ELECTRON MICROSCOPY 



nuclear sol is obtained. Further growth is coagula obtained can be determined from 



slow and the sol shows a fairly narrow parti- micrographs. These are found to vary con- 



cle size distribution. siderably according to the system and coagu- 



An important additive to the silver halide lating agent employed, typical examples 



sols formed for coating photographic plates being edge to edge adhesion of flat plates, 



is gelatin. An electron microscope examina- irregular clumps, chains and massive "lace- 



tion of the growth of silver bromide particles like" aggregates of particles embedded in 



in gelatin has been carried out by Ammann- additive (see Fig. 13). 

 Brass (29), and the effect of a number of 



other polymers of high molecular weight by ^he Structure of Sol Particles 



Perry (30). In the latter case the nature of Micro -diffraction Examination. With 

 the polymer was found to have a profound an electron beam, as with an x-ray beam, a 

 effect on the growth process and on the regular crystal lattice acts as a diffraction 

 morphology of the crystals obtained. grating and gives rise to a diffraction pat- 

 Studies on Flocculation. The floccula- tern. Thus the electron microscope may be 

 tion of sols is usually considered to include used as a diffraction camera. In earlier 

 both the process of coagulation, i.e., the ac- machines a different specimen holder was 

 tual adhesion of the particles, and the subse- often used for diffraction but in many mod- 

 quent processes of recrystallization etc. ern machines the specimen is left in the same 

 Coagulation can often occur during the dry- position and hence diffraction and micros- 

 ing down of specimens for electron micro- copy can be carried out consecutively on 

 scopic examination and as such is of purely the same specimen. Moreover, with the elec- 

 nuisance value. Direct electron microscopic tron optical arrangements available it is 

 studies, however, do provide a useful method possible to select a specimen area of diameter 

 of obtaining information on coagulation down to 2,000 A and to obtain a diffraction 

 processes provided precautions are taken to pattern from this region alone. It is also pos- 

 eliminate artefacts occurring during the sible to modify instruments so that diffrac- 

 drying down process. tion patterns are produced under the same 

 Electron microscope studies on the coagu- illuminating conditions as used for micros- 

 lation of silver iodide sols have been carried copy (32). Hence by this means, single col- 

 out by Mirnik, Strohal, Wrischer and Tezak loidal particles can be isolated and diffraction 

 (31) and by Home, Matijevic, Ottewill and patterns obtained directly from them; in the 

 Home (28). The former authors studied the case of thin plates with a cross-sectional dis- 

 formation and coagulation of the sol as a tance greater than 2000 A it is possible to 

 function of time and the latter authors the isolate particular regions for examination, 

 effect of dodecylpyridinium iodide, at vari- Thus with colloidal particles which are single 

 ous concentrations, on the sol formation crystals a symmetrical spot pattern is ob- 

 process. tained (see Fig. 14). If the inclination of the 

 So far electron microscopy does not appear single-crystal specimen to the electron beam 

 to have been employed to carry out quanti- is changed, the diffraction pattern alters ac- 

 tative studies on the kinetics of coagulation, cording to the amount and direction of the 

 but it does form a useful method of confirm- inclination. This effect has been studied in 

 ing whether effects observed in quantitative detail bySuitoandUyeda (33) using lamellar 

 measurements by other methods, e.g., spec- gold crystals. For the detailed interpretation 

 trophotometry, are really to be attributed to and analysis of micro-diffraction patterns 

 coagulation or to other effects such as re- the relationship between the reciprocal lat- 

 crystallization. Moreover, the form of the tice of the crystal and the Ewald sphere must 



136 



