Secretion—We have examined microscopically the bark of stems of Fraxinus Ornus that had been incised for manna at Capaci. It exhibits no peculiarity explaining the formation of manna, or any evidence that the saccharine exudation is due to an alteration of the cell-walls as in the case of tragacanth. The bark is poor in tannic matter; it contains starch, and imparts to water a splendid fluorescence due to the presence of Fraxin.
Description—Various terms have been used by pharmacological writers to designate the different qualities of manna, but in English 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 unctuous or gummy appearance, are less esteemed. Good manna is crisp and brittle, and melts in the mouth with an agreeable, honey-like sweetness, not entirely devoid of traces of bitterness 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 manna, at least of the better sorts, is Manna-sugar or Mannite, C₆H₈(OH)₆ which likewise occurs, though in much smaller quantity, in many other plants besides Fraxinus. Artificially, it is produced by treating glucose, C₆H₁₂O₆, with sodium amalgam, and indirectly in the fermentation of glucose or of cane-sugar. It is isomeric with dulcite or melampyrin; crystallizes in shining prisms or tables, belonging to the rhombic system; melts at 166° C., and in very small quantity may by careful heating be sublimed and decomposed. It dissolves in 6·5 parts of water at 16° C., less freely in aqueous alcohol, very sparingly in 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, though not so easily as sugars belonging to the group of carbo-hydrates. The quantity of mannite in the best manna varies from 70 to 80 per cent.
When a solution of manna is mixed with alkaline cupric tartrate, rapid reduction to cuprous hydrate takes place even in the cold. This effect is due to the presence of a sugar which, according to Backhaus (1860), consists of ordinary dextro-glucose. It may amount to as much as 16 per cent., and is found in the best flake manna, but most abundantly in the unctuous varieties. Buignet[1511] has pointed out that the rotatory power of this sugar being inconsiderable, it probably consists of a mixture of Cane-sugar and Levulose. He found however that an aqueous solution of manna deviates powerfully to the right, a fact which he considers due to the presence of a large proportion of Dextrin. The best kinds of manna, according to Buignet, contain about 20 per cent. of dextrin; the inferior much more.
In our experiments we have not succeeded in isolating either dextrin or cane-sugar. There is present, even in the finest manna, a small amount of a dextrogyre mucilage, which is precipitated by neutral acetate of lead, and yields mucic acid when boiled with concentrated nitric acid.
Ether extracts from an aqueous solution of manna a very small quantity of red-brown resin, having an offensive odour and sub-acrid taste; together with traces of an acid which reduces silver salts and appears to be easily resinified. The quantity of water in the inferior kinds of manna often amounts to 10 or 15 per cent. The finest manna affords about 3·6 per cent. of ash.