COLLOID CnEMTSTRV. 215 



lyte, and when ' salted out ' by larger quantities tlie effect seems often 

 more osmotic than electrical. In some cases, however, when the colloid 

 has acquired a de6nite positive or negative (ionic) charge it becomes 

 much more sensitive to electrolytes, and where organic sols mutually 

 precipitate each other, the presence of a trace of electrolytes is probably 

 essential to communicate the necessary charges. It is stated that gela- 

 tine free from inorganic constituents oidy gives a slisjht opalescence with 

 tannin, and in ordinary cases the rapidity and completeness of the 

 precipitation are much increased by the presence of salts. 



Many aqueous organic colloid solutions are flocculated by additions of 

 alcohol, ether, itc, and, conversely, alcoholic by addition of water, which 

 have usually no action on inorganic sols. These effects are probably 

 purely osmotic and dependent on the solubility of the substance in the 

 different media, but such additions often change the electrical charge of 

 the solvent. 



In most cases organic sols are simply viscous liquids which on con- 

 centration become more and more viscous till they reach a practically 

 solid form without any break of continuity ; but others, usually by 

 change of temperature, pass into the semisolid state known as jelly. 

 This solidification generally occurs with comparative suddenness, and is 

 sometimes irreversible, but usually jellies have pretty definite melting- 

 points at or slightly lower than their points of solidifi'^ation, and varying 

 only slightly with the concentration of the solution, but, like the melting- 

 points of fats, not reaching their full value till some time aftpr apparent 

 solidification. Like crystallisation also, the change to the solid form is 

 accompanied by evolution of heat, and liquefaction by its absorption. 



Conversely, organic colloids in the solid form are capable of absorbing 

 water or some other solvent, sometimes passing back without break to 

 the viscous condition, but often reaching a maximum absorption and 

 retaining the jelly state. Gelatine, for instance, dissolves direct to a 

 viscous solution at temperatures above 25° or 30", but swells to a jelly at 

 lower ones, which melts if the temperatore is raised. Cellulose and 

 many of its derivatives and many animal and vegetable tissues swell, 

 but do not dissolve without hydrolysis or other chemical change, and may 

 be viewed as irreversible iellies. Thus dyeing, tanning, and many other 

 industries may be regarded as branches of colloidal chemistry. 



The absorption of the solvent is accompanied by swelling or increase 

 of volume, but especially in the early stages by marked contraction of 

 the sum of the volumes of solvent and colloid, and considerable evolution 

 of heat ; and, conversely, in dehydration heat is absorbed. Consequently 

 cold promotes swelling (as distinguished from solution), and heat has the 

 opposite efFect, though in dilute jellies, which can usually only be sub- 

 jected to a small range of temperature, these effects are not very marked, 

 and are often masked by other influences such as bacterial fermentation. 

 As an illustration, it may be mentioned that gelatine is most rapidly and 

 satisfactorily dissolved for culinary purposes by swelling in cold water, 

 which may be thrown away with the dissolved impurities, and then by 

 melting the swollen jelly by setting the vessel in hot water. 



The absorption of water-vapour by most solid organic colloids is a 

 cyclical and reversible process, the colloid returning by evaporation to its 

 original condition, and it therefore follows that Clausius' equations can 

 be applied in this case as well as in the expansion of bodies by heat. 



