CARBOHYDRATES 



51 



some neighboring kinds of light to pass through. If the light 

 is spread out in a spectrum, a portion will be missing. Many 

 substances in solution have the property of absorbing particular 

 kinds of light in this way, thus leaving a blank dark area in 

 the spectrum if the emerging light is analyzed by spreading it 

 out into its spectrum. The property is so constant that it may 

 be made use of to identify compounds. The spectra resulting 

 are called absorption spectra, and each absorption spectrum is 

 characteristic of a particular substance. The lines or dark 

 areas are charted with reference to the dark lines always ob- 

 served in the spectrum of sunlight. These lines are called the 

 Frauenhofer lines. The important ones are as follows: 



C D Eb F 



Red Yellow Green Purple 



By looking through a solution with a spectroscope it thus is 

 possible to identify many compounds of biological importance. 



Hexoses. C 6 H 12 6 



Glucose. (Dextrose, grape sugar). Glucose is found in both 

 plants and animals. In the plant world it occurs in grapes and 

 other sweet fruits, in seeds, roots, etc., and as a constituent of 

 di- and polysaccharides and glucosides is much more widely 

 distributed. In animals it is found in the blood and lymph, and 

 occasionally in the urine. If the amount in the urine is more 

 than a trace and it is found regularly, the condition is pathologi- 

 cal and no time should be lost in consulting a physician. Glu- 

 cose also is found in honey, an animal product. It may be ob- 

 tained by boiling starch, glycogen, dextrins, etc., with dilute 

 acid. Glucose crystallizes readily. It is soluble in water. The 

 solution is less sweet than that of cane sugar. It is dextrorota- 

 tory, the specific rotation varying somewhat with the concen- 



