GUM—MUCILAGES. 169 
Considerable differences, which have, however, been demon- 
strated as yet only by a few examples,’ are also presented by the 
various kinds of mucilage from an optical point of view, some of 
them rotating the plane of polarized light to the left, in the 
same manner as ordinary gum, while other mucilages rotate it 
to the right. 
With regard to their chemical character, gums and mucilages 
are but little known, and are with difficulty freed from inor- 
ganic constituents and nitrogenous substances.” Gum arabic 
appears to be composed of the calcium, potassium, and magne- 
sium salts of arabic acid. Ifthe formula Ca(C,,H,,0,,),+3H,O is 
assigned to gum arabic, it must contain 13.3 per cent of water 
and 1.9 per cent of calcium; these numbers nearly correspond to 
the actual proportions. 
It is also scarcely possible to characterize gums micro- 
chemically. They mostly swell in water (not the gum produced 
by wounds) and are not rendered blue by iodine, or by iodine with 
sulphuric acid. The plant mucilages are colored yellow or blue 
by iodine, and blue or violet-brownish by iodine with sulphuric 
acid. Both are insoluble in ammoniacal oxide of copper. The 
amyloid of Schleiden, which should also be considered here, is 
colored blue by iodine, yellow by iodine water, and is soluble in 
boiling water. In some cases, for instance, in Cydonia and 
salep, the mucilage retains the capability of being colored from 
reddish to blue by iodine, after treatment with sulphuric acid, 
and in this respect stands one step nearer to cellulose. It does 
not follow from this, however, that mucilages always originate 
from cellulose. In Semen Cydonia, Sem. Lint, Sem. Sinapis albe, 
and also in the seeds of Plantago Psyllium, before they ripen, and 
1 Wiggers-Husemann’s Jahresbericht, 1869, 154, top, 
2 Tragacanth affords three per cent of ash. In mucilage of Irish 
moss, even after repeated purification, there are still contained sixteen 
per cent of inorganic substances and 0.88 per cent of nitrogen (= six 
per cent of albumen): Wiggers-Husemann’s Jahresbericht der Pharm., © 
1868, 88. With reference to many other varieties of mucilage compare 
Frank, Pringsheim’s Jahrb. fiir wissenschaftliche Botanik., V. (1866), 
161. 
