CHAPTER XIII. 



FURTHER PHYSICAL METHODS IN BLOOD EXAMINATION. 



FREEZING POINT AND ELECTRICAL CONDUCTIVITY. 



THE HEMATOCRIT. 



OSMOTIC PRESSURE. 



In many of the phenomena of the body the osmotic pressure 

 of dissolved substances plays an extremely important part. 

 This is especially true in the study of the blood as a whole, 

 and it is therefore proper at this point to enter upon a short 

 explanation of what is meant by osmotic pressure and what 

 its relations are. 



Nature of Osmotic Pressure. Solids in solution exert a 

 pressure in all directions quite analogous to that observed with 

 gases, and in general the laws connecting increase in pressure 

 with concentration and temperature are the same as for gases. 

 With many solids, however, dissociation in solution or separa- 

 tion into ions takes place and each separate ion behaves as a 

 whole molecule as far as pressure is concerned. 



Some of the simple effects of this pressure are easily ob- 

 served. When a drop of a strong solution of blue vitriol is 

 placed carefully on the surface of a weak solution of potassium 

 ferrocyanide a precipitate of copper ferrocyanide forms as a 

 sheath or membrane around the vitriol drop and holds it in 

 nearly spherical form. If the drop is properly deposited, 

 which requires some care, it will gradually enlarge by the en- 

 trance of water, which dilutes the enclosed copper sulphate. 

 None of the latter passes out and the ferrocyanide solution 

 evidently does not enter since no more precipitate forms within 

 the drop. The copper ferrocyanide membrane must possess 

 therefore some interesting properties; it is permeable for 

 water, it is not permeable for either of the salt solutions. Sim- 

 ilar membranes may be made with a number of substances and 



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