990 



DIFFUSION AND OSMOSIS. 



601. Different salts in solutions of equal strength diffuse in 

 unequal times. Thus potassic lydrate difi'uses with double the 

 rate of potassic sulphate, and the latter with double the rate 

 of crj'stallized sugar. But these substances have a compara- 

 tively higli rate of diffusion. A solution of caramel (sugar 

 heated till it becomes brown) diffuses very slowly ; the sugar in 

 this case has been so changed in its character tliat its rate of 

 diffusion has been reduced from a high to a verj- low one. Gela- 

 tin maj- be taken as the representative of the almost " fixed" or 

 slowly diffusible class of substances ; most crystalline substances, 

 as lepresentatives of the highly diffusible class. The former are 

 collectively' known as colloids (KoXXa, glue), the latter as crystal- 

 loids. It must be noted that Graham's use of this word " crys- 

 talloid " is different from that in which it has been employed in 

 speaking of the protein bodies (177). 



602. With each salt the rate of diffusion increases at a 

 slightly higher rate than the temperature of the solution. 



603. Tlie members of certain chemical groups are equallj' dif- 

 fusible. Thus hydrochloric, hydrobromic, and hydriodic acids ; 

 the chlorides, bromides, and iodides of the alkaline metals, etc., 

 have equal rates of diffusion into pure water. 



604. The diffusion of a solution of a salt into the dilute solution 

 of another salt takes place nearly' as rapidlj- as into pure water ; 



The difference in the rates of diffusion of ten per cent solutions of different 

 substances experimented upon in the manner described on the preceding page 

 is clearly shown by the auuexed table. 



The first series of experiments are described in Philosophical Transactions, 

 1850 ; the second, in 1861. 



