Chemical Constituents of the Animal Body. 2 1 1 



Mannose is a hexose isomeric with glucose which is 

 obtained by the hydrolysis of a polysaccharose (mannane) 

 which occurs in several plants. The sugar obtained from 

 natural products is dextro-rotatory. 



Galactose is also a hexose isomeric with glucose, and 

 like this sugar and mannose also an aldehyde. It is 

 obtained from the disaccharose milk-sugar or lactose 

 (see p. 214) by hydrolysis with acids. The more complex 

 sugar by this treatment undergoes scission into equi- 

 molecular amounts of dextrose and galactose. It is also 

 dextro-rotatory. 



Synthesis of Sugars. Attention has been already 

 called to the methods of building up the more complex 

 from the simpler sugars. Particular interest attaches to 

 some other methods of synthesis, which tend to throw 

 light on the processes by means of which these substances 

 are formed in plants, which are the chief agents in nature 

 for their production. In 1861 Butlerov found that 

 trioxymethylene, a solid polymerization product of for- 

 maldehyde, on treatment with lime water, yielded a sweet 

 syrupy substance which possessed many of the reactions 

 of a sugar. The observation was extended in 1886 by 

 Loew, who showed that formaldehyde itself, on treatment 

 with lime water, also yielded a sugar-like substance. 

 This observation is an important one, for Baeyer, in 1870, 

 had expressed the view that carbon dioxide, when assimi- 

 lated by green plants, is first reduced to formaldehyde, 

 CH 2 O ; this latter substance on polymerization was thought 

 to be capable of yielding a sugar, C 6 H 12 6 , from which by 

 condensation of several molecules and the withdrawal of 

 water, starch can be produced. The substance obtained 

 by Loew was not isolated in a pure state, and subsequent 

 researches by E. Fischer and his collaborators have shown 

 that Loew's product was in fact a mixture. Later, it was 



