266 SCIENCE PROGRESS 



chloric acid. Since all material bodies have at least three kinds 

 of surface, that which separates them from other objects, 

 their atomic surface and that of the electrons of which the 

 atoms are composed, the study of the manifestations of surface 

 energy embraces a universal field, to which v. Weimarn has 

 given the name of " Dispersoidology." 



Like most other fundamental precepts of science, v. Wei- 

 marn 's theory is capable of simple expression. The form 

 in which a substance is deposited from the gaseous or liquid 

 state is conditioned by the number, N, of condensation centres 

 produced, as determined by the equation : 



N = K r I 



where P is the mass of the body about to be deposited, in 

 excess of that which will remain in solution, and L is its solu- 

 bility, or partial pressure, in the particular state of subdivision 

 concerned. K is a factor which is proportional to the degrees 

 of aggregation of the substances taking part in, or present 

 during, the reaction, and to the viscosity of the reaction medium. 

 Of these, solubility is the quantity which has the greatest 

 influence on the form of the precipitate, and is, in fact, the 

 decisive factor. Thus bodies like soda and copper sulphate, 

 which are easily soluble, occur naturally in large crystals ; 

 while, at the other extreme, compounds which are almost 

 insoluble, such as aluminium hydroxide and silicic acid, result 

 in the gelatinous form. 



For the production of the colloid condition it is essential 

 that, in the medium chosen, L should be exceedingly small. 

 Then, by varying the value of P, v. Weimarn has shown that 

 five distinct states are produced : 



I. Corresponding to solutions which deposit no pre- 

 cipitate for a long time, until after some years 

 the solid phase appears in the form of macro- 

 crystals. 

 II. Corresponding to solutions which deposit macro- 

 crystals in a short time. 

 III. In which a precipitate separates in the form of 

 crystalline growth figures (crystalline skeletons 

 and needles). 



