SECT. m. CRYSTALLOIDS. 109 



and equal, the three jars exactly the same in size and 

 form, and contained the same quantity of water; but 

 the first vial contained a solution of gum arabic, the 

 second a solution of Epsom salt, and the third a solu- 

 tion of common salt. After fourteen days the diffusion 

 of the gum had risen through one half of the superin- 

 cumbent water, while the particles of both the salts 

 had risen to the surface. However the common salt 

 would have risen much higher, for when the strata of 

 water at the two surfaces were drawn off by a siphon 

 and evaporated to dryness, there was fifteen times as 

 much common salt as Epsom salt. The three solutions 

 are heavier than water, yet they rise notwithstanding 

 their gravitation, whence Mr. Graham thinks that 

 there is probably an attraction between the particles 

 of the dissolved substances and those of the water. 

 The force of molecular attraction is more powerful 

 than gravitation, hence the -particles must rise by the 

 difference of the two forces. 



After many comparative experiments the professor 

 concluded that most substances differ in diffusibility, 

 and that crystalloids or crystalline substances such as 

 salts, sugars, &c., are much more diffusible than colloids 

 or amorphous sticky bodies, such as gum, caramel, 

 jellies, and substances that combine with the hydrogen ^ y, 

 of the water to form gelatinous hydrates. 



The partial decomposition of definite chemical com- 

 pounds may be effected by diffusion. Alum, which is a 

 double sulphate of the two metals potassium and 

 aluminium, furnishes an example ; when allowed to 

 diffuse itself from its aqueous solution, the diffusive 

 tendency of potassium compounds is so much greater 

 than the diffusive property of aluminium compounds, 

 that a portion of the sulphate of potassium actually 

 breaks away from the sulphate of aluminium with which 

 it was combined, in order to diffuse itself in the super- 



