36 INTRODUCTION. 



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, as a general rule, 

 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. This regularity 

 in the diffusion of gases is expressed in the Law of Graham, thus : 

 'The velocity of the diffusion of any gas is inversely proportional to 

 the square root of its density. 



Indestructibility. All matter is indestructible i. e., cannot pos- 

 sibly be destroyed by any means whatever, and this property is known 

 as indestructibility. Form, shape, appearance, properties, etc., of 

 matter may be changed in many different ways, but the matter itself 

 can never be annihilated. Apparently, matter often disappears, as, 

 for instance, when water evaporates or oil burns; but these apparent 

 destructions indicate simply a change in the form of matter; in both 

 cases gases are formed, which become invisible constituents of the 

 atmospheric air, and can, therefore, not be seen for the time being, 

 but may be recondensed or rendered visible in various ways. 



Not only is matter indestructible, energy also partakes of this 

 property. Energy may be converted from one form into some other 

 form. Motion may be converted into heat, and heat into motion, or 

 this motion into electrical energy and chemical energy. In fact, all 

 the different forms of energy are convertible one into the other with- 

 out loss. This fact is spoken of as the Law of the conservation of 

 energy. 



To repeat : The total quantity of matter in the universe is con- 

 stant, and the same is true of energy. 



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 explana- 

 tion 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 osmose. 39. Which substances are most apt 

 to dialyze, and which have no such tendency ? 40. What is meant by saying 

 that matter and energy are indestructible? 



