MEDIUMS AND ADHESIVES 55
as the resin decomposes and an oil distills off. As samples of the same variety of
resin may give different results, the melting point can not be used to identify
a particular resin.
When exposed to ultra-violet light, resins tend to decompose and to yellow.
Generally, the natural gums and resins are said to be less sensitive to ultra-violet
rays and to be more transparent, especially in the shorter wave-lengths, than are
synthetic resins. The relatively transparent resins remain stable under ultra-
violet radiation, Kauri being found the most stable.
From x-ray diffraction studies resins have been found to be amorphous and
to give patterns that are similar to those obtained from liquids. The intermolecu-
lar spaces are larger in the soft resins than in the hard ones. It is possible that
the hardness of resins like Zanzibar copal and Congo copal depends upon the
strength of the forces preventing the larger particles from becoming scattered.
This theory would also explain the wide variations in solubility of different resins.
Kauri, for instance, easily dissolves in drying oils, and shows a weak inner ring
which contracts and becomes diffuse when the resin has been melted; Zanzibar
copal, which is soluble only with difficulty, shows a sharply defined, inner halo.
The solubility of a particular resin varies with its age and with its handling
after collection. In general, the hard resins are most difficult to dissolve. No
resins are soluble in water. All may be classified according to solubility: those
which are completely soluble in one or more organic liquids, and those which are
partially or completely insoluble. The balsam resins, such as colophony, are
entirely soluble in alcohol. The soft resins, such as dammar, mastic, and sandarac,
are more or less readily dissolved in alcohol; but the fossil resins, amber and
copal, are soluble only with difficulty, though more readily so if they are first
melted. Prolonged contact of the resin with the solvent results in a dispersion
rather than in a true solution. Some of the hard resins, however, may be com-
pletely dissolved in alcohol on standing for several weeks. It does not necessarily
follow that, upon evaporation of a dissolved resin, a satisfactory varnish film
results- Often an uneven and opaque mass is obtained, as many resins are pre-
cipitated when a certain concentration is reached.
There appears to be little available information concerning the autoxidation
of resins. No doubt oxidation plays an important part in the drying and hardening
of oleo-resins and balsams, and autoxidation may take place, to a limited extent,
in the drying of soft resins. Tschirch and co-workers (I, 309) have found that
when such resins as colophony, mastic, and sandarac are powdered and exposed
freely to the air there is a definite amount of oxygen absorption which is accom-
panied by a lessened solubility of the resins in petroleum ether. The aging,
embrittlement, and final destruction of soft resin films, however, do not appear to
be caused primarily by autoxidation but rather to be the result of molecular
re-arrangement and association and loss of volatile essential oil. Tschirch and
Stock (he. «*., pp, 315-317) report that many of the resins which have been found