 

 flow 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 spectropliotometer 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 be twice as great as that near E, then if the urine contained 

 but one pigment, this same ratio would 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 "Hsemoglobin," p. 213. 



3 This argument only holds for colouring matters which do .not undergo dissociation in 

 solution. 



