ULTRAMICROSCOPY 85 



per cm., can be determined. In the above example, this 

 velocity would be y2$ x io -3 mm. per second. 



The Inverse Layers. — Using a thick preparation (about 

 o"i mm.), and observing at some distance from the walls, all 

 the particles of a given colloid move in the same direction ; 

 this direction is reversed when the field is reversed ; for positive 

 and negative liquids the movements are in opposite directions. 



Leaving the central portion of the liquid, and observing 

 parts near the walls, some differences are found. Here the 

 granules move in a direction opposite to that taken by the 

 rest of them, whatever the nature of the colloid. The thickness 

 of this " inverse layer," as it is called, depends on the thickness 

 of the preparation, and increases as that of the latter diminishes. 



Alternating Electromotive Zones. — Returning to preparations 

 showing the normal movements, it was mentioned that the 

 direction of motion of the ultramicroscopic particles is reversed 

 when the potential difference is reversed. It is interesting to 

 observe the effect of reversing this potential at short intervals, 

 which may be done by using an alternating electromotive force. 

 The magnitude of the force in this case will usually be con- 

 tinually changing, and at intervals the force reverses sign. 

 Under these conditions the spot of light seen in the microscope 

 is drawn out into a band of light, slightly thickened at the 

 ends, and somewhat resembling a dumb-bell. 1 This linear 

 appearance is, of course, due to persistence of vision, the 

 particle performing linear oscillations parallel to the electro- 

 motive force. If the preparation is rapidly moved at right 

 angles to these lines, the lines are changed into curves. 



This vibratory motion, which has its maximum amplitude 

 in the central part of the liquid, decreases as we approach 

 the walls, and may be entirely eliminated. Knowing the 

 mobilities of the particles, the amplitude under a given alter- 

 nating force of known frequency can be calculated, and it is 

 found that the value thus obtained agrees very well with the 

 observed values. 



The Form of the Ultramicroscopic Particles. — As has been 

 mentioned above, from the appearance of a spot of light (nearly 

 always circular) seen in the microscope we cannot at once 

 determine either the size or the form of the particle. The 

 method of determining the size has been discussed above. 

 1 Compare the movements of smoke particles under similar conditions. 



