Lecture III. 21 



case of the dissolved crystalloid the particles are so minute 

 and the union between the solvent and solute is so intimate, 

 that the phenomena which are characteristic of a surface do 

 not occur. It is evident then, that the mixture of a colloid in 

 water is very different from a true solution. To mark this 

 difference a colloid is said to be dispersed, while a crystalloid 

 is said to be dissolved in the medium which contains it. 



The difference in size of the particles of matter in the two 

 states may be further demonstrated by filtration. It is found 

 that characteristic colloids will not pass through a vegetable 

 or animal membrane, whereas a solution of crystalloids may 

 be transmitted easily. This fact, which is utilised in dialysis 

 to separate crystalloids and colloids, shows clearly the different 

 state of subdivision characteristic of the two states. Thus certain 

 colloids especially the proteins may be broken into smaller 

 aggregates which will pass through a cellulose membrane, but 

 yet are not small enough to exhibit all the characters of dis- 

 solved crystalloids. A complete series of preparations, grading 

 into each other, may be made, in which the lowest members of 

 the series are true solutions (diffuse, exercise osmotic pressure 

 and penetrate organic membranes) while the upper members 

 have all the characteristic properties of true colloids. 



Colloidal particles although invisible with ordinary microscopic 

 methods may be rendered visible by the ultra-microscope, an 

 apparatus by which particles may be microscopically viewed while 

 reflecting a very intense beam of light. 



Particles having a diameter of io~ 5 cm. are scarcely visible 

 with ordinary microscopic methods and at this size their surface 

 is so large compared to their volume that they begin to exhibit 

 phenomena unlike larger visible particles. The smallest col- 

 loidal particles must exceed molecular dimensions (viz. about 

 10 ~ 8 cm.). Therefore the diameter of colloidal particles may 

 be said to range between io~ 5 and io~ 8 cm. Particles having 

 a greater diameter form suspensions. And when subdivision 

 is as fine or finer than the lowest limit mentioned the mixture 

 has the properties of a solution. 



It is evident that, as the proportion of surface to bulk rapidly 

 increases with diminution in size (the volume of a sphere de- 

 pends on the cube while its surface depends on the square of 

 its radius), the surface of a given mass of matter in the col- 

 loidal state is relatively enormous and we may expect peculiarly 

 enhanced effects in dispersed colloids of all actions which take 

 place at surfaces. Thus when a dissolved substance tends to 



