210 CHEMISTRY OF PLANT LIFE 



On the other hand, gels when exposed to the air lose water by 

 evaporation, shrink in volume, and finally become hard inelastic 

 solids, as in the case of the familiar forms of glue, gelatin, agar- 

 agar, gum arabic, etc. 



The difference in the relation of gels and that of non-colloidal 

 solids to water may be illustrated by the different action of peas, 

 beans, etc., and of a common brick, when immersed in water. Each 

 of these substances, under these conditions, absorbs, or " imbibes," 

 water; but the peas and beans swell to more than twice their 

 original size and become soft and elastic, while the brick under- 

 goes no change in size, elasticity, or ductility. In all cases of col- 

 loidal swelling, the swollen body possesses much less cohesion, 

 and greater ductility, than it had before swelling. The essential 

 difference in the two types of imbibition is that in the case of 

 the non-swelling substances the cohesion, or internal attraction 

 of the molecules of the material, is too great to permit them to 

 be forced apart by the water; while in colloidal swelling, the 

 particles are forced apart to such an extent as to make the tissue 

 soft and elastic. It is possible, of course, to make this separation 

 go still further, until there is an actual segregation of the mole- 

 cules, when a true solution is produced; for example, gum arabic 

 when first treated with water swells into a stiff gel, then into 

 a soft gel, and finally completely dissolves into a true 

 solution. 



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 recon- 

 verted 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 under- 

 gone some chemical change during the peptization, and the coag- 



