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- I. General and Physico-chemical
- I. Osmotic Pressure
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PHYSIOLOGICAL CHEMISTRY.
CHAPTER I.
GENERAL AND PHYSICO-CHEMICAL.
I. OSMOTIC PRESSURE.
When certain substances are placed in contact with water they
dissolve therein and finally a liquid is obtained which contains an equal
quantity of the dissolved substance in each unit volume. There exists
between the water and the soluble body a certain attractive force. Upon
this force depends also the so-called diffusion, which manifests itself when
two different solutions of the same or different substances are brought
into immediate contact with each other. The dissolved molecules and
the water intermingle with each other so that finally the dissolved
bodies are equally divided in the entire quantity of water. Imagine
a cane-sugar solution in contact with pure water; the equilibrium or the
homogeneity of the system can then be brought about in two ways;
namely, the sugar molecule can migrate in part into the water, and
secondly, the water can pass into the solution. If the two fluids at the
beginning are in immediate contact with each other then the two
processes take place simultaneously.
The conditions change when the two liquids are separated from
each other by a membrane, which allows of the passage of water but
not of the dissolved substance (in this case cane-sugar). In the presence
of such a so-called semipermeable membrane the equilibrium can only
be established by the water passing into the cane-sugar solution.
Semipermeable membranes have been artificially prepared, and they also
occur in nature, or conditions exist which give results like those of the
membranes. To the first group belong Traube’s so-called precipitation
membranes.[1] Such a membrane, for example can be produced by
carefully dropping a concentrated solution of copper sulphate into a dilute
[1] Arch. f. (Anat. u.) Physiol., 1867, pages 87 and 129.
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