294 H. K. SCHACHMAN AND R. C. WILLIAMS 



counting its numbers and by weighing a dried, aliquot volume of the suspen- 

 sion. The spray-drop method is demonstrably the most accurate of the three 

 as far as counting is concerned, but it requires that the virus suspension 

 contain at least 5 X 10^ paritcles/milliliter. The other two methods allow 

 concentrations to be used that are some 100- to 1000-fold less. 



m. Sections of Infected Cells. In exploring the morphological aspects of 

 viruses within their host cells it is necessary to use thin sections cut through 

 such infected cells. Except in their extreme peripheral regions, even tissue- 

 cultured cells are far too thick for direct electron microscopic examination. 

 The purely technical aspects of fixation, embedding, and microtomy of cells 

 for electron microscopy are now well worked out. [See Gelber (1957) for a 

 summary of techniques.] Serial sections as thin as 200-400A are obtained 

 without great difficulty, allowing the electron microscope to be used at its 

 full power. 



The fixative and stain almost universally employed in tissues prepared 

 for electron microscopy is osmium tetroxide, and its use reveals an impres- 

 sively great wealth of fine structures within cells. Unfortunately, it seems 

 not to have any highly selective staining properties as far as viruses are 

 concerned; in fact, its mechanism of reaction with viruses is poorly under- 

 stood. Nor are the possible leaching effects of osmic acid, of the dehydrating 

 alcohols, and of the monomeric form of the embedding material (butyl 

 methacrylate) adequately investigated. About all that can be said at present 

 is that, if cells known to be infected with a large virus are sectioned, the 

 sections exhibit particles that are not unlike what one would anticipate to 

 be the appearance of the virus. In a very few cases sections of pellets of 

 purified, large viruses confirm this anticipation. It is clear, however, that 

 there is great need for more stains that would be effective in enhancing the 

 the contrast of the electron image, and that would be more specific for virus 

 particles. 



5. Radiation Inactivation 



a. The Effects of High-Energy Radiation. The effect of the interaction of 

 high-energy radiation with matter is to create ionization and excitation of 

 the atoms. If the radiation is electromagnetic in character, such as X-rays 

 and y-rays, ionization is produced by the ejection of photoelectrons and 

 Compton-recoil electrons. When particles of high velocity are used for 

 bombardment, such as electrons, protons, a-particles, and neutrons, the 

 ejection of electrons from the atoms of the bombarded m; fcerial comes about 

 by direct or near collision. In either type of production of ionization the 

 ejected electrons from the 'primary ionization have considerable velocity 

 and, as a consequence, are capable of producing further, or secondary, ion- 

 izations along their tracks. Atoms that have been ionized by the ejection 



