Osmosis. 41 



by gaseous bodies like air or steam, upon the walls of 

 confining chambers or vessels, is due to the combined 

 energy of the blows of the molecules against these walls. 

 The greater the number of molecules in a given space 

 and the more rapidly they move the greater is the' pressure 

 they exert. If the temperature of a gas is increased, 

 leaving the volume the same, the pressure is increased in 

 the same ratio, because the velocity with which the mole- 

 cules are moving is increased. So, too, if the number of 

 molecules of a given gas in a given space is increased the 

 pressure is increased in- a like ratio, if the temperature 

 remains the same, because then there are more molecules 

 to strike a unit area in a given time. 



To double the pressure on a gas will reduce its volume 

 one-half and to double the volume of a gas will reduce its 

 pressure one-half. So, too, will doubling the absolute tem- 

 perature of a gas double its pressure if it is not allowed to 

 expand. It is on these accounts that the higher the steam 

 pressure in a boiler the hotter it is and the more work it is 

 capable of doing. 



57. Osmosis. Abbe Nollet, who lived between 1700 and 

 1770, appears to have been the first to record that, if 

 a glass vessel be filled with wine and covered with a 

 bladder and then immersed in water, the contents of the 

 vessel would increase and sometimes to such an extent 

 as to rupture the membrane. Such a phenomenon has 

 been named osmosis, and there are many familiar phe- 

 nomena of every day experience which are of the same 

 nature. 



When dry beans, peas or grain of any kind are put into 

 water they swell, increasing in volume as the wine in 

 ISTollet's covered dish did when placed in water. So, too, 

 if dried raisins, prunes or apples are placed in water 

 they increase in size, thus exhibiting the process of osmosis. 



On the other hand if fresh fruit of almost any kind 

 is placed in a strong solution of sugar it at once begins 

 to shrivel and decrease in size ; this again is due to osmosis, 



