INTRODUCTION. 
XV 
The most distinct secretions of vegetables, we shall class 
under distinct heads. 
1. Gum, or Mucilage. A viscid substance of little flavour or 
smell, soluble in water, and exuding from many trees in the 
form of drops or lumps, as in the plum, cherry, peach, &c. 
and in many species of the Mimosa, as the Mimosa Nilotica^ 
which yields the gum arabic of the Pharmacopoetjbri^tS 
2. Jelly. Somewhat allied to the former, but of less con- 
sistency, being soluble by heat, and remaining fluid in a very 
low temperature. It is procured from ripe pulpy fruits. " 
3. Resin. Soluble in spirits — insoluble in water, as the 
turpentine of the fir and juniper, &c. 
4. Gum Resin. Vegetable exudations for the most part 
partake of a nature between resin and gum, being partly 
soluble in spirits, and partly in water. 
5. Milky Juice. The milky juice of the fig, spurge, &c. 
is an emulsion or combination of watery fluid with oil or resin, 
and when evaporated in the air becomes gum-resins, as the 
gum euphorbiura. 
6. Essential Oils. The volatile resinous secretions, are 
called essential oils, and are often highly aromatic and odori- 
ferous, as the essential oil yielded by the bark of cinnamon, 
and those procured from the effluvia of flowers, as lavender, 
&c. 
7. Bitter Secretion. The nature of this secretion is not very 
well understood, some chemical botanists suppose it to be of a 
resinous nature, but as it is often perfectly soluble in water, 
this opinion does not seem to be well founded. The Cinchona 
Officinalis is a familiar instance of this secretion. 
8. Acid Secretions. These are found to be very abundant 
in plants, as the oxalic procured from oxalis, or wood sorrel, 
the malic, citric, &c. 
9. Alkaline. Of these there are two kinds, the vegetable 
alkali, termed l^ali, and the fossil alkali or soda, procured 
mostly from marine plants. 
10. Saccharine Secretions. Sugar more or less pure is very 
generally found in plants ; ia various roots, as the carrot, beet, 
parsnip, &c. it abounds also in the Saccharum Officinarum, 
or sugar cane, so well known in commerce, as to need no 
mention here : in fruits likewise the saccharine principle con- 
