COLLOIDS 505 



The investigations as to the size of particles all lead to two gen- 

 eral conclusions : first, suspensions and colloidal solutions in general 

 differ from one another mainly in degree of dispersion, at least up 

 to the limit of individual detectibility of the particles under the 

 ultramicroscope, beyond which point at present all is speculation, 

 although the presumption is strong and the belief is growing that 

 there is also no other fundamental distinction to be drawn between 

 colloidal and so-called true solutions ; second, it is always found that 

 unless special purification is resorted to a colloidal solution contains 

 particles of widely differing sizes side by side. 



2. THE BROWNIAN MOVEMENT. About a century ago the Eng- 

 lish botanist, Brown, noticed that very small spores and other bodies 

 when suspended in water, and observed under the microscope, were 

 in a state of rapid oscillatory and rotary motion. This motion of 

 small masses of matter has come to be known as the Brownian move- 

 ment. It is noticed in colloidal solutions whose particles are not too 

 large, and at the same time are large enough to be individually de- 

 tectible under the ultramicroscope. As a result of the theoretical 

 considerations of Einstein, 7 of Smoluchowski 8 and of Corbino, 9 

 and of the experimental researches of Svedberg 10 and of Perrin, 11 

 the Brownian movement has come to be considered as nothing more 

 nor less than a manifestation of that kinetic energy with which all 

 matter is endowed, and which forms the basis of the kinetic theory 

 of gases. A rapidly gyrating and oscillating colloid particle is there- 

 fore looked upon as a large scale picture of the state of the molecules 

 themselves. These investigations have probably done more than any- 

 thing save the development of the kinetic theory itself to place 

 molecular and atomic speculations on a firm basis of plausibility. 



Svedberg's investigations were instituted to determine the mean 

 velocity of colloid particles whose mass could be determined by the 

 ultramicroscopic method above referred to. Computing from these 

 factors the average kinetic energy of the particles, this, according to 

 Svedberg, gives the same value which would be computed for the 

 particle on the basis of the kinetic theory. Perrin attacked the prob- 

 lem from a somewhat different point of view. The number of gas 

 molecules in the atmosphere decreases from the surface of the earth 

 outward at a rate which is determinable by computations based on 

 the kinetic theory. Perrin set himself the task of determining the 

 rate of decrease in the concentration of the particles of a colloid solu- 

 tion, in which the particles were of uniform size, the concentrations 

 being determined at different levels in a cylinder in which the solu- 



7 Ann. der Phys., 9, 417; 11, 170; 17, 549; 19, 371. 



8 Ann. der Phys., 21, 756. 



9 Nuovo Cimento, 20, 5. 



10 Ztschr. f. Elektrochem., 12, 853, 1906. 



11 C. R., 146, 967, 1908. 



