POROSITY. 85 



brane with much greater facility or in larger quantities than 

 others, and that some do nof pass through at all. As a general 

 rule, crystal lizable substances pass through more freely than 

 amorphous substances. Those substances which do not pass 

 through membranes in the process of dialysis are known as 

 colloids, those which diffuse rapidly crystalloids. 



Capillary attraction, or, more generally speaking, surface- 

 attraction, is undoubtedly to some extent the cause of the phen- 

 omena of endosmosis, the surface of the diaphragm exercising 

 an attraction upon the liquids. 



Diffusion of gases. A diffusion similar to that of liquids takes 

 place also when two different gases are separated from each 

 other by some porous substance, such as burned clay, gypsum, 

 and others. 



It has been found that specifically lighter gases diffuse with 

 greater rapidity than the heavier ones. The quantities of two 

 different gases which diffuse into one another in a given time, 

 are inversely as the square roots of their specific gravities. 

 Oxygen is sixteen times as heavy as hydrogen ; when the two 

 gases diffuse, it will be found that four times as much hydrogen 

 has penetrated into the oxygen as of the latter gas into the 

 hydrogen. 



QUESTIONS. 



31. What is porosity? 



32. What two meanings may be assigned to the word surface ? 



33. Mention some phenomena caused by surface-action. 



34. Explain the term adhesion. 



35. Under what circumstances can capillary attraction be noticed, 

 and how does it manifest itself? 



36. Give an explanation of the word absorption, and mention some 

 instances of the absorption of gases by solids or liquids. 



37. What do we understand by diffusion of gases or liquids ? 



38. Define the word endosmosis. 



39. Which substances are most apt to dialyze, and which have no such 

 tendency ? 



40. State the law regarding diffusion of gases through porous sub- 

 stances. 



