208 CHEMISTRY OF PLANT LIFE 



The property of forming an emulsoid seems to be much more 

 definitely a characteristic of the substance in question than does 

 the formation of sols from solids which, under other conditions, 

 may form true solutions. This difference may be due to the fact 

 that the liquids which easily form emulsoids (usually those of 

 organic origin) have very large molecules, so that the transfer 

 from molecular to colloidal condition involves much less change 

 in such cases than it does in the case of solid (inorganic) substances 

 of relatively low molecular weight. This view of the matter is 

 further borne out by the fact that solids which have very large 

 molecules (generally of organic origin) take on the colloidal form 

 much more readily than do those of small molecular size. 



At the same time, a given liquid may form a true emulsoid 

 when introduced into one other liquid and a true solution when 

 introduced into another. Thus, soaps form emulsoids with water 

 (true hydrosols) ; but dissolve in alcohol to true solutions, in which 

 they affect the osmotic pressure, the boiling point of the liquid, 

 etc., in exactly the same way that the dissolving of other crystal- 

 loids in water affects the properties of true aqueous solutions. 

 Again, ordinary " tannin," when dissolved in water, produces a sol, 

 which froths easily, is non-diffusible, etc. ; but when dissolved in 

 glacial acetic acid, it produces a true solution. 



The concentration of the disperse phase may be much greater 

 in the case of emulsoids than it can be in suspensoids. This is 

 probably because the dispersed particles do not carry so large 

 an electric charge and are not in such violent motion. 



GEL-FORMATION 



The one property which most sharply distinguishes sols from 

 true solutions is their ability to " set " into a jelly-like, or gela- 

 tinous semi-solid, mass, known as a "gel," without any change in 

 chemical composition, or proportions, of the two components of 

 the system. In the gel, the two components are still present in the 

 same proportions as in the original sol; but the mixture becomes 

 semi-solid instead of fluid in character. Thus, an agar-agar sol 

 containing 98 per cent of water sets into a stiff gel; while many 

 other gels which contain 90 to 95 per cent of water can be cut into 

 chunks with a knife and no water will ooze from them. The 

 water is not in chemical union with the solid matter in the form of 



