There are fluids on both sides of the membrane. The circumstances which determine their passage are mainly five. 1. _The densities of the liquids:_ other things being equal, the lighter of the two tends to pass through to the heavier, more than the heavier to the lighter. 2. _Their attraction for the intervening membrane._--That one passes through most which has the greatest affinity for the membrane. 3. _The affinity of the fluids for each other._--A fluid passes through more rapidly when it is readily taken up and dissolved by that on the opposite side. By virtue of the last two laws, but contrary to the first, water passes through to alcohol more rapidly than alcohol to water. 4. _The motion of the fluid on one side_ promotes the passage through of that on the other, because it is carried off as fast as it permeates the membrane. This also will cause endosmosis in defiance of the first law. This may be proved by a simple experiment. Let a large vein, cleanly dissected, be attached at one end to the stopcock of a vessel containing pure water. Let it then pass through a basin containing a strong solution of Ferrocyanide of Potassium, and let the other end hang over a jar filled with a solution of the Sesquichloride of Iron. If the cock be now turned, and water be allowed to pass through the vein into the vessel beyond, the solution of Iron will quickly acquire the tint of Prussian blue; for the heavy solution in the central vessel has passed through to the simple water of the vein, mainly by virtue of the motion of the latter. It will be seen directly that this is a matter of considerable importance. 5. The last law is, that _any pressure on the fluid on one side of the membrane_ has a powerful influence in determining the passage of the current from that side. (_Vide Liebig's Animal Chemistry_, p. 72; _Liebig on the Motion of the Juices_; and the _Lectures of Professor Matteucci of Pisa_.) The mucous membrane and the coat of the capillary vessel beyond it are probably subject to about the same physical conditions of absorption as those which are thus found by experiment to regulate the passage of fluids through dead animal membranes.