236 A TEXTBOOK OF BOTANY [Ch. V, 5 



soft cells more tense and explosive ; but warm water is not 

 used because it tends to fill the air spaces and thus deaden 

 the explosions. Sugar and salt are effective preservatives of 

 fruits and meats, though not in themselves deadly to the 

 living organisms (germs) which cause decay; and the fact 

 that those substances must be used in great strength sug- 

 gests the explanation, that they inhibit the activity of the 

 germs by osmotically robbing them of water. Beans or 

 rice are cooked more quickly and perfectly if not salted until 

 nearly done, and indeed if placed in water too strongly 

 salted at the start may refuse to swell at all. The sensation 

 of thirst which follows the eating of much sugar or salt has 

 apparently this basis, that those substances withdraw water 

 from the stomach, thus causing the thirst sensation. The 

 student will be able to cite other examples of osmotic phe- 

 nomena in daily experience. 



Closely connected with osmosis, of which it is part, is 

 diffusion. When the molecules of a substance are re- 

 moved beyond the range of one another's cohesive attrac- 

 tion, as in a gas or a solution, they acquire an active 

 back-and-forth motion from the kinetic energy of the 

 heat waves reaching them from the surroundings. Thereby 

 they strike and rebound from one another, and hence are 

 worked outward, exerting pressure if confined, but spreading 

 indefinitely if not. Accordingly by diffusion any substance 

 as a gas or a solution always tends to work away from places of 

 greater to places of lesser concentration, and away from a place 

 where it is being produced, and towards a place where it is 

 being absorbed, each substance diffusing in general as though it 

 alone were concerned. Familiar phenomena of gaseous dif- 

 fusion occur in the spread of odors through a house, of floral 

 fragrance through gardens, and of smoke through the air; 

 while solution diffusion is illustrated by the spread of ink 

 or sugar through water. This is the way that carbon dioxide, 

 in photosynthesis, passes from the great reservoir of that gas, 

 the atmosphere, through the stomata and along the air- 



