340 PLANT AND ORGANIC CHEMISTRY 
converted into the osazones; for each sugar gives identically 
the same osazone, and this compound affords a means of pass¬ 
ing from a sugar of one class to a sugar of the other class. 
The other division of natural sugars, or the ketone sugars, 
of which fructose is the type, includes, up to the present time, 
three representatives only. Of these three sugars, only the 
dextro-fructose combines with prussic acid; consequently, 
the synthesis in this division by Kiliani’s method is limited. 
It has not gone beyond the fructo-heptose, or a ketone sugar 
containing 7 atoms of carbon. 
Fructose is the fruit sugar to which the sweetness of fruits 
is chiefly due. This sugar crystallizes from alcohol in crystals 
belonging to the rhombic system, whilst the crystals of glu¬ 
cose are obtained as fine needles. Fructose occurs in three 
modifications. The inactive modification is of historical in¬ 
terest, since it was the first synthetic sugar made out of ma¬ 
terials obtained by synthetical means. 
Of the methods which serve for the direct synthesis of 
sugar may be mentioned: (1) the polymerizing of formyl- 
aldehyde by bases; (2) a valuable synthetical means is, like¬ 
wise, the reaction which corresponds to aldol condensation; 
and (3) Kiliani’s method has been of untold value in this field; 
(4) the artificially made sugars may be separated from solu¬ 
tion in form of their osazone compounds, and, in most cases, 
the sugars can be regenerated from these compounds. 
In all cases a mixture of these osazones arises. The inactive 
phenyl-glucosazone is the direct source of the inactive fructose; 
for, by reducing the osone derived from the osazone, a ketose 
arises which agrees in all respects with the inactive form of 
fructose. It possesses all the properties of the natural fruit 
sugar. 
By the reduction of this inactive fructose, obtained syntheti¬ 
cally, arises the inactive mannite. The synthesis of the active 
glucose and fructose (the natural sugars) may be made from 
the inactive mannite in the following way: By treating the 
inactive mannite with the suitable oxidizing agent, it will be 
oxidized to the inactive mannose and the inactive mannonic 
acid; the inactive mannonic acid can be split into its active 
