I IO PHYSIOLOGY OF NUTRITION 



Whether the dissolved substance is a gas, liquid or solid under ordinary 

 conditions, it forms an aqueous solution when it is dissolved in water. The 

 dissolved substance is usually called the solute and the water in which it 

 dissolves is the solvent. A solution may contain many different kinds of solutes, 

 all dissolved in the common solvent. All dissolved substances diffuse in all 

 directions within the limits of the solution or solvent, and tend to become equally 

 distributed throughout its volume. If two solutions having a common solvent 

 but different solutes be brought into contact, the two solutes diffuse into each 

 other's region and they eventually become completely mixed, so as to form a 

 single solution of two solutes. The solvent itself exhibits a corresponding 

 tendency to diffuse in all directions; if a mass of pure water be brought into 

 contact with an aqueous solution, water enters the solution and dilutes it, while 

 the solute or solutes enter the water and convert it into a solution, this process 

 continuing until the resulting solution becomes uniform throughout. (If the 

 solute be another liquid — as alcohol, glycerine, etc. — the solute may become 

 the solvent when it predominates. Thus we may have a solution of glycerine 

 in water or a solution of water in glycerine, etc.). It appears that the solute 

 and solvent attract each other and that the latter enters between the particles 

 of the former, thus hastening their outward diffusion. If a membrane that is 

 permeable to water but relatively impermeable to the solute be placed around 

 the solution and be, in turn, surrounded by the pure solvent, a pressure, called 

 osmotic pressure, is developed, which tends to drive the membrane outward 

 before the outwardly diffusing solute, thus stretching — or even rupturing — the 

 membrane. This phenomenon of osmotic pressure was discovered by Dut- 

 rochet, 1 as early as 1827, who observed the escape of zoospores from an alga and 

 tried to arrive at an explanation for the bursting of the sporangium. He 

 supposed that an increased absorption of water by the sporangium was brought 

 about by water-attracting substances within, and that this caused the rupture. 

 If an animal bladder filled with aqueous sugar or salt solution is placed in water, 

 the solvent enters, and the outwardly directed osmotic pressure simultaneously 

 developed may become so great as to rupture the membrane itself. The rupture 

 of the alga sporangium as observed by Dutrochet, was caused in a similar way f 



[ l Dutrochet, Rene Joachim Henri, Nouvelles observation sur l'endosmose et l'exosmose, et sur la cause 

 de ce double phenomene. Ann. chim. et phys. 35: 393-400. 1827.] 



{ It is still commonly stated or implied that the entering water turns on itself after entrance, 

 and, thus tending to return, presses outwardly upon the membrane and causes the rupture. 

 But the bladder membrane is, in itself, as permeable to water diffusing in one direction as to the 

 same substance diffusing in the other, and more water enters than passes out, so that if there is 

 a pressure of water in either direction it should tend to collapse the bladder, not to explode it 

 from within. A logical picture may represent the osmotic pressure causing the rupture as 

 directly due to a tendency of the solute particles (as of sugar or salt, or ions), or of any combina- 

 tions of solute particles with water particles (in so far as these are unable to pass the septum), to 

 diffuse outward into the surrounding solvent. This, in turn, may be considered as brought 

 about or made possible by the entrance of water (at least it cannot occur without this entrance), 

 which, finally, may be due to an attraction exerted upon the water by the solute. Such a 

 simple picture may still serve the purposes of physiology, although serious complications appear 

 to arise sometimes when a complete appreciation of osmotic and related phenomena is 

 attempted. The most thorough discussion of osmotic pressure so far available is that given 

 by Washburn [see note e, p. 109]. — Ed. 



