Sec. LI. Of Gum arabic, and Gum Tragacanth.
Gum arabic, which exudes from a tree that grows in Egypt and Arabia (Mimosa nilotica) when pure is transparent, and nearly colourless. There are several varieties of this gum; the gum senegal, for instance, which is of a reddish colour, and occurs in larger pieces. Other mucilaginous substances, the peach tree gum, the cherry tree gum, &c. which exist only in small quantities, are analogous to the gum of the Mimosa.
Gum arabic is brittle, and for that reason may be easily reduced to powder. It is readily dissolved in water, with which it forms mucilage. In this state, it is employed in fire-works, chiefly as a vehicle for the mixing of pastes, matches, &c.
Gum is a vegetable oxide, composed of carbon, hydrogen, and oxygen. It does not crystallize. It is precipitated by some metallic salts, as acetate of lead. It is insoluble in alcohol, which distinguishes it from resins. Nitric acid decomposes it, and changes it into the saclactic or mucous acid. With sugar, the same acid produces oxalic acid.
Gum tragacanth, or gum dragon, is the produce of a thorny shrub, which grows in Candia, and other islands of the Levant, called astragalus tragacantha. The gum obtained from this shrub has many properties in common with gum arabic, and is, therefore, used as a paste. It dissolves readily in boiling water; but is insoluble in alcohol, or ether.
It consists, almost entirely, of a peculiar vegetable principle, which is called cerasin by Dr. John. Cerasin has the adhesive qualities of gum arabic, but in a greater degree. It is said to constitute a part of the gummy matter, that exudes from the prunus cerasus, prunus avies, prunus domestica, &c.
Sec. LII. Of Cotton.
The soft down, which envelopes the seeds of different species of gossypium, or cotton plant, is the cotton of commerce. These plants are natives of warm climates. Cotton when bleached is perfectly white. It is extremely combustible, and burns with a clear lively flame. The ashes left behind contain potash.
Cotton is the substance, usually employed in making match rope, for the communication of fire. It has also other uses in pyrotechny. Cotton match is much used in fire-works for exhibition, not only for single cases, but also for a series of cases of artificial fire, either for fixed or moveable pieces; and serves to communicate fire, either singly, or from one case to another, or to the whole piece at one time. Matches, so used, are called leaders, and are generally confined in paper tubes.