COLLOIDS, LYOPIIOBIC 



and the speed with which they occurred are 

 illustrated in Fig. 23. Particles are shown 

 which have undergone considerable changes 

 within the period of two frames of cine film 

 (He sec). The electron-transparent regions 

 moved very rapidly inside the crystal with- 

 out effecting any change in the external 

 shape, and continued to do so indefinitely 

 under constant beam conditions. 



With certain types of silver iodide parti- 

 cles, mainly the tetrahedral variety, fila- 



r 



i 



I- 



lOOO A' 



H 



Fig. 23. Sequence from a cine film showing a 

 silver iodide particle undergoing changes of con- 

 trast. 



^_OJj^ 



Fig. 24. Filament growth from a single tetra- 

 hedral particle of silver iodide. 



ments appeared to be pushed outwards from 

 the interior as the intensity of the electron 

 beam was increased. Fig. 24 illustrates typi- 

 cal filament growth from the interior of a 

 particle. In many cases the filaments ap- 

 peared to be ribbons with widths as low as 

 30 A; these remained, however, quite rigid 

 and were able to push holes in the support- 

 ing membrane. Some filaments also show^ed 

 well-defined contrasting bands (see Fig. 24). 

 Filament growth was also observed in some 

 rather coagulated regions which received 

 strong electron irradiation. A sequence from 

 a cine film of filament growth is given in Fig, 

 25. 



Many other dynamic processes should be 

 amenable to investigation by electron mi- 

 croscopy using this type of technique. 



General Morphology of Colloidal Par- 

 ticles. The number of substances which can 

 be prepared in the colloidal state is very 



143 



