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18 GENERAL AND PHYSICO-CHEMICAL.
closer must the filter be Extensive experiments on the filtering of
colloids have been carried out by Bechhold.1
He used paper filters
which were impregnated with collodion dissolved in glacial acetic acid.
According to the concentration of the collodion solution filters of dif-
ferent porosity were obtained. The colloid solutions were pressed
through the filter by a pressure up to five atmospheres. It was shown
that all colloid solutions contained particles of various sizes. Never-
theless for every solution a filter could be prepared whose pores were
small enough to retain all the particles. In this manner Bechhold was
able to classify the colloids in a series according to the size of the smallest
particles. He found that in general the inorganic colloids (Prussian
blue, platinum, iron oxide, gold, silver) form larger particles than the
organic colloids (gelatin, haemoglobin, seralbumin, proteoses, dextrin).
Still it must be remarked that according to Zsigmondy2
the size of the
particles of the same colloid are larger in one preparation than in another
and that the size can change on keeping.
On filtering proteose solutions through filters of unequal thickness
Bechhold was able to show that the larger the particles of the proteoses,
the easier are they precipitable by ammonium sulphate.
Diffusion. We have already seen that the osmotic pressure of a
colloid solution is very small and also that the osmotic pressure of a solu-
tion is the cause for the diffusion of the particles, therefore it is evident
that the diffusion ability of colloids can only be very slight. This is
not only true for the free diffusion but also for the diffusion through a
membrane. Both of these was first studied by Graham. The first was
found very slight but measurable in several cases while the fact that
the colloids did not diffuse through membranes (non-dialysable) was
given as the most constant difference between colloids and crystalloids.
Nevertheless, there does not exist any sharp boundary and dialysis depends
principally upon the size of the particles as well as upon the character of
the membrane.
Internal Friction. By the internal friction of a fluid we mean the
force whiph resists the displacement of the particles of the fluid among
one another. The internal friction is therefore an expression for the
great thickness or viscosity of the fluid.
For physiological purposes the internal friction is determined by
measuring the time which a given volume of the fluid requires to flow
through a capillary tube under a pressure of its own weight.
It is generally accepted that the internal friction of suspension col-
’ ZeilM hr. f. physik. Chem., 60, 257 (1907).
2
Zur Erkenntnis d. Koll., (1905), 104 as well aB Zeitschr. f. Elektrochem., 12,
681 Ifl
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