2 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY 



OILS specimens of carbohydrates which are presented for 

 study. 



Test for the constituent elements as follows: 



(a) Carbon. 1. Heat cautiously some of the substance 

 on a platinum foil. The piece will char, owing to the sepa- 

 ration of the carbon in the substance. Further heating ren- 

 ders the carbon capable of combining with the oxygen of the 

 air, with the result that the former passes off as C0 2 , and 

 in the case of carbohydrates, where combustion is com- 

 plete, no residue is obtained. A substance containing oxy- 

 gen in sufficient quantities to form C0 2 with all the carbon 

 present will not carbonize. 



2. Mix thoroughly some of the dried substance with 

 powdered CuO and place the mixture in the bottom of a dry 

 test-tube. Upon warming, the carbon of the substance is 

 oxidized by the oxygen of the CuO and escapes as C0 2 . 

 This C0 2 may be detected by holding a glass rod moistened 

 with lead acetate at the mouth of the test-tube. 



(6) Hydrogen. 1. In the latter experiment, moisture will 

 have collected on the cold part of the test-tube. The hydro- 

 gen of the substance, in the presence of heat, has combined 

 with the oxygen supplied by the CuO, forming H 2 0. 



PENTOSES, C 5 H 10 5 . 

 ARABINOSE. XYLOSE. 



The pentoses do not occur free in nature, but exist in the 

 form of pentosanes bodies which are found in the fruits and 

 polysaccharide gums (e.g., gum arabic, cherry gum) and from 

 which the pentoses may be obtained by hydrolysis with 

 weak acids. The ingestion of pentosanes also causes pento- 

 ses to appear in the urine. Chemically they are aldehydes 

 and as such reduce Fehling's solution. With regard to their 



