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16 GENERAL AND PHYSICO-CHEMICAL.
behave in many respects like suspension colloids. Suspensions of mastic, 1
colophony,2
and cholesterin3
belong to this class.
The hydrophile colloids stand closer to the crystalloids than do the
suspension colloids, and the transition between the crystalloids and the
hydrophile colloids is only gradual. At the boundary we find the pep-
tones and proteoses which belong to the proteins, but at the same time
dialyze rather well. On the other hand, we also have colloids which to a
certain extent form intermediary steps between the hydrophile colloids
and suspension colloids. Finally, there are also numerous intermediary
members between the suspension colloids and the finely divided substances
suspended in water (kaolin).
Osmotic Pressure. As above stated, the osmotic pressure of solu-
tions of crystalloids can be determined only in exceptional cases by
means of the semipermeable membrane, because it is very difficult to
prepare membranes which are impermeable for crystalloids. As pre-
viously stated, most membranes are impermeable for colloids, and the
osmotic pressure of the colloids can be best directly determined by the
aid of a membrane in a so-called osmometer. As shown bjr Moore and
Roaf, in such an apparatus changes in pressure can be determined which
are not detectable by the determination of the freezing-point.4
Equimolecular solutions of various non-electrolytes give the same
osmotic pressure. From this it follows that when different non-elec-
trolytes exist in solutions with the same percentage concentration, the
osmotic tension of these solutions must be in inverse proportion to their
molecular weights. Certain colloids which will be discussed in another
connection (proteins, glycogen, etc.) must have a very large molecule.
From this it follows that these bodies must exert a very low osmotic
pressure. The proteins always contain a small amount of salts which
«xist either in a sort of combination with the colloids or are to be con-
aidered as contaminations which are difficult to remove. For this reason
it has been repeatedly stated that these salts are responsible for the
small differences in the osmotic pressure. By carefully washing crys-
stalline proteins from serum and egg-white, Reid was able to prepare
bodies which gave finally no osmotic pressure in the osmometer.5
In
opposition to this, Mooue and Roaf as well as Lillie call attention to
the fact that the osmotic pressure of protein solutions is influenced by
the tn-atmcnt which the protein received before the determination.
1
Zeitsc.hr. f . physik. Chem., 57, 47 (1906).
2
Ibid., 38, 385 (1901).
Bioch. Zeitschr., 7, 152 (1908).
* Bioch. Journ., 2, 34 (1906).
• Journ. of Physiol., 31, 438 (1904).
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