mediums and adhesives	69
general darkening of surfaces. Among the synthetic resins, there is a wide choice in
respect to refractive index. Polymerized vinyl acetate and cellulose acetate have
very low refractive properties. If in viscous solution so that they will not penetrate
deeply, they may be used to coat paper, textiles, and granular surfaces so that
very little darkening or change in value results. The refractive indices of several
clear, synthetic resins and cellulose derivatives are given in the table below. These
data have been collected from various sources: manufacturer's bulletins and
journal articles.
SYNTHETIC   RESINS,  INDICES   OF   REFRACTION
Polyvinyl acetate	n = 1.46-1.47
Cellulose acetobutyrate		1.47
Ethyl cellulose		1.47
Cellulose acetate		1.48
Cellulose acetate—plasticized		1.48-1.51
Cellulose nitrate—plasticized		1.45-1.50
Cellulose nitrate		1.51
Polymethacrylate		1.48-1.52
Polyvinylchloracetate		1.53
Glycerol phthalate		1.56-1.58
Polystyrene		1.50-1.75
Conspicuous among the properties of many of the synthetic resins is their
behavior when subjected to heat and pressure. On the basis of their response to
heat, they may be divided into two main classes: (a) those that are * thermoplas-
tic/ and (b) those that are * thermosetting.' For molding purposes, the powdered
or granulated resin is placed in a form and when this is heated (usually with
pressure) the resin fuses and flows; when the mold is cooled, it sets to a hard, solid
substance. A thermoplastic resin is permanently plastic when repeatedly heated
and cooled. The thermoplastic resins are, for the most part, linear polymers like
the polyvinyl, polystyrene, polymethacrylate, and alkyd resins. On the other
hand, certain of the synthetic resins, when heated and formed in a mold, set to
hard and permanently infusible compounds. These are called * thermosetting
resins.' The resins that belong to this class are three-dimensional polymers, or
condensates, and the phenol-formaldehyde resins are good examples. Houwink has
discussed the colloidal structure of linear and three-dimensional polymers in their
various stages of formation, and he has made a comparison between the thermo-
plastic and thermosetting resins on the basis of their intermolecular bonds.
Tempera (see also Egg Tempera). The meanings of the word, * tempera/ as
used to define a painting medium, have been many and have changed from time
to time in the history of the art. As late as the XV century this term probably
included all mediums but, with the gradual prevalence of oil, its limits were
narrowed until it has often meant only a medium prepared from egg. A broader
definition which allows it to include albuminous, gelatinous, and colloidal mate-