ADVANCED PHYSIOLOGICAL CHEMISTEY 485 



concentration of the unknown strength, and C l the concentration of the 

 standard, then C 2 may be ascertained from the equation 



log tan - log tan 2 

 2 = ' log tan B- log tan 6^ 



In using the spectrophotometer the mean of at least three observa- 

 tions as to the position of the pointer in any particular case should be 

 taken. If the substances under examination give marked absorption 

 bands it is desirable to work in a part of spectrum where such bands 

 are absent. With oxy haemoglobin the green about A510 is best. 



CHAPTER XVII. 

 THE PIGMENTS OF URINE. 



WHEN fresh normal urine is examined by means of the spectroscope 

 it usually presents no absorption bands, a diffuse absorption of the 

 violet end being alone conspicuous. 



The yellow colour of the urine is to be regarded as due almost 

 entirely to the presence of a preformed pigment, urochrome. If this 

 pigment be removed from the urine, the colour of the urine is largely 

 lost. It may be separated from urine by saturating urine with am- 

 monium sulphate and filtering. The nitrate which contains the 

 pigment is shaken with alcohol, and by such repeated extractions from 

 the saline solution practically all the pigment may be removed. 

 The urochrome may now be precipitated by adding an excess of 

 ether. The substance is readily soluble in water and when examined 

 by the spectroscope shows no absorption bands. 



Urobilin is present in very small quantities in normal urine and 

 the amount normally present is generally in the condition of a 

 chromogen. In abnormal conditions the urine may tend to have 

 a brownish tint added to the ordinary rich orange colour and such 

 urine frequently contains urobilin. A solution of urobilin or urine 

 rich in urobilin will present the spectrum shown in Fig. 277, 1. If 

 a concentrated solution of urobilin in sodium hydrate be taken and 

 hydrochloric be added till the mixture is slightly acid, a turbid 

 condition of the liquid results owing to imperfect re-solution of 



