COIJ.OIDS, LYOPIIOBIC 



in water, the carbon film floats on the sur- rectly, concentration can be achieved by- 

 face and can be transferred to the grid. If means of electrodecantation (3). 

 the colloidal dispersions are spread on the Deposition of Sols on Supporting 

 glass slide and allowed to dry before applying Membranes. The simplest method of trans- 

 the carbon, the particles remain embedded in ferring a sol sample to the supporting mem- 

 the film on removal and can be used for direct brane is by the use of a fine loop of platinum 

 viewing. Although carbon forms a very wire. The wire can readily be cleaned by 

 stable film it is not suitable for the examina- flaming before transference of the specimen, 

 tion of all materials. Electrostatic effects are An alternative method, often useful for 

 often encountered which make it difficult to quantitative measurements, is to spray the 

 obtain drop adhesion and solutions contain- sol on to the supporting membrane by means 

 ing surface-active agents usually disrupt the of a nebulizer. Freeze-drying of the sample 

 film; in these cases nitrocellulose-carbon films is often useful and this can be carried out 

 are the most useful with the liquid applied simply by placing the grids on a copper block 

 to the nitrocellulose side. immersed in a freezing mixture; the aqueous 



Silicon monoxide has also been used to drops then freeze immediately on making 



obtain a stable supporting membrane, with contact with the supporting membrane, 



little structure (2). Specimen Contrast. In order to obtain 



Preparation of Colloidal Dispersions, an image of the particle in the electron mi- 

 Where colloidal dispersions are produced by croscope, electrons must be scattered out of 

 the interaction of two ionic reagents, the the field so that they do not reach the pho- 

 sol produced usually contains considerable tographic plate. The amount of scatter gen- 

 quantities of extraneous electrolyte. This, if erally depends upon the atomic number and 

 left in the sol, tends to crystallize on the sup- the density of the specimen, and it is for this 

 porting membrane and the resultant crystals reason that materials composed mainly of 

 may be confused with the colloidal particles carbon, hydrogen, nitrogen and oxygen, i.e., 

 during examination. Even if actual crystal- of the same composition as a nitrocellulose 

 lization does not occur poor backgrounds supporting membrane, are difficult to ob- 

 may result, or flocculation of the sol may serve. In the latter case the specimen con- 

 occur due to the high concentration of elec- trast can usually be increased by staining or 

 trolyte reached during evaporation . Removal by shadowing with a heavy metal vapor such 

 of any extraneous electrolytes is therefore ^s that of chromium, gold or uranium in a 

 advisable either by dialysis or electrodialysis; ^^Sh vacuum. 



care must be taken, however, not to remove ^^^'"^^ ^^ ^^^ low penetrating power of 



stabilizing potential-determining ions. Small electrons it is necessary to use very thin 



samples may be rapidly dialysed against specimens if mtenor details are to be ob- 



distilled water in cellophane dialysis sacs; served (see later). With crystalline specimens 



f i.i-i- 1 r-i- considerations other than random scatter 



lor electrodialysis a number of simple pieces , ... 



c , , , , ., 1 1 . V have to be taken into account (2). 



ot apparatus have been described which can „ , . /• r- i, . , , « . , 



, ,., , /..N T., , Kesolution ot Colloidal Particles. In 



be readily constructed (3). Electrolyte may j- ^ • .• r n -j i ^- i 



. , , , ^ ^ -^ -^ direct examination of colloidal particles, 



also be removed by passage of the sol ^^^jy ^^e two-dimensional aspects of the 



through a suitable ion-exchange resin pro- panicle are seen. In this connection it was 



vided that precautions are taken to avoid realized by von Borries and Kausche (4) that 



contamination of the sol by particles or crystalline colloidal particles, which should 



complex ions from the resin. be bounded by plane faces intersecting in 



WTiere sols are too dilute to be used di- geometrical lines, should be revealed as well 



125 



