412 OLEACE^. 



commerce they are not now employed; and the better kinds of the 

 drug are called simply Flake Manna, while the smaller pieces, usually 

 loosely agglutinated and sold separately, are termed Small Manna or 

 Tolfa Manna. 



Owing to the gradual exudation of the juice and the deposition of 

 one layer over another, manna has a stalactitic aspect. The finest 

 pieces are mostly in the form of three-edged sticks, sometimes as much 

 as 6 to 8 inches long and an inch or more wide, grooved on the inner 

 side, which is generally soiled by contact with the bark ; of a porous, 

 crystalline, friable structure and of a pale brownish yellow tint, 

 becoming nearly pure white in those parts which have been most 

 distant from the bark of the tree. The pieces which are of deeper 

 colour, and of an unctuons or gummy appearance, are less esteemed. 

 Good manna is crisp and brittle, and melts in the mouth v.'ith an 

 agreeable, honey-like sweetness, not entirely devoid of traces of bitter- 

 ness and acridity. Its odour may be compared to that of honey or 

 moist sugar. 



Manna of the best quality dissolves at ordinary temperatures in about 

 six parts of water, forming a clear, neutral liquid. It contains besides 

 mannite, a small proportion of sugar and gum. 



The manna which exudes from the older stems and from the lower 

 parts of even young trees, contains more or less considerable quantities 

 of gum and fermentable sugar, as well as extraneous impurities. The 

 less favourable weather of the later summer and autumn promotes an 

 alteration in the composition of the juice, and impairs its property of 

 concreting into a crystalline mass. 



Chemical Composition — The predominant constituent of mauna, 

 at least of the better sorts, is Manna-sugar or Mannite, ^^'(OH) 

 which likewise occurs, though in much smaller quantity, in many other 

 plants besides Fraxinns. Artificially, it is produced by treating 

 glucose, C«ff2on^ ^viti^ sodium-amalgam, and indirectly in the fermenta- 

 tion of glucose or of cane-sugar. It is isomeric with dulcite or melam- 

 pynn; crystallizes in shining prisms or tables, belonging to tlie 

 rliombic system ; melts at 166° C, and in very small quantity may by 

 careful heating be sublimed and decomposed. It dissolves in Go parts 

 of water at 16° C, less freely in aqueous alcohol, very sparingly m 

 absolute alcohol, and not in ether. The solution has an extremely 

 weak rotatory power, and is not altered by boiling with dilute acids or 

 alkalis, or with alkaline cupric tartrate. 



Berthelot has shown that mannite is susceptible of fermentation, 

 tliough not so easily as sugars belonging to the group of carbo-hydrates. 



■Ihe quantity of mannite in the best manna varies from 70 to SO per 

 cent. 



When a solution of manna is mixed with alkaline cupric tartrate, 

 rapid reduction to cuprous hydrate takes place even in the cold. A^"^ 

 elect IS due to the presence of a sugar which, according to Backhaus 

 (l^OO), consists of ordinary dextro-glucosc. It may amount to as m"cn 

 f.l^^ P"'' ""f/"^-' ^''^ ^^ f"^»^d in the best flake manna, but most abun- 

 ciantiy in the unctuous varieties. Buio-net^ has pointed out that tne 

 rotatory power of this sugar being inconsiderable, it probably consist. 



^Journ. dePhann. vii. (1867) 401 ; viii. (18C8) 5. 



