50 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



bottom at once, others sink more slowly, but the very 

 fine colloidal particles (clay) remain suspended indefi- 

 nitely, forming a colloid sol with the water. 



Microscopic and Ultramicroscopic Particles. — The 

 larger of these very fine colloid particles, such as those 

 of an Indian ink sol, may be clearly seen under the 

 high power of the microscope. When they are quite 

 freely suspended, that is not adhering to the glass 

 slide or coverslip, they may be seen to be in continuous 

 oscillation, or intermittent jerky movement. This is 

 due to the continuous bombardment of the disperse 

 particles by the molecules of the liquid, which are of 

 course themselves in constant motion. At any given 

 moment the probability is that this bombardment will 

 be unequal on different sides of the suspended particle, 

 and this will therefore move away from the side on 

 which it is most heavily bombarded. If the bombard- 

 ment happens to be momentarily equal on all sides 

 the particle will remain motionless. This kind of 

 movement is called Brownian movement, from its dis- 

 coverer, the botanist Robert Brown. 



The disperse particles of the finer sols, such for 

 instance as a Congo Red sol, are too small to be seen 

 under the highest powers of the microscope, and the 

 sol appears clear. With the highest powers and the 

 best definition the smallest particles that can be clearly 

 defined under the microscope are those not less than 

 about 'IS jLi ' = 'ooois mm. in diameter. But by 

 means of an instrument called the uUramicroscope- — 

 which consists essentially of a high power microscope 

 with lateral instead of vertical illumination — when 

 lighting from the side and viewing against a dark 



' fi is one-thousandth part of a millimeter and is the ordinary unit 

 of microscopic measurement. 



