ELECTRON >IICROSCOPY 



such as proteins, enzymes, viruses and high 5G00 A) the maximum resohition obtainable 

 polymers, which ha^•e molecular weights in is of the order of 2000 A, while with ultra- 

 the region of 1,000 to several million and violet light resolution of the order of 800 A 

 exist in molecular solution, and smokes, may be obtained; such resolution is totally 

 mists, gels, etc.. In this size range, many inadequate for the examination of most col- 

 inorganic materials can be prepared as two- loidal dispersions. However, the wavelength 

 phase systems of small particles in liquid of an electron beam produced at an accelerat- 

 media, and are spoken of as colloidal dis- ing potential of 80 kV is 0.043 A and there- 

 persions or sols. The latter groups are usually fore theoretically resolution of the order of 

 termed hjophobic colloids, and it is with this one A unit should be possible. Final resolu- 

 class of colloidal material that this article is tion, however, is limited by the difhculty of 

 mainly concerned. correcting the lens aberrations and with 

 The most important properties of a lyo- earlier microscopes the resolving power was 

 phobic colloidal dispersion are: only of the order of 30 to 50 A. With many 



(a) the size and shape of the particles, modern instruments the resoh^ing power is 

 and whether the system is dispersed or floe- of the order of 5 A and hence it is possible to 

 culated, resolve objects of the order of atomic di- 



(b) the chemical structure of the particles, mensions. 



(c) the nature and structure of the sur- 



£„_g Experimental Techniques 



(d) the mode of nucleation and growth of Preparation of Specimen Supports. 



the particles, and the possible production of The essential criteria for a supporting mem- 



monodisperse sols, brane are that it should be rigid enough to 



(e) the electrical charge on the surface withstand manipulation, remain stable in 

 (electrical double layer), and its relation to the electron beam during examination and 

 stability. have a thickness of the order of 100 A. Many 

 Moreover, in phenomena such as nucleation, types of materials have been suggested as 

 growth, coagulation and aging of sols the supporting membranes (1) and probably the 

 dynamic aspects of the system have to be membranes most commonly used are pre- 

 considered and a knowledge of changes in pared from "Formvar" or nitrocellulose. The 

 the system with time is required. The elec- "Formvar" or nitrocellulose membrane is 

 tron microscope may be employed to obtain formed on a dish of distilled water and then 

 answers, or some of the answers, to all these picked up on the surface of a copper mesh 

 questions with the exception of (e). It must grid (2). These films, however, are not suffi- 

 be remembered, however, that as specimens ciently stable for high -resolution work; un- 

 are subjected to a high vacuum (ca. 10~^ mm less they are carefully prepared, they have a 

 mercury) in the electron microscope they tendency to drift under the influence of the 

 cannot be investigated in their natural liciuid electron beam. Greater stability can be 

 environment. The exposure of the specimen achieved by evaporating a thin layer of car- 

 to a beam of high energj^ electrons also means bon on to the plastic film, but care must be 

 that suitable precautions must be observed taken not to increase the support thickness 

 to prevent heating, with subsequent sublima- beyond the limits required for high resolu- 

 tion or decomposition of the specimen; tion. 



examination of large specimens may be pre- One of the most stable supporting mem- 

 cluded by this factor. branes can be obtained by evaporating ear- 

 In the case of an optical microscope, when bon onto carefully cleaned glass slides or 

 viewing by white light (average wavelength freshly cleaved mica. On immersing the slide 



124 



