=Reversibility of Gel-formation.=--In some cases, the change of a sol to a gel is an easily reversible one. Glue, gelatin, various fruit jellies, etc., "melt" to a fluid sol at slightly increased temperatures and "set" again to a gel on cooling, and the change can be repeated an indefinite number of times. On the other hand, many gels cannot be reconverted into sols; that is, the "gelation" process is irreversible. For example, egg-albumin which has been coagulated by heat cannot be reconverted into a sol; casein of milk when once "clotted" by acid cannot again be converted into its former condition, etc. Irreversible gelation is usually spoken of as "coagulation." Some coagulated gels, by proper treatment with various electrolytes, etc., can be converted into sols, the process being known as "peptization"; but in such "peptized" hydrosols, the material usually exists in a different form than originally, having undergone some chemical change during the peptization, and the coagulation and peptization cannot be repeated, that is, the process is not a definitely reversible one. =Importance of Gel-formation.=--From the physiological point of view, gel-formation is undoubtedly the most important aspect of colloidal phenomena. In the first place, the ability to absorb and hold as much as 80 to 90 per cent of water in a semi-solid structure is of immense physiological importance. In no other condition can so large a proportion of water, with its consequent effect upon chemical reactivity, be held in a structural, or semi-solid, mass. But a vastly more significant feature of the conditions supplied by the gel lies in the fact that the non-water phase, or phases, of the system are spread out in a thin film, or membrane, thus giving it enormous surface as compared with its total volume. This effect is easily apparent if one thinks of the enormous surface which is exposed when a tiny portion of colloidal soap is blown out into a "soap-bubble" several inches in diameter. This condition brings into play all the phenomena resulting from surface boundaries between solids and liquids, liquids and liquids, liquids and gases, etc., from surface tension, surface energy, etc. Among these effects may be cited those of adsorption, increased chemical reactivity due to enlarged areas of contact, permeability and diffusion, etc., the importance of which in the vital phenomena of cell-protoplasm will be discussed in detail in the following chapter. GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS =Non-diffusibility.=--The most characteristic property of all sols is the failure of the suspended particles to pass through a parchment, or any similar dialyzing membrane.