MEDIUMS AND ADHESIVES 25
which seems to be a compound of formaldehyde with gelatin. The formogelatin is
decomposed by repeated washing with boiling water, by heating to 110° C, and
by cold 15 per cent hydrochloric acid. Although formogelatin is nearly insoluble
and swells much less in water than does untreated gelatin, it is not, strictly
speaking, a very water-resistant adhesive. Bogue (p. 319) says that joints made
from it fail to retain their strength when subjected to drastic treatment with either
cold or hot water. Casein and blood albumin glues are much more highly water-
resistant.
Gelatin (see also Glue and Tempera) belongs to the complicated class of or-
ganic compounds known as proteins and is composed of carbon, hydrogen, oxygen,
and nitrogen. Its exact composition is not known but, like all proteins, it is made
up of large molecules of high molecular weight. Though an animal product, it is
not itself found in the animal organism, except under pathological conditions.
The parent substance of gelatin is collagen, of which the organic material of the
bones, the tendons, the cartilage, and the skin is largely comprised. Gelatin is
slowly formed from collagen by heating that in water to 80° or 90° C. It is a nearly
colorless, transparent, amorphous substance. In its normal, dry state, in which it
still retains 15 to 18 per cent of water, it is flexible and horny, and has a slight
yellow cast. Precipitated from alcohol or from salts, it is pure white and nearly
water-free.
Gelatin is a typical colloid of the emulsoid type, and the viscosity of its solu-
tions is high and variable with slight alterations in temperature, concentration,
hydrogen-ion concentration, etc. It swells to many times its normal volume when
immersed in cold water or in dilute acids or alkalis; a slightly acid solution is the
most favorable for maximum swelling. Above 35° C., the swollen jelly goes readily
into solution. A firm jelly is formed when a solution containing I or more per
cent of gelatin is allowed to stand at 10° C. Gelatin, either pure or in the impure
form known as glue, is used extensively as an adhesive. »
Glair (see also Tempera and Egg Tempera) is the white of egg (see Egg White)
and is a term now largely employed with reference to the painting medium pre-
pared from this substance.
Glaze. When the quantity of medium is so great in relation to the quantity of
pigment that light is refracted through the film produced by the mixture of these
two and is reflected from the surface beneath it, such a film is commonly called a
* glaze.' The term has no precise meaning but usually indicates a coating in which
there is some pigment content. Its main characteristic is transparency. When the
pigment used is opaque and pale, films of this general type are called ' scumbles.'
Glue (see also Gelatin and Tempera) is an adhesive consisting largely of
gelatin, but the collagen from which gelatin or glue is prepared is invariably
associated with other protein material such as keratin, elastin, mucin, chondrin,
etc., in addition to non-protein, organic material and inorganic salts which may
or may not remain in the glue. Glue and gelatin merge into one another by im-