chap, xxii.] FILTRATION. 267 



of fluids through membranes under pressure, the 

 membrane requiring to be permeable. This is entirely 

 different from the passage of fluids through a mem- 

 brane when the membrane is bathed by two different 

 fluids on opposite sides. 



Elaborate researches on filtration have been made 

 by various experimenters. A large number of great 

 interest and importance are published by Dr. Wilibald 

 Schmidt of Voigtland, in Po^gendorfFs " Anna! en des 

 Physic uncl Chemie," for 1856 (p. 337), and 1861 

 (p. 337): Schmidt used animal membranes, specially 

 the pericardium of the ox. Briefly put, Schmidt's 

 more important results are, that each substance has 

 its own rate of filtration, that crystalloids filter more 

 quickly than colloids, that the amount of a colloid which 

 will filter through a membrane from a liquid containing 

 colloid in solution increases with the concentration of 

 the liquid in the filter, and with the pressure that it 

 diminishes with the weakness of the solution in the 

 filter, with diminished pressure, and with increased 

 temperature ; if the liquid containing colloid in 

 solution contains also crystalloids, the quantity of 

 colloid filtered through is less than it would have 

 been had the crystalloids been absent, and the filtrate 

 is richer in crystalloids than the liquid in the filter 

 (i.e. the presence of crystalloids diminishes the speed 

 of filtration of colloids). In regard to the filtration 

 of mixed solutions of crystalloids and colloids, Schmidt 

 corroborates previous results of von Wittich, that the 

 change in a liquid by filtration is a quantitative, not a 

 qualitative, one ; the filtrate, that is, contains the 

 same substances as the liquid on the filter, though in. 

 different proportions. 



Transudation under pressure is seen in the living 

 body. Pressure on a vein, obstructing the course 

 of the blood, will cause in time filtration, owing to 

 the accumulating pressure behind the obstruction. 



