62 AQUEOUS AGENCIES. 



proposition that ice is a viscous substance seems at first palpably ab- 

 surd. It is necessary, therefore, to show that this proposition is not so 

 absard as it seems. 



The properties of solidity and liquidity, though perfectly distinct 

 and even incompatible in our minds, nevertheless, in Nature, shade into 

 one another in the most imperceptible manner. Malleability, plasticity ', 

 and viscosity, are intermediate terms of a connecting series. The idea 

 which underlies all these expressions is that of capacity of motion of 

 the molecules among themselves without rupture : the difference among 

 them being the greater or less resistance to that motion. In the case 

 of malleable bodies, like the metals, great force is required to produce 

 motion ; in plastic bodies, like wax or clay, less force is required ; in 

 viscous bodies, like stiff tar, motion takes place spontaneously but 

 slowly ; while in liquids it takes place freely and with little or no resist- 

 ance. In all these cases, if the pressure be sufficient, the body will 

 change its form without rupture — in other words, will fioiv. Now, by 

 increasing the mass we may increase the pressure to any extent. 

 Therefore, all malleable, ductile, plastic, or viscous bodies, if in suffi- 

 ciently large masses, will flow like water. Thus, a mass of lead, suffi- 

 ciently thick, would certainly flow under the pressure of its own weight. 



But solid bodies may be divided into two great classes, viz., bodies 

 which are malleable, plastic, or viscous, and bodies which are brittle ; 

 the very idea of brittleness being that of total incapacity of motion 

 among the particles without rupture. Now, ice belongs to the class of 

 brittle bodies. Forbes attempts to remove this difficulty by showing 

 that many apparently brittle bodies will also flow under their own 

 weight ; for instance, pitch, so hard and brittle that it flies to pieces 

 under a blow of the hammer, will, if the containing barrel be removed, 

 flow and spread itself in every direction. So, also, molasses-candy, 

 made quite hard and brittle, will still flow by standing. A remarkable 

 pitch-lake, about three miles in circumference, occurs in Trinidad. 

 The pitch is described as in constant, slow-boiling motion, coming up 

 in the center, flowing over to the circumference, and again sinking 

 down. Yet this pitch, in small masses, would be called solid and 

 brittle. Struck with a hammer, it flies to pieces like glass. In fact, 

 the essential peculiarity of a stiff, viscous body, in which it differs 

 from malleable or plastic bodies, is, that it yields only to slowly -applied 

 force. 



Forbes, therefore, thinks that glacier-ice is an exceedingly stiff, vis- 

 cous substance, which, though apparently brittle in small quantities and 

 to sudden force, yet, under the slow-acting but powerful pressure of 

 its own weight, flows down the slope of its bed, squeezing through 

 narrows and spreading out into lakes, conforming to all the larger and 

 gentler inequalities of bed and banks, but not to the sharper ones. 



