1894.] of the Ye/low Colouring Matter of the Urine. 405 



faintly tinted. If acid is added they have a brown colour like that 

 of crystals thrown down on the addition of acid to urine. 



The converse experiment to this was performed some years ago by 

 Ord,* who showed that, on repeatedly redissolving and reprecipitating 

 urinary uric acid, the crystals lost their colour, and, at the same 

 time, tended to assume the tabular forms of those of pure uric acid. 



The above result lends strong support to the view that the pigment 

 ( is isolated by the alcohol process in the form in which it actually 

 exists in the fresh urine, and confirms the statement that it plays an 

 important part in determining the forms which the crystals assume." 



Another fact which is demonstrated by this experiment is that the 



yellow pigment is one of those which colours the urinary crystals, 



although it does not stand alone in this respect. I do not, however 



propose to enter further into this subject here, as I hope to deal with 



it at length in a separate paper, but I may mention that crystals of 



uric acid which are deposited from a solution of urobilin are colour- 



j less and exhibit no modification of form, resembling, in every 



respect, those thrown doAvn from pure aqueous solutions of urates. 



Summary and Conclusions. 



There cannot, I think, be any doubt that the substance isolated 

 from the normal urine by the process here described is that to which 

 its colour is almost entirely, if not entirely, due, and, since solutions 

 of this substance do not fluoresce with zinc chloride and ammonia, 

 show no absorption bands, and cannot be got to show a urobilin band 

 by any process to which it was subjected, it follows that urobilin is 

 ; not the chief colouring matter of normal urine. Moreover, there is 

 every reason to believe that the product obtained has not undergone 

 any notable change in the process of extraction, although its solu- 

 bility in various media appears to be somewhat impaired. 

 ' The question whether the yellow colouring matter so obtained is a 

 definite chemical entity is one to which it is very difficult to give a 

 conclusive answer, chiefly on account of its physical properties. 

 However, the uniform course of events observed on each of the many 

 occasions on which the alcohol and ether process was carried out, 

 strongly suggested that the product was a definite compound. 



This view also received support from its behaviour .towards its 

 solvents and its precipitation by ether, as well as by its effect upon 

 uric acid crystals, which is hardly what might be expected from a 

 mixture of pigmentary substances. 



The only fact with which I am acquainted which appears to be 

 opposed to this idea is the impossibility of completely decolorising its 

 solutions by certain metallic precipitants, which throw down the 



* " The Influence of Colloiis upon Crystalline Form and Cohesion," 1879, p. 52. 



2 r 2 



