ULTRAMICROSCOPY 75 



to be slower. It has been shown that some of these particles 

 have a radius about forty times as large as that of a molecule, 

 which for hydrogen, oxygen, and nitrogen is about 3 x io~ 8 cm. ; 

 and it can be understood how a membrane, which is permeable 

 to particles of molecular size, may be impermeable to these 

 larger ones. 



Brownian Movements 



Any one observing hydrosols and other colloidal liquids 

 with the ultramicroscope cannot help being struck by the fact 

 that the small particles are not at rest but are constantly in 

 motion. The movements are quite irregular and neighbouring 

 particles appear to move quite independently ; it is only when 

 they come very close together, as they do in some flocculi, that 

 the movement diminishes or ceases. The movements can often 

 be seen with the microscope using ordinary illumination ; it 

 was in this manner that Brown first observed them as early 

 as 1830. Later, they were described and studied in some detail 

 by Gouy. 



The introduction of ultramicroscopic methods, however, has 

 greatly facilitated their study. This depends on the fact first 

 observed by Gouy, that the movements increase in intensity, as 

 the size of the particles diminishes. With very small particles, 

 in addition to an oscillatory motion, there is observed also 

 a translatory one. 



The movements are shown very clearly by the particles 

 in the hydrosols of gold and silver. The particles not only 

 move, but show variations in brightness ; at one instant a 

 particle is quite bright and then it disappears, only to reappear 

 again after a short interval. These scintillations are often very 

 conspicuous. 



Szigmondy gives a very interesting description of the 

 Brownian movements, after first observing them. He says : 

 " The small particles (of gold) no longer oscillate ; they move 

 and with a marvellous rapidity. He who has seen a swarm of 

 flies dance in the sun's rays can picture the movements of the 

 particles of the hydrosol of gold. They frisk, dance and leap, 

 approach and retreat, so that one scarcely knows where one 

 is in this tumult. For the smallest particles, the movements 

 seem to be different from the ordinary Brownian movements 

 (oscillatory) ; they have a translatory motion as well. After 



