26 REPORTS ON THE STATE OF SCIENCE.—1917. 
Tyndall effect after the lapse of considerable time, which may even extend 
to weeks, but below 60°, given time, the process seems completely re- 
versible. Above 60°-70° some permanent change takes place (hydrolysis 
or depolymerisation) which results in lowered viscosity, Tyndall effect, 
and setting power. These facts are best explained by the hypothesis that 
below 60° the gelatine solution is one of molecules or small molecular 
aggregates, which, as temperature falls, gradually unite to form larger 
ones, and at the setting point unite to a complete molecular network 
analogous to a mass of tenuous crystals. Cases are known in which such 
crystalline masses closely simulate colloid jellies. Time is of course needed 
for this rearrangement, as it is for actual crystallisation, and owing to 
the size and comparative immobility of the particles, rearrangement 
is very slow. Kundt‘* has shown that under the influence of rapid 
flow at 18° (which is below the setting point) even very dilute and quite 
liquid solutions of gelatine show the polarisation effects of strain, while 
no such effect could be observed with glycerine or sugar solutions of much 
higher viscosity. The writer proposes to repeat these experiments at 
higher temperatures, but in the meantime it is clear that the viscosity 
of such solutions is not due simply to liquid friction, but includes an 
element of strain. 
Proteids, among which gelatine must be included, are now known to 
consist of open or closed chains of amino acids, linked by the carboxyl 
group of one to the amino group of the next with elimination of OH». In 
closed chains, groups within a single molecule, forming terminal amino and 
carboxyl groups are also similarly united ring-structures. In this case the 
molecule is electrically neutral, and non-reactive till the ring is broken, 
while the open chains are amphoteric—basic by their terminal amino 
group and acid by their carboxyl. A very useful practical distinction is 
that ring proteids are unattacked by trypsin alone, while pepsin is able 
to open the ring.> Gelatine can be digested by trypsin, but collagen 
is only attacked by pepsin, hence the view, supported by other facts, 
that collagen is the ring or anhydride form of gelatine into which it is 
converted by continued boiling or by the action of acids or alkalies. 
If gelatine (or hide fibre) be placed in dilute acid, it swells very much 
more than in water alone, and at the same time a considerable amount 
of free acid disappears (7.e., is no longer capable of reddening methyl 
orange). The effect is most readily investigated with a strong mono- 
basic acid such as hydrochloric acid. In this case the maximum swelling, 
which may reach an absorption of 50 c.c. of liquid for 1 grm. of dry 
gelatine, occurs at an acid concentration under 0.005 N, from which it 
rapidly falls in a curve of hyperbolic type as the concentration is increased, 
the equilibrium being completely reversible up to about 0.25 N, beyond 
which some secondary reaction, probably a further breaking up of the 
proteid chain, begins to take place. At the same time the total absorption 
of acid steadily increases with concentration in a curve which may be 
closely represented by the ordinary adsorption formula, a=ka? (where 
a is total acid, x the concentration of external solution, and k and p are 
4 Wied. Ann., 1881, 18, 110. 
5 Plimmer, Chemical Constitution of the Proteids, Part II. p. 11 (Sec. Ed., Long- 
mans, Green, & Co.). The statement seems to require confirmation. 
