222 



DIFFUSION AND OSMOSIS. 



601. Different stilts in solutions of equal strengtli diffuse in 

 unequal times. Thus potassic hydrate diffuses with double the 

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

 of ciystallized sugar. But these substances have a compara- 

 tivel}' high 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 that its rate of 

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

 tin may be taken as the representative of the almost " fixed " or 

 slowly diffusible class of substances ; most crystalline substances, 

 as representatives of the highly diffusible class. The former are 

 collectively known as colloids (/coAAa, 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 emploj'ed 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. The members of certain chemical groups are equally 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 rapidly 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 annexed table. 



The first scries of experiments are described in Philosophical Transactions, 

 1850 ; the second, in 1861. 



