CHEMICAL BASIS OF THE ANIMAL BODY. 1307 



This alkaline solution to which the zinc salt has been added 

 also shews, in marked distinction to the acid solutions, a bril- 

 liant fluorescence which is most characteristic of the substance, 

 being of a bright rosy-red colour by transmitted and bright green 

 by reflected light. 



Previously to the discovery of hydrobilirubin, a well-char- 

 acterized urinary pigment had been isolated and described under 

 the name of urobilin (see below), while about the same time a 

 pigment had been obtained from faeces and described, under 

 the name of stercobilin, as identical with urobilin. Careful 

 comparison of hydrobilirubin with urobilin has led to the view 

 of the close relationship if not identity of the two substances. 

 This view has been most generally adopted and is correct as a 

 broad statement of facts. When haematin in acid solution is 

 reduced by means of tin and hydrochloric acid it assumes a 

 pale yellow colour and a substance is obtained which is char- 

 acterized by an absorption band between b and F. A similar 

 result follows when haematoporphyrin is treated in the same 

 way. The product to which the absorption band is due may 

 be regarded as practically identical with urobilin or hydro- 

 bilirubin, and if this be so we have here the best and most direct 

 chemical evidence of the relationship between the colouring 

 matters of the blood and bile. For if one and the same sub- 

 stance, viz. urobilin, can be prepared by the same means, 

 namely reduction (hydrogenation) from both haematin (htemo- 

 globin) and bilirubin, these two substances must be themselves 

 closely related. It has not however as yet been found possible 

 to produce a bile-pigment directly from haemoglobin or haematin 

 by any artificial process outside the animal body. Solutions 

 of haemoglobin when injected into the subcutaneous tissue of 

 the horse become after a few days partially converted in situ 

 into granules and flakes which are of a yellow or orange colour 

 and yield an intense Gmelin's reaction. Finally by the action 

 of phenylhydrazin on rnematin and on bilirubin products are 

 obtained which in each case exhibit a similar and marked play 

 of colours under the action of fuming (yellow) nitric acid. 



THE PIGMENTS OF URINE. 

 1. Urobilin. C 32 H 40 N 4 7 . (?) 



This is the best known and most definitely characterized of 

 the urinary pigments. In fresh normal urine the amount is 

 frequently extremely small, but increases on standing exposed 

 to the air (oxygen), a result due to the probable presence 

 in the urine of some chromogen or mother-substance of the 

 urobilin. 



