158 SOILS: FBOPEBTIES AND MANAGEMENT 



Inorganic: Gold, silver, iron, ferric hydrate, arsenious 

 oxide, zinc oxide, silver iodide, Prussian 

 blue, etc. 



105. Colloidal phases. — In general, two conditions 

 are necessary for the colloidal state — a dispersive medium, 

 and a material that will disperse, the latter being usually 

 designated as the disperse phase. Three materials may 

 function as a dispersive medium — a liquid, a soKd, or a 

 gas. In the same way, with each dispersive medium there 

 may be three disperse phases — a liquid, a soHd, or a gas. 

 This gives nine general phases to be considered in colloidal 

 chemistry. From the soil standpoint, the Kquid-solid 

 and the liquid-liquid phases are by far the most important 

 and will be the only ones to receive detailed attention 

 here. In the Uquid-solid phase, as with colloidal gold 

 or ferric hydrate, the particles are suspended in water 

 as the dispersive medium. In the case of gelatin, another 

 hquid-solid example, the jelly surrounds the dispersive 

 medium, or liquid. An emulsion may exhibit the liquid- 

 liquid phase, and possibly exists in soils rich in humus. 



In these colloidal phases under discussion and of such 

 particular interest in soil study, two general classes of 

 materials are found, which seem to differ radically from 

 each other and yet are likely to lead to considerable con- 

 fusion unless special pains are taken to distinguish between 

 them. As types, gelatin and a colloidal suspension of 

 ferric hydrate may be cited. The gelatin is considerably 

 more viscous than water, while the ferric hydrate does 

 not differ from water in this respect. The former gelatin- 

 izes on cooling or on loss of moisture, but will become 

 dispersed again on the addition or presence of water. In 

 other words, it will pass again and again, back and forth, 



