INTERNAL, OR MOLECULAR FORCES. 27 



The property of Hardness 13 quite distinct from Density. Gold and lead 

 possess great density, yet tliey are among the softest of metals. 



What is Brit- 51. Brittleness is a property in virtue of 

 tieness? wliicli Lodics are easily broken into fragments. 

 It is a characteristic of most hard substances. 



In a brittle body, the attractive force between the atoms exists within such 

 narrow limits, that a very slight change of position, or increase of distance 

 among them, is sufiBcient to overcome it, and the body breaks. 

 ' 52. The modifications of the force of cohesive attraction between, the par- 

 ticles of matter, which give rise to tlie properties of malleability, ductility, 

 flexibility, pliability, hardness, and brittleness, seem to be intimately con- 

 nected with, or depend upon the particular form of the atoms of the sub- 

 stance, and the particular manner in which they are arranged. 



Every one knows that it is easier to split wood lengthwise than across the 

 fibers ; hence, the force whicli binds tlie particles of the wood together is ex- 

 erted in a less degree in one direction than in the other. 

 Explain how -^^ changing the form or arrangement of the atoms of a 



the force of substance, we can in many instances apparently renew or de- 

 pends on the stroy the various modifications of the attractive force. The 

 arrangement following is a familiar illustration of this principle : 



Steel, when heated and suddenly cooled, is rendered not 

 only veiy havd, but very brittle ; but if heated and cooled gradually, it be- 

 comes soft and flexible. We may suppose that when the atoms of steel are 

 expanded — forced apart from each other by the action of heat, and then sud- 

 denly caused to contract — forced in upon each other — by cooling, that no op- 

 portunity is afforded them for airangement in a natural manner. But when 

 the steel is cooled slowly, each atom has an opportunity to take the place best 

 adapted for it, ^Wthout interfering witli its neighbor. According to one ar- 

 rangement of the atoms, the steel is brittle, or the atoms will not admit of 

 any motion among themselves without breaking ; but according to a difiercnt 

 arrangement, the attractive force is modified, and the steel is soft and flexible. 

 In a similar manner, bricks stacked up irregularly, may be made to fall 

 easily, but if piled in a regular manner, the}' retain their stability. 



It is a very singular circumstance, that the same operation of heating and cool- 

 ing suddenly, whicli hardens steel, should soften copper. A piece of steel which 

 has been hardened in this way is not c ondensed — made smaller — as we might 

 have supposed it would be, but is actually expanded, or made larger. This proves 

 that the arrangement of the atoms, or particles, has been changed. Any ono 

 may satisfy himself of this by taking a piece of steel, fitting it exactly into a 

 guage, or between two fixed points, and then hardening it. It will then bo 

 found that the steel will not go into the guage, or between the fixed points. 



What is An- 53. The process of rendering metals, glass, 

 neaiing? ^^^^^ ^^^^ ^^^ flcxiblo by heating and gradually 

 cooling, is called Annealing, and is of great importance 

 in the arts. 



