396 JAMES HILLIER 



At first sight it would appear that these conditions place quite 

 stringent restrictions on the range of objects that can be examined 

 with the electron microscope. Actually, that has not been the case 

 and while many of the techniques developed have required consider- 

 able imagination and skill, it has been possible nevertheless to satisfj^ 

 these conditions in a surprisingly large number of cases. After sev- 

 eral years of seeing the "impossible" problems of specimen prepara- 

 tion solved by simple means, this writer is convinced that these con- 

 ditions can be fulfilled for most of the materials that may be en- 

 countered. The conditions discussed above limit the use of the elec- 

 tron microscope to the examination of solid specimens. At the same 

 time it now appears that few, if any, solids cannot be studied with 

 that instrument. Obviously the field of application is an enormous 

 one and a thorough discussion is beyond the scope of this article. It 

 is hoped that the general discussion in the above and in Avhat follows 

 will allow the reader to judge for himself the utility of the electron 

 microscope in any particular problem. 



Solids to be examined by means of the electron microscope can be 

 divided into two broad groups- — the particulate and the compact 

 solids. This subdivision is made almost entirely on the basis of the 

 method of preparation of the specimen, since the information sought 

 in either group is the same. 



3. Examination of Particulate Specimens 



Many specimens to be examined with the electron microscope 

 are naturally particulate; others are made particulate by fragmen- 

 tation of a compact structure. 



There are many methods of preparing particulate specimens, 

 which vary according to the material. All of them involve obtaining 

 a suitable dispersion on or in a thin supporting membrane or on some 

 other support without changing the particles in any way. For 

 smokes, soils, pigments, colloids, catalysts, macerated fibrous mate- 

 rials, etc., we find that condensation from a vapor stage, thermal or 

 electrostatic precipitation from an aerosol on a plastic membrane, 

 milling into a membrane-producing solution, centrifugation or electro- 

 phoretic deposition from liciuid suspension, or the simple evaporation 

 of a liquid suspension, are the main methods used. For large organic 

 molecules and viruses, centrifugation or evaporation from a purified 

 suspension are useful methods. For bacteria or other single cells, 



