The resinous products of the Norway spruce, though yielded by the tree in less abundance than those furnished by the pine, are of considerable economic value.
Its duplex character, and the fact that the electricity produced by rubbing glass and vitreous substances was different from that produced by rubbing sealing-wax and resinous substances, seemed to necessitate the assumption of two kinds of electric fluid; hence there arose the conception of positive and negative electricity, and the two-fluid theory came into existence.
De C. du Fay (1699-1739) made the great discovery that electricity is of two kinds, vitreous and resinous (Phil.
Externally they are brown and marked with small transverse paler scars, and internally they present a dirty white resinous or starchy fracture.
In the temperate uplands of the interior, as about Luang Prabang, Himalayan and Japanese species occur - oaks, pines, chestnuts, peach and great apple trees, raspberries, honeysuckle, vines, saxifrages, Cichoraceae, anemones and Violaceae; there are many valuable timber trees - teak, sappan, eagle-wood, wood-oil (Hopea), and other Dlpterocarpaceae, Cedrelaceae, Pterocarpaceae, Xylia, ironwood and other dye-woods and resinous trees, these last forming in many districts a large proportion of the more open forests, with an undergrowth of bamboo.
In other districts lack of water impedes cultivation, though after the rains pasturage is abundant, and resinous plants are so varied and numerous as to justify the ancient name of the region.
It has a strong and characteristic odour, and a hot sweetish taste, is soluble in ten parts of water, and in all proportions in alcohol, and dissolves bromine, iodine, and, in small quantities, sulphur and phosphorus, also the volatile oils, most fatty and resinous substances, guncotton, caoutchouc and certain of the vegetable alkaloids.
Their soft, straight-grained, resinous and often durable wood gives to many kinds a high economic value, and some are among the most esteemed of timber trees.
There are several species of palms, flowering trees, trees with beautifully coloured foliage, tree ferns, resinous trees and trees bearing tropical fruits.
Resins when soft are known as oleo-resins, and when containing benzoic or cinnamic acid they are called balsams. Other resinous products are in their natural condition mixed with gum or mucilaginous substances and known as gum-resins.