
attraction. 

physical Jaws. 
To the second class belong both Hardy’s and Wo. 
Ostwald’s theories that gels are composed of two 
liquid phases. Theories of this type present the same 
difficulty as those of the first class in Sepa the 
increase of viscosity during gelation. o emulsions 
are known having properties really like those of gels. 
Nor can liquid-liquid systems be imagined with the 
elastic properties of gels. Moreover, no hypothesis 
in either of these classes is sufficient to allow the 
deduction of the various properties of gels, and there 
is little direct evidence for any of these suppositions. 
The third type of theory, that gels contain both 
liquid and solid, is the most natural, and was the 
earliest to be proposed. The first definite suggestion 
parents to have been made by M. L. Frankenheim in 
1835, who thought that jellies were aggregates of small 
crystals with pores between them. A similar view 
was adopted in 1879 by K. von Nageli, that such 
bodies were composed of molecular complexes, or 
micelle, with crystalline properties, separated by 
skins of water and forming meshes (or interstices) 
in which the water was contained by molecular 
From the use of the German word 
Maschen it has been inferred that von Nageli 
intended a geometrical framework. But it is not 
necessary to assume that he meant more than that 
the water was held by molecular attraction in the 
interstices of the aggregates (within and between), 
and that the aggregates were separated by capillary 
skins of water. This view is almost exactly that 
which must be accepted as the result of recent experi- 
ments. However, since von Nageli, a number of 
unsuccessful attempts have been made to devise a 
mathematical network which would account for the 
elastic and thermal popes of jellies; it has 
scarcely been recognised that such a framework must 
conform to the facts that the elastic properties of 
different gels differ greatly, and that the different 
directive forces inherent in the ultimate particles of 
different jellies must have some effect on their struc- 
ture. On this account it seems unlikely that a single 
framework would be found to satisfy the different 
properties of different gels ; it appears more probable 
that the structure of gels will be found to vary accord- 
ing to the nature of the gel substance. 
O. Biitschli’s extensive researches on foams and 
gel structure are well known. He came to the 
opinion that the properties of gels might be explained 
on the basis of a honeycomb structure, in which the 
walls were permeable to liquids because of their 
extreme thinness, although they might be porous. 
To this it must be objected that the use of alcohol 
and tanning reagents to bring out the microstructure, 
adopted by Biitschli, and later by Moeller, is open.to, 
objection, as being likely to alter the structure of the 
s or to modify the gelation process. Moreover, 
Zsigmondy and Bachmann have demonstrated, from 
the vapour pressure isotherms, that gels must contain 
fine pores with a radius of from 2-5 to 5 uu, some 300 
times smaller than Biitschli’s honeycombs. From 
microscopic work on soap curds and gels, these workers 
and McBain have favoured a fibrillar structure. This 
view has also been adopted by Moeller for gelatin, and 
is supported by Barratt from experiments on fibrinogen 
gels. For the soaps and fibrinogen there is direct 
microscopic evidence that fibrils can be formed by 
the cooling solutions, indicating that the ultra- 
microscopic structure of their gels may be fibrillar. 
In other cases a globulitic structure is indicated. 
Menz observed the development of submicrons in 
gelating 2 per cent. gelatin, which increased from 4, 
showing Brownian movement, in a square division of 
the field with a side of 9 4, to 80 or 100, at rest, in the 
NO. 2780, VOL. 111] 
ey Ae Ae Che 1 . 
hey wt ne ; . : 
FEBRUARY I0, 1923] NATURE 201 
acids can be explained by simple chemical and | same area. Hardy describes the appearance of micro- 
scopic spherites of ro u in gels of 5-dimethylaminoanilo 
3 : 4-diphenylcyclo-pentene-1 : 2-dione, and Bachmann 
showed that the ultramicroscopic appearance of 
gelatin gels deprived of water is globulitic. 
Thus there is much evidence for the liquid-solid 
type of theory. Nor is Debye and Scherrer’s X-ray 
analysis sufficient to show that the ultimate particles 
of gels are not crystalline, because the radial elements 
of the spherites, in which form experiments show 
gelatin and agar-agar to be deposited from solution, 
cannot be composed of many layers of molecules, and 
it is doubtful whether such complex molecules could 
eg appreciable interference of monochromatic 
X-rays. 
But none of the theories mentioned is sufficiently 
definite to permit the deduction of the properties of 
gels and the explanation of the reversible sol-gel 
transformation. Nor do they suggest a reason why 
such substances as gelatin and agar-agar should occur 
invariably in the colloid state. The latter question 
proves to bea crucialone. Investigation shows that, 
in this respect, there is no fundamental difference 
between gelatin and other substances—that the same 
laws govern their solution and precipitation; and 
gelation is merely a limiting case a copstelnagtivan 
In this connexion the researches of von Weimarn 
are of fundamental importance. From a great many 
experiments with such substances as barium sulphate 
and aluminium hydroxide he deduced an empirical 
formula, 
me 
N Kp 
which expresses a relation between N, the “ form 
coefficient ’’ of the precipitate, and K, P, and L, 
respectively functions of the viscosity of the reaction 
medium together with the size and structure of the 
particles in solution, the excess concentration of the 
substance to be precipitated, and its solubility. Von 
Weimarn was able to show that as N increases, the 
precipitate passes through stages in which it appears 
as (1) large complete crystals only after some years, 
(2) ordinary crystals in a short time, (3) growth figures 
or needles, (4) amorphous precipitates frequently 
showing microscopic spherical grains, and (5) as a 
gel which cannot be differentiated by the microscope. 
The formula suggests at once that gelation is merely 
an extreme case of crystallisation, and that gelatin 
is a substance the properties of which lead naturally 
to a high value of N. This is completely borne out 
by experiment. Not only do the properties of gelatin 
sols coincide with those of supersaturated solutions, 
but by reducing the value of N, gelatin is readily 
obtained as a precipitate, with pore microscopi- 
cally visible. The solubility of ashless gelatin in 
water is found to be 0-12 grm. per 100 grm. solution 
at room temperature, i.e. about 18°C. More recently 
Fairbrother and Swan found the value 0-07 per cent. 
at 18° for another brand containing 2-24 per cent. of 
ash. Such a solution is perfectly clear. At 0-13 per 
cent. gelatin forms a metastable solution, which re- 
mains in the supersaturated stage on account of the 
very low diffusion constant of the substance. This 
solution has a beautiful bluish opalescence and may 
be regarded as a typical sol. A further slight increase 
in concentration brings about the precipitation of the 
excess of gelatin as a gelatinous mass appearing in 
the microscope like grains of sand. Many of the 
particles can be separately distinguished ; they are 
spherical in form and up to about 2 u in size. With 
increase of concentration, the bulk of the precipitate 
grows and the particles decrease in size until, at 
about o-7 per cent., the precipitate fills the solution 
and forms a white, slightly opaque jelly. The 
