318 COLOURING MATTERS. 



is not the part of the organism in which the bile- pigment is formed; 

 we shall, however, discuss this question in the second volume, when 

 treating generally of the origin of the bile. 



URINE-PIGMENT. 



Considered either in a chemical or in a physiological point of 

 view, there is scarcely any substance in the whole range of physio- 

 logical chemistry regarding which our knowledge is in so unsatis- 

 factory a state as the urine-pigment. 



Experiments have often been commenced upon this substance, 

 but the difficulties which present themselves in the investigation are 

 so numerous that most experimentalists have soon resigned it, and 

 directed their labours to some more productive department of 

 chemistry. It unfortunately happens that no certain chemical 

 differences can be detected between urines presenting the most 

 striking difference of colour to the eye of the clinical physician. 



The difficulties of this investigation are dependent on the fol- 

 lowing circumstances. 



The amount of this substance in the urine is extremely minute ; 

 a very small quantity of the pigment giving a colour to an extremely 

 large amount of other matters. 



It begins to decompose even during the most cautious evapora- 

 tion of the urine : to be convinced on this point we need only com- 

 pare urine concentrated by evaporation, with a spec'.men from which 

 a great part of the water has been removed by congelation. 



Even on exposure to the air, or under the air pump, the decom- 

 position of this substance commences. 



Like many other pigments, it adheres tenaciously to other 

 substances, sharing their solubility or insolubility. 



Besides the pigment, there are other substances in the urine 

 which have the same degree of solubility, which do not crystallise, 

 and are not volatile ; as they neither combine in definite propor- 

 tions with other bodies, nor differ in solubility from the pigment, 

 they cannot be separated from it. 



The pigment occurs in the urine under various modifications, 

 on which are dependent the different tints presented by morbid 

 urine and its sediments. 



Finally, this pigment is very readily acted on by chemical 

 reagents, especially by acids and alkalies. 



Scherer's* investigations on this subject especially show that this 

 * Ann. d. Ch. u. Plianti. Bd. 57, S. 180, 195. 



