32O NATURE OF COLLOIDAL SOLUTIONS 



versely through a true solution placed in the field of a powerful 

 microscope, the ray passing at right angles to the optical axis. 

 No light enters the lens, and the whole field remains in darkness. 

 If, however, a colloid solution be similarly treated, these particles 

 will reflect the light in every direction, and some rays will enter 

 the lens. The result is that the particles are seen as minute 

 luminous points on a dark background. The process is analogous 

 with the examination of the stars with a telescope, w r hich, no 

 matter how powerful, never shows the star as a disc, but, merely 

 by collecting more light, renders it brighter, and shows stars so 

 small as to be invisible to the naked eye. 



The particles in question are so small that they are prevented 

 from falling to the bottom of the fluid by friction ; if, however, 

 they clump together, the particles become heavier, whilst the 

 surface which they expose to the fluid does not increase at so great 

 a rate, until at last aggregates of molecules are formed, which fall 

 more or less rapidly. This is what takes place in coagulation of 

 a proteid, whether brought about by heat or by the addition of 

 electrolytes, etc. The factors which inhibit and which cause this 

 clumping of molecules thus come to be of vital importance. The 

 force which tends to cause this clumping is probably surface 

 tension (Hardy, Bredig, Perrin), which is developed at the junc- 

 tion of the molecule and the water, and which, as explained in the 

 section dealing with agglutination, tends to draw together any 

 particles within a certain distance of one another, so that the 

 surface may become as small as possible. This, of course, is not 

 a peculiarity of colloidal solutions, but occurs in all emulsions of 

 insoluble particles. Surface tension, therefore, is constantly tend- 

 ing to make the particles of colloid in a solution approach one 

 another, form aggregates, and so cause precipitation or coagula- 

 tion. 



This action is counterbalanced in a stable solution by a force 

 of electrical repulsion. The colloid particles all carry a feeble 

 charge of positive or negative electricity, and therefore tend to 

 repel one another. The existence of this charge and its nature 

 can be shown by passing an electrical current through the fluid. 

 Under these circumstances the colloids do not undergo dissocia- 

 tion with passage of the + ions to the - pole, and vice versa, like 

 the electrolytes ; instead, the molecules pass bodily to one or other 

 pole, according to the sign of the electric charge they carry. Thus 

 Field and Teague find that antitoxin is carried towards the cathode, 



