ORGANIC MATTER. 15 



panied by a decrease of those dimensional contrasts to which 

 })olarity is ascribable. A sphere is the fi'gure of equilibrium 

 which any aggregate of units tends to assume, under tlie 

 influence of simple mutual attraction. Where the number 

 of units is small and their mutual polarities are decided, 

 this proclivity towards spherical grouping will be overcome 

 by the tendency towards some more special form, determined 

 by their mutual polarities. But it is manifest that in pro- 

 portion as an aggregate molecule becomes larger, the effects 

 of simple mutual attraction must become relatively greater; 

 and so must tend to mask the effects of polar attraction. 

 There will consequently be apt to result in highly compound 

 molecules like these organic ones, containing hundreds of 

 elementary atoms, such approximation to the spherical form 

 as must involve a less distinct polarity than in simpler mole- 

 cules. If this inference be correct, it supplies us with an ex- 

 planation both of the chemical inertness of these most com- 

 plex organic substances, and of their inability to crystallize. 



§ 6. Here we are naturally introduced to another aspect of 

 our subject — an aspect of great interest. Professor Graham 

 has published a series of important researches, which promise 

 to throw much light on the constitution and changes of 

 organic matter. He shows that solid substances exist under 

 two forms of aggregation — the colloid or jelly-like, and the 

 crystalloid or crystal-like. Examples of the last are too 

 familiar to need specifying. Of the first may be named such 

 instances as " hydrated silicic acid, hydrated alumina, and 

 other metallic peroxides of the aluminous class, when they 

 exist in the soluble form; with starch, dextrine and the 

 gums, caramel, tannin, albumen, gelatine, vegetable and 

 animal extractive matters." Describing the properties of 

 colloids. Professor Graham says : — " Although often largely 

 soluble in water, they are held in solution by a most feeble 

 force. They appear singularly inert in the capacity of acids 

 and bases, and in all the ordinary chemical relations." 



