278 W. Rallodk—Flow of Solids. 



Whether rupture or flow takes place in any given case, when 

 the deforming overcomes the resisting force, depends upon the 

 nature of the substance, its limiting conditions, and the time 

 allowed for the accomplishment of the motion. 



For convenience I would classify substances as: True solids, 

 viscous solids, viscous liquids and true liquids. True solids 

 retain their shape under ordinary circumstances indefinitely. 

 Yiscous solids gradually yield to gravity and flatten out. Vis- 

 cous liquids "fill " their containing vessels only after a fraction 

 of a second. True liquids fill their containing vessel practi- 

 cally immediately. Examples of each class are steel, lead or 

 paraffine, tar and water, respectively. It goes without saying 

 that we have no examples of absolute or perfect liquids or solids. 

 Absolute rigidity is as unknown as absolute liquidity. If these 

 ideas are correct true "liquefaction" is a diminution of the 

 rigidity of a substance until its molecules change their relative 

 positions as easily as in the " true liquid." above defined. 



In order that a substance may undergo a change in its chemi- 

 cal or crystalline character, it would seem necessary that it should 

 be in the condition, at least, of a viscous solid, in order that 

 the molecules may rearrange themselves if there be any force 

 urging them thereto. A priori it is inconceivable to me how 

 or why pressure should impart this freedom to the molecules. 

 With the exception of a few substances at isolated points, as 

 water between 4° C. and ice at 0° C, an increase of liquidity, 

 or a diminution of rigidity is simultaneous with an increase of 

 volume, i. e., with the increase of the intermolecular distances, 

 which is usually accomplished by heating the substance. In 

 general for one and the same substance over considerable ranges of 

 condition, the rigidity diminishes as the intermolecular distances 

 increase. How can pressing the molecules nearer together be 

 expected to give to them a property which always accompanies 

 their separation ? 



Apparatus. — Through the kindness of the Chief of Ordnance 

 we were permitted to use in its leisure moments the beautiful 

 testing machine built for that department by A. H. Emery,* 

 and situated at the U. S. Arsenal, Watertown, Mass. I also 

 gladly acknowledge my debt of gratitude for kindnesses and 

 many good suggestions to Col. F. H. Parker and Capt. John 

 Pitman of the Ordnance Corps, and to Mr. J. E. Howard, the 

 engineer in charge of the machine. 



Fig. 2 shows a vertical section across the holder on the plane 

 B B of fig. I, which gives a vertical section on A A of fig. 2. 

 E E are the two halves in contact at h A, enclosing the cylin- 

 drical hole F, in which the substance to be pressed is placed. 



* Report of the Board on Testing Iron, Steel and other Metals, vol. ii, 1881. 

 Government Printing office. 



