286 



The Properties of Colloidal Systems. 



these acts as an individual and equivalent element in the production of 

 osmotic pressure. 



Summary. 



Congo-red, although a colloid in the sense of not being diffusible through 

 parchment-paper and exhibiting certain other colloidal properties, has an 

 osmotic pressure equal to that which would be given if it were present in 

 true solution in single molecules. 



The solutions are not resolvable into particles by the ultra-microscope. 



The theoretical osmotic pressure is only to be obtained in the complete 

 absence of extraneous electrolytes. Even the carbonic acid present in 

 ordinary distilled water is sufficient to cause a marked fall in the osmotic 

 pressure. 



The manner in which electrolytes produce this fall is by causing aggrega- 

 tion of molecules to particles. This is the same whether acid, alkali, or 

 neutral salt be in question. 



The action of a stable colloid in protecting against the effect of electrolytes 

 is shown to consist, in the cases of congo-red and arsenious sulphide, in the 

 production of minute aggregates, which, although causing fall in osmotic 

 pressure by diminution of effective concentration, are not of sufficient size to 

 precipitate. Hence the protective power can only be regarded as a limited 

 one, due probably to the formation of complex colloids. 



The free acid of congo-red forms a blue colloidal solution when dialysed. 

 This is easily resolvable under the ultra-microscope, but gives a definite and 

 measurable, though small, osmotic pressure, about 14 mm. Hg for a 1-per-cent. 

 solution. Assuming the kinetic theory to be correct, this means that the 

 aggregates contain, on an average, 20 molecules. 



Estimation of molecular dimensions are given on the basis of enumeration 

 of the number of particles in unit volume by means of the ultra-microscope. 

 The values found are considerably larger than the accepted ones for water, etc. 



The whole of the results are capable of explanation on the assumption that 

 colloidal particles possess the kinetic energy of molecules, but do not lend 

 support to any view which postulates the necessary presence of foreign 

 electrolytes. 



(The expenses were defrayed by a grant from the Government Grant 

 Committee of the Eoyal Society.) 



