ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 259 



higher concentration from that of lower concentration by a 

 semipermeable membrane. As was pointed out before, this 

 means a membrane which will not give passage to a dis- 

 solved substance but only to its , solvent. The dissolved 

 substance in its movement through the smvent is then stopped 

 by this wall, in consequence of which it exerts a pressure 

 (osmotic pressure) upon it which evidences itself, if the 

 membrane is not so supported as to prevent it, by a bulging 

 of the membrane toward the region of lower osmotic pres- 

 sure. 



Just as gas pressure varies with different external con- 

 ditions, so does osmotic pressure (VAN'T HOFF). Of great 

 physiological importance is the fact that at constant tem- 

 perature the osmotic pressure of dilute solutions is proportional 

 to the concentration of the dissolved substance. By the con- 

 centration of the dissolved substance is meant the number 

 of particles of this substance contained in the unit volume. 

 This, the first law of VAN'T HOFF, means that if a solution of 

 a certain concentration has a certain osmotic pressure, the 

 osmotic pressure will be doubled if in the same volume of 

 solvent twice the amount of soluble substance be dissolved, 

 or trebled if three times the original amount goes into solu- 

 tion in it (see below). 



VAN'T HOFF 's second law is also of physiological importance. 

 The osmotic pressure of a dilute solution is proportional to the 

 absolute temperature, in other words, increases as the tem- 

 perature is increased, even when all other external conditions, 

 are left unchanged* The third law of VAN'T HOFF, that at 

 the same temperature equal volumes of all dilute solutions which 

 have the same osmotic pressure contain the same number of 

 molecules does not interest us in a discussion of the immediate 

 problem. 



We have purposely spoken above of the osmotic pressure 

 of dissolved particles. We can say now that wherever this 

 term has been used above we may substitute the word 

 molecules, but this only when the substance under con- 



