1855] WRITINGS OF JOSEPH HENRY. 353 



ever was presented at the last hour; the facts were not fully 

 stated and have never been published. I will therefore 

 briefl}^ occupy your time in presenting some of the facts I then 

 intended to communicate, and which I have since verified by 

 further experiments and observations. 



In a series of experiments made some ten years ago I 

 showed that the attraction of the particles for each other — of 

 a substance in a liquid form — was as great as that of the same 

 substance in a solid form.* Consequently, the distinction 

 between liquidity and solidity does not consist in a difference 

 in the attractive power occasioned directly by the repulsion of 

 heat ; but it depends upon the perfect mobility of the atoms, — 

 the loss of adhesion, or lateral cohesion. We may explain this 

 by assuming an incipient crystallization of atoms into mole- 

 cules, and consider the first effect of heat as that of breaking 

 down these crystals and permitting each atom to move freely 

 around each other. When this crystalline arrangement is 

 perfect, and no lateral motion is allowed in the atoms, the body 

 may be denominated perfectly rigid. We have approxi- 

 mately an example of this in cast-steel, in which no slipping 

 takes place of the parts on each other, or no material elonga- 

 tion of the mass; and when a rupture is produced by a tensile 

 force a rod of this material is broken with a transverse fracture 

 of the same size as that of the original section of the bar. In 

 this case every atom is separated at once from the other, and 

 the breaking weight may be considered as a measure of the 

 aggregate attraction of cohesion among the whole transverse 

 series of the atoms of the metal. 



The effect however is quite different when we attempt to 

 pull apart a rod of lead. The atoms or molecules slip upon 

 each other. The rod is increased in length and diminished 

 in thickness until a separation is produced. Instead of lead 

 we may use still softer materials, such as wax, putty, &c., 

 until at length we arrive at a siibstance in a liquid form. 

 This will stand at the lower extremity of the scale, and be- 

 tween extreme rigidity on the one hand, and extreme liquid- 



*[Proceedings Am. Philosophical Society. See ante, page 217.] 



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