Tie Inaugural Address. 279 



hand, a colloid body presents great indifference in this respect. 



Albumen and isinglass are colloid bodies, for they are incapable 

 of crystallisation ; so also is glass, and Graham cites as a curious 

 illustration of the property just alluded to, that isinglass allowed to 

 dry in a glass vessel adheres to it so firmly and itself contracts so 

 resolutely as to tear the glass asunder as it dries and contracts upon 

 it ; whereas it will not adhere with any force to quartz crystal or to 

 a plate of mica. The silica contained in solution in water continually 

 tends to separate in a colloid condition, and to contract and ulti- 

 mately become hard, but this separation takes place more slowly as 

 the solution is more dilute and cold, provided there is no gelatinous 

 colloid to draw the dissolved portion to itself. The phenomena 

 presented by agate direct the mind at once to this process of dialysis 

 as the clue to the enigma of agate formation. 



I have stated that agates are found in igneous rocks, and that 

 they occur in round or oval almond-shaped masses. To explain them 

 we have to suppose that the amygdaloidal cavities formed by gases 

 confined in the flowing lava have, after the rock has cooled, been 

 left empty of all but gas, and during a long succession of ages these 

 cavities have been continually filled and refilled with water per- 

 meating the rock, perhaps intermittently, according as wet and dry 

 periods or seasons may have succeeded each other, and this water, 

 perhaps thermal, would from the nature of the rock it has traversed 

 be highly charged with silica, or at least with silicates in solution. 



The laminae that we see following the form of the cavity generally 

 exhibit an apparent streaming towards a tube-like entrance into the 

 cavity from without; sometimes there are several of these. By 

 these the water in the rock must be supposed to have entered the 

 cavity; and the circulation or introduction of fresh supplies of 

 mineral-charged water into the cavity will correspond to the changes 

 in saturation in the rock, and probably also in the nature of the 

 chemical substances it may hold in solution. Intermittent conditions 

 in these latter respects may cause repetitions of exchange by trans- 

 fusion between the liquid in the cavity and that in the rock, 

 through the open feeding tubes. But while this goes on 

 through the tube-entrances, another process is in action on the walls 



