THE PLASTIC STATE. 43 



cause a downward curvature of the rod. If a ball of hard 

 and brittle tar be left standing for a time the splierical 

 shape will gradually disappear, and the tar will settle down 

 until it becomes a circular disc. 



A substance which exhibits the phenomena of flow to a 

 striking degree is ice. Under sufficient pressure, this 

 otherwise solid and even brittle substance can be made to 

 flow out of an orifice much in the same way as a viscous 

 fluid. 



A still more striking instance of this molecular move- 

 ment was quoted some years ago by Professor Blake, of 

 Connecticut, U.S.A., who drew attention to certain 

 geological phenomena which he had had under observation. 

 What he noticed was that in certain conglomerates, the 

 pebbles were all found to be of an elongated, flattened, 

 ellipsoidal form. He was forced to believe, from other 

 evidence which he held to be conclusive, that these shapes 

 were entirely due to an elongation and compression having 

 taken place under very great force. After admitting that 

 in all the cases quoted, a movement of the particles or 

 molecules must have taken place, what is called the flow of 

 metals is not very difficult to recognise. 



In the first place, we are quite familiar with the fact 

 that under conditions of high temperature, when wrought 

 iron and steel are at a red heat, they assume a very visibly 

 plastic condition, and can be hammered, pressed, or rolled 

 into all manner of shapes. To effect these changes of 

 shape and dimensions, certain stresses need to be applied 

 or now will not take place, at any rate in the short space 

 of time required, unless these stresses exceed a certain 

 minimum value. 



If the metal be treated in .a similar manner when in 

 the cold condition, similar effects may be produced, with 

 certain modifications, but the applied stresses must be much 

 greater before any flow will take place. Under this heading 

 we have the cold stamping of iron and steel, wire drawing, 

 the cold drawing of pipes, in all of which there is a most 

 decided flow of the metal taking place, from one part of 

 the mass to another. With the softer metals, such as lead, 

 the appearance of fluidity is even more noticeable, and 

 very frequent use is made of this property in the manufac- 

 ture of lead for commercial purposes. For instance, pipes 

 of lead are made by forcing the metal under very high 

 pressure through annular dies, the metal flowing out 

 easily and smoothly, and retaining the form given to it. 



