202 

opalescence gradually disappears as the gel particles 
become smaller with increasing concentration. Gela- 
tin jelly is therefore a gelatinous precipitate of gelatin 
of at least 0-7 per cent. concentration. 
The size of the particles can be increased by allow- 
ing them to grow by spontaneous evaporation of the 
solution, subject to the necessary precautions. After 
one month the precipitate is buff-coloured, and 
appears as a mass of perfectly spherical microscopic 
grains exactly like Perrin’s grains of mastic. From 
these and many similar experiments, in conjunction 
with the ultramicroscopic appearance, it may be 
concluded that a gelatin jelly is a mass of ultra- 
microscopic spherites of gelatin in which, as von 
Nageli suggested, the water is held by molecular 
forces. These forces are the cause of the swelling 
in water, and the heat of swelling can be calculated 
roughly on this supposition. The structure fits 
exactly Zsigmondy’s analysis of the vapour pressure 
isotherms. Experiments on the relation between the 
excess concentration and the size of particle are being 
made and may lead to a more definite form of 
von Weimarn’s equation. But, in its present state, the 
formula is sufficient to explain the occurrence of 
gelatin in the colloidal state. The molecular weight 
is unknown, but Dakin’s recent analyses suggest 
that it may be as great as 10,000, or more, a value 
which would correspond to a molecular diameter of 
0-75 ““ and bring its molecules up to colloidal size. 
But, though the molecular weight should be much 
less, there is no doubt that the molecules are very 
complex, for this is the reason for the very low diffu- 
sion constant ; moreover, as the viscosity also of the 
sols is considerable, the factor K must be very large. 
In addition to this, the solubility, L, is very small, 
and the excess concentration, P, is usually large,, so 
that everything conspires to produce a maximum 
value of N corresponding to the colloid condition. 
The permanence of the jelly is due to the very small 
diffusion constant, which prevents recrystallisation. 
But this does occur slowly, as is shown by the gradual 
appearance of opalescence, and even of microscopic 
spherites, in gels kept for a long time in sealed tubes. 
Since agar-agar also separates from solution in 
the form of spherites, it appears that the structure 
NATURE 
[ FEBRUARY 10, 1923 

of gels of this substance and of gelatin is probably 
that of a pile of shot, while soap and fibrinogen gels 
may be fibrillar. Such a fine-grained structure is 
compatible with all the known properties of gels, 
except the heat of swelling of gelatin, 5-7 cal. per grm., 
and the so-called thermalanomaly. Re-determination 
of the former gave 33:25 cal., and investigation of 
the latter showed that it was unfounded. Two 
questions remain undecided : “(a) The nature of 
the spherites and (b) whether they are joined 
together to produce a framework in the jelly. Spher- 
ites are known in every gradation, from the obviously 
crystalline form, built up of coarse radiating crystalline 
needles separately visible, through stages showing 
only a more or less radiating formation, but giving 
the well-known shadow-cross in polarised light, 
to apparently homogeneous spherical bodies giving 
no definite evidence of crystalline structure. Gelatin 
and agar-agar spherites appear to belong to the last 
class. Experimental evidence suggests that the 
spherites coalesce during gelation. They would seem 
either to aggregate crystallographically or to adhere 
by their mutual attraction ; or, the apparent attraction 
may be due to the water molecules having a greater 
mutual attraction than the spherites. 
mitted, however, that, in the case of such small par- 
ticles, there can be very little difference between the two 
former methods of attachment, since the union must 
be due to the forces between the few molecules in the 
surface of contact. With two grains only, the coupling 
would be unstable, but it would become firm as more 
grains were added. 
Since writing the above, evidence has been obtained 
that the gelatin spherites are really crystalline. 
Some of these, grown to a size of about 3 #, by 
methods previously described, and mounted in 
glycerin, were examined with polarised light. When 
the Nicols were crossed they became brilliantly 
coloured and many showed shadow crosses, while 
grains of mastic prepared by Perrin’s method and 
mounted in the same way became invisible. These 
experiments are being continued, but, without evi- 
dence to the contrary, it will be difficult to deny that 
gelation is merely an extreme case of crystallisation. 
Physical Properties of Clay and Clay-Mud. 
M UD and clay are materials, the properties of 
which are not only of concern to the meticulous 
housewife and to the children who make mud pies 
and clay engines; the geologist has found interest 
in their formation, and from the study of them is 
able to trace a large part of the history of the earth’s 
crust. They have played their part in the esthetic 
development of the race. They have been the 
architect’s and engineer’s friend for the making of 
building materials, and have filled them with concern 
and not infrequently dismay when they have desired 
to build upon them or when they desired to support 
them. The story of the development of buildings, 
bridges, and other types of structures, tells of many 
failures, because of the treacherousness and un- 
certainty of these materials, and partly, at least, 
because engineers and architects had not attempted 
to determine their properties in a scientific manner. 
Mr. A. S. E. Ackermann has presented, during 
recent years, four papers to the Society of Engineers 
in which’-he has described experiments to determine 
the physical properties of clay and the effect of 
water content upon their properties.1_ He has shown 
that, like certain metals, clays have a certain measure 
of fluidity. When a disc resting on clay is loaded 
4 Society of Engineers Transactions, 1919-20-21-22. 
NO. 2780, VOL. 111] 

the disc sinks into the clay, the amount it descends 
depending on the load and on the time allowed; and 
when the load exceeds a certain amount, which 
depends upon the amount of water present, the rate 
and extent of penetration are considerably increased. 
The stress at which this occurs, Mr. Ackermann has 
called the pressure of fluidity. Mr. Ackermann’s 
experiments have been directed toward determining 
the bearing power of soils, and the loads that can 
safely be applied to them. : 
The difficulty of reconciling experimental data on 
the properties of these materials is evidenced by 
comparing the results of experimenters. Mr, Acker- 
mann states that the friction angle for wet mud 
varies as the square root of the pressure, while 
Crosthwaite says it is proportional to the square 
root of the pressure. A special committee of the 
American Society of Civil Engineers to codify present 
practice on the bearing values of soils for foundations, 
has issued a series of reports, and has emphasised 
the importance of the colloid content of clay, which 
consists of non-crystalline, hydrated, gelatinous 
aluminium silicates, gelatinous silicic and hydrated 
ferric oxides; rarely aluminium hydrate may also 
be present. Most of the grains of the minerals in 
the clay are enveloped by colloid, but quartz grains 
It will be ad- | 
