612 SPECIAL PHYSIOLOGY. 



animal membrane. It is this diffusion of crystalloids away from col- 

 loids through a membrane which is termed, by Prof. Graham, dialysis. 

 It is through this second essential condition, diffusion, that the cur- 

 rent is completed. When the fluid having the greater affinity for the 

 membrane has passed through its pores and arrived at the opposite 

 side, if it meets there with a fluid with which it is chemically compati- 

 ble and into which it is capable of diffusing itself, it will so diffuse 

 itself from the membrane, where it is replaced by a fresh supply. 

 Thus, a constant current will be established from one side of the mem- 

 brane to the other, producing the so-called first current* which, it will 

 be seen, depends upon affinity and diffusibility, and whose direction 

 is thus determined without regard to the density of either, while the 

 rapidity of the current evidently depends upon diffusibility. 



It has been stated, however, in the definition of osmosis, that there 

 is an interchange or mutual action between the two liquids, that is, that 

 a second or reverse current is produced in an opposite direction to the 

 first. With this, affinity, at least for the membrane, has nothing to 

 do, because the affinity of the first fluid being greatest has superseded 

 any which the second might have. Two explanations are usually given 

 to account for it. In the first, the reverse current is explained by 

 the supposition that a diffusion of the second fluid takes place through 

 the first already occupying the capillary pores of the membrane. Ac- 

 cording to the second explanation, suggested by Prof. Graham, it is 

 not true capillary attraction which causes the flow of liquids through 

 animal membranes, but it is due to a combination and decompo- 

 sition taking place in the membrane. When a colloidal membrane is 

 in contact with pure water on one side, and a saline solution on the 

 other, it combines with the water, but the saline solution having a 

 stronger attraction for the water than the membrane has, takes it 

 away, and thus by a constant hydration and de-hydration of the inter- 

 vening colloid, the motion of the currents is established (Youmans 

 New Chemistry]. It may, perhaps, be more clearly indicated as fol- 

 lows : Suppose the membrane to be indicated by m, the first fluid by 

 a, and the second by b, then, although a has a greater affinity for 

 m than has 6, yet a may have a greater affinity for b than for m. 

 Hence, b is taken up by a and carried to the opposite side. Practi- 

 cally, there appears little difference in these two modes of explanation, 

 while the simplicity of the first recommends itself. 



Although these two may be said to be the essential conditions of 

 osmosis, there are certain other influences which may modify it in 

 varying degrees. 



1. To secure the most favorable condition of osmosis, a certain de- 

 gree of temperature is necessary. If the temperature be depressed to 

 the freezing-point, the current is almost entirely stopped; if it be much 

 above 100, the process is also interfered with, while a temperature of 

 from 90 to 100 seems to be most favorable. 



2. A certain amount of motion of the fluids is also necessary to 

 rapid osmosis. If there be no such motion, those portions of the 

 fluid in immediate contact with the membrane soon become of equal 

 density on either side (unless diffusion is very rapid so as to produce 



