CHAPTER II. 



PHYSICAL CHEMISTRY IN BIOLOGY. 



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 sec- 

 ondly, the water can pass into the solution. If the two fluids at the 

 beginning are in immediate contact with each other then the two proc- 

 esses 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. Semi- 

 permeable 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 For example, such a membrane can be produced by care- 

 fully dropping a concentrated solution of copper sulphate into a dilute 

 solution of potassium ferrocyanide. Thereby the drop of copper sulphate 

 is surrounded by a membrane of copper ferrocyanide, which is imper- 

 vious to copper sulphate as well as to potassium ferrocyanide, but allows 

 water to pass. The drops retain their blue color in the yellow solution 

 but increase in volume, due to the taking up of water, until the tension of 

 the membrane prevents the further increase in size. If the difference in 



1 Arch. f. (Anat. u.) Physiol., 1867, pages 87 and 129. 



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