CA RBOH YDRA TES 5 1 



hydrogen and oxygen in the same proportion as in water; 

 or, a compound of carbon and water, thus : Dextrose = 

 6C + ()II:>(). This definition, while not exactly correct, will 

 hold in a majority of cases and serves very well to dis- 

 tinguish the group. The exceptions to this statement are 

 principally in the case of acetic acid, whose empirical formula 

 is C2H4O2, but whose graphic formula is CH3COOH, showing 

 the acid or carboxvl group; and lactic acid, C 3 IIgO.<, but 

 otherwise written CH3.CHOH.COOH. 



A carbohydrate may be defined more accurately as a 

 compound containing always one or more hydroxyl (Oil) 

 groups, and usually either an aldehyde 



H O 



II w I II I 



(— c— c=0) or ketone (— c— C— C— ) group. 



I I I 



The presence of the aldehyde group indicates that the 

 carbohydrate is easily oxidized, whereas the presence of the 

 ketone group usually, though not always, indicates that the 

 carbohydrate is not easily oxidized. The principal carbo- 

 hydrates contain carbon atoms to the number of six or a 

 multiple of six. The carbohydrates are divided into two 

 general classes, the Sugars including the monosaccharides 

 and disaccharides; and the Non-Sugars or Polysaccharides. 

 51. Sugars. — As a class the sugars are colorless, odor- 

 less, crystalline compounds, soluble in water, and, ordinarily, 

 sweet in taste. Their most characteristic property is that 

 of optical activity, that is, they rotate the plane of polarized 

 light either to the right or to the left. 1 The simple sugars, 



1 Optical Activity : An ordinary ray of light is supposed to consist of 

 particles vibrating in every direction at right angles to the direction of the 

 ray. When such a ray is passed through a properly cut crystal of Iceland 

 spar called a Nicol prism, it is separated into two rays, one of which is 

 reflected out to one side of the prism, and the other passes through, its 

 particles now vibrating in only one plane. This ray is said to be polarized, 

 and substances which have the power of rotating this plane of polarized 

 light in one direction or the other are said to be optically active, either 

 dextrorotatory (+) or levorotatory ( — ) as they turn the plane of polarized 

 light to the right or to the left. The amount of rotation depends on the 

 specific property of the substance and the number of molecules through 

 which the light passes. The amount of rotation can be measured, and by 



