50 PHYSIOLOGICAL CHEMISTRY 



substances will give similar colors, it is necessary to confirm the 

 result by observing the absorption spectrum of the colored sub- 

 stance after it has been dissolved out by shaking the liquid with 

 amyl alcohol. The orcin test 'gives an absorption band between 

 the C and D lines, the phloroglucin test, between the D and E 

 lines. See discussion of absorption spectra below. 



This procedure does not distinguish pentoses from glucuronic 

 acid, however. 



Arabinose is obtained by the hydrolysis of gum arabic, cherry 

 gum, peach gum, etc., with dilute acid. It sometimes occurs in 

 the urine. It has a sweet taste, and its solution is dextrorotatory 

 (1-arabinose has a specific rotation + 104.5). Its melting point 

 is 160. Its osazone melts at 163-164. Xylose is obtained by 

 hydrolyzing wood, gum, straw, bran, etc. It often is called wood 

 sugar. The pentose isolated from the nucleoprotein of the pan- 

 creas is said to be xylose. Its solution is dextrorotatory. The 

 specific rotation of 1-xylose is + 18.1. Its phenylosazone melts 

 at 155-158 C. 



Absorption Spectra. 



White light is made up of a great many different colored 

 lights as may be demonstrated by passing a beam of white light 

 through a prism and observing the spectrum resulting from 

 spreading out the different colored components of the original 

 ray. Colored light may be either light of a single color or wave 

 length, or it may be a mixture of many different colored lights, 

 the sum of which, however, falls short of making a complete 

 spectrum, or in which one color greatly predominates in in- 

 tensity over the other colors present. If light passes through 

 water, it still appears as white light, for the water has allowed 

 the ray to pass through intact. If a ray of white light passes 

 through a solution of hemoglobin, the red pigment of the blood, 

 or of an amyl alcohol solution of the compound made by heating 

 a pentose with orcin and hydrochloric acid, the ray no longer 

 looks white, because the solution has absorbed or reflected a por- 

 tion of the colored light, allowing only the red and perhaps 



