CHEMICAL COMPOSITION OF THE HUMAN BODY 19 



charides being sweeter than the monosaccharides. Sugars 

 are soluble in water and alcohol, but insoluble in ether; 

 they crystallize and dialyze. The polysaccharides are in- 

 soluble in water or form only colloidal solutions ; they 

 neither crystallize nor dialyze. 



The monosaccharides (hexoses, glucoses) have the fol- 

 lowing constitution: 



Aldehyde sugar (aldoses): 



CH 2 OH.CHOH.CHOH.CHOH.CHOH.COH. 



Ketone sugar (ketoses) : 



CH 2 OH.CHOH.CHOH.CHOH.CO.CH 2 OH. 



Characteristics of monosaccharides : 



1. They are optically active, i.e. their solutions rotate 

 the plane of the polarized light ; most of them turn it to the 

 right; only fructose turns it to the left (hence called levu- 

 lose). The optical activity of the sugar is due to the pres- 

 ence of asymmetrical carbon atoms, i.e. carbon atoms whose 

 four valences are united to four different radicles or atoms. 



2. The aldehyde sugars, like all aldehydes, are easily 

 oxidized, forming first monobasic and then dibasic acids. 

 The ketoses are also oxidized, whereby they are, at the 

 same time, split up into bodies containing less carbon. 

 Upon this fact, that the sugars are oxidizable, depends the 

 detection of sugar by the so-called reduction tests. Of these 

 the following are the most important : 



(a) Trout mer 'JT test: Sugar solution mixed with potassium 

 hydrate and cupric sulphate, on boiling, gives a red precipi- 

 tate, the cupric oxide having been reduced to the insoluble 

 cuprous oxide. 



(//) fiottgcr' s test: Basic bismuth nitrate, by heating with 

 sugar in an alkaline solution, is reduced to metallic bismuth 

 (black precipitate). 



(c) Mulder 's test: Weak alkaline indigo solution, heated 

 with sugar, loses its color through reduction. 



