STUDIES IN GELS 121 



Technique of making preparations (Ruska, 1939; Wyckoff, 1949). The 

 penetrating power of electron rays being small, it is difficult to find 

 adequate specimen holders. The most suitable holders are nitrocellulose 

 films of submicroscopic thickness. These can be made by spreading a 

 drop of a collodion solution in amyl acetate on water, which is saturated 

 with this solvent. After evaporation of the amyl acetate, a nitrocellulose 

 film, which in favourable cases is only lo m/z, is left on the water surface. 



When investigating suspended objects (bacteria, viruses, colloid particles), 

 a drop of the suspension is left to dry on the specimen holder. Only dried 

 objects can be placed in the apparatus, because the exposure has to be made 

 in vacuo. This rules out the observation of living organisms in the electron 

 microscope, and it is also impossible to image cytological objects in their 

 natural swollen state. 



The methods described are appropriate for the investigation of corpuscular 

 colloids. But in general those methods are unsuitable for reticular colloids 

 with a coherent structure and a different microtechnique had to be devised 

 for these objects. Sometimes gel solutions or gels can, admittedly, be dried 

 on a specimen holder to be imaged (Fig. 86a, p. 128). Before it was possible 

 to prepare sections thin enough for the electron microscope, like those 

 obtained in ordinary microscopy by means of microtomes, all kinds 

 of expedients had to be resorted to. In some cases the thin edge of a wedge- 

 shaped section is thin enough for use in electron microscopy, but no images 

 of suitable dimensions can be obtained in this manner. Thick objects, such 

 as cell walls and fibres, can be teased into small fragments after being 

 allowed to swell (Wergin, 1942), or else they can be crushed into sub- 

 microscopic splinters in a vibrating ball mill. These splinters are suspended 

 and finally dried on the specimen holder (Hess and co-workers, 1941). 

 Some gels can be divided into submicroscopic flocculates by means of special 

 vibrators (O'Brien, 1945). Ultrasonic waves have proved to be particularly 

 suitable for this purpose; e.g., by this method microscopic fibres can be 

 disintegrated into submicroscopic fibrils (Wuhrmann, Heuberger, and 

 MiJHLETHALER, 1 946) without damage to the structure such as the vibrating 

 mill inflicts. 



Several dif^culties arise when these preparations are irradiated. The 

 electrons absorbed impart a negative charge to the object, resulting in 

 repulsive forces between the structural elements, and may cause inflation of 

 the fibrils at the points irradiated. The changes brought about by this effect, 

 however, are as a rule less striking than those suffered by the object as a 

 result of the heat evolved. On absorption, the great energy content of the 

 fast electrons is mostly converted to heat. Silver and gold can be fused 

 together or even melted completely in the electron microscope. Obviously, 

 therefore, organic compounds become charred if exposed too long. Many 

 objects, such as bacteria, appear brownish after exposure in the electron 

 microscope, even if precautions are taken to protect them. Naturally, the 

 thicker the object, the greater is the heat evolved. The object is not easy 



