Of the physical characteristics of the resins generally, the first to be considered are their form and size. The hard resins have a drop-like, stalactitic, or knotty form. When the resin spreads over the surface of the plant and there collects, it is usually drop-like or stalactitic ; when it flows into the ground, it becomes knotty. Other forms are rare. The drops, as met with in commerce, are generally pretty constant in shape for each kind, but the stalactitie and knotty form vary much in that respect. Some commercial resins are shaped by artificial means—dragon's-blood in sticks or tears, gamboge in cylinders, shellac in laminas.
The surface of many resins forms an important characteristic. In red xanthorrhosa, the surface which was in contact with the stem is rough, dull, unresinitied, and possesses structural peculiarities tLat will be detailed when speaking of it in particular. Many resin-surfaces present polygoinal excrescences, with regular crystallographic form. In Zanzibar copal, these are visible by the naked eye ; and in sandarach, by the aid of a gla,ss.
Resins seldom present a definite structure, as they occur for the most part in thick homogenous masses. But benzoin, yellow xantliorrhasa, and the softer kinds of dragon's-blood exhibit a structure known as amygdaloidal to mineralogists. lt is produced by a number of rounded grains imbedded in the mass, and differing from it in colour. Some sorts of turpentine-resin are permeated by globular pores of various size.
Most resins are quite amorphous ; few contain crystalline substances, and still fewer occur as crystals thenaselves. The turpentines often hold abietic acid in the form of crystals. In the turpen tine-resins, a crystalline body is frequently visible. In tbe elenai-resins, which are very rich in crystal line components, it often happens that the optical properties of the mass so nearly approach those of the crystals, that the latter only become visible after dissolving the amorphous portion in alcohol.
The optical properties of the resins seldom afford any guide.
The colours of the resins are in many cases pronounced, as the yellowish-brown of gamboge, the red of dragon's-blood and red xanthorrlicea, the white of Siam benzoin, and the black of some rosins. Many resins are colourless ; the tints of most lie between yellow and brown.
In transparency, the resins vary much. Some are as transparent as glass (certsin copals) ;
others are quite opaque (xanthorrhoea, dragon's-blood). Most are merely translucent to the naked eye. Microscopic sections, even of the quite opaque, are at least translucent, and often communicate their own colour to the transmitted light.
The lustre of most resins is ahnost adamantine, and constitutes a degree termed "resinous" by mineralogists. There also occur those of fatty (small sorts of benzoin, and dragon's-blood) and of waxy (the "alnaonds" of benzoin) lustres, and some are lustreless (certain elemi-resins).
The fracture is mostly glassy, often conchoidal; sometimes also smooth, granular, earthy, and splintery.
The hardness of most resins lies between gypsum and rock salt ; only the best copals are harder than the latter, and for the verification of thesc, hardness is one of the best tests.
The density (sp. gr.) is for many resins a distinctive feature, e. g. many copals, In general, the sp. gr. of resins is sonaewhat greater than that of water, while the guna-resins are considerably heavier, e. g. asafcetida 1.3., The balsams are mostly lighter than water. The specific gravities of some of the resins are shown in the annexed table :— In tenacity, most resins are friable ; some are soft (stick-lac, shellac), some pliant (eletni). The degree of friability can be tested by scratching the surface with a needle: the most friable show a splintery scar ; the least, a smooth line.
Many resins, both coloured and colourless, give a white streak, even the dark-hued colophony. Some resins, finely disseuainated through water, exhibit a rapid molecular movement ; others a very indolent one. The meet pronounced occurs in the case of gamboge ; copal powder, on the other hand, moves vory slowly.
Several resins possess highly characteristic odonr and flavour.
The secretion or imprisonment of organic delris in resins is much more frequent than ie com monly supposed. The fragments are mostly invisible, except through the microscope, and chiefly consist of particles of bark and wood from the stems of the plants, which have become imbedded during the concretion of the resin. The recognition of these remains in a resin is often of impurt anee, the more or less decomposed tissue illustrating the characteristics of the resin, as well as indicating its origin and derivation.