THE PROPERTIES OF MATTER. 19 



It is measured by the degree of difficulty with which it 

 is scratched by another substance. Fluids are not said to 

 have hardness. 



(a.) Hardness does not imply density. The diamond is much 

 harder than gold, but gold is four times as dense as diamond. 



48. What is Tenacity ? Tenacity is that prop- 

 erty of 7natter by virtue of which some bodies re- 

 sist a force tending to pull their particles asunder. 



(a.) Like hardness and the other characteristic properties of 

 matter, it is a variety of cohesion which is the general term for the 

 force which holds the molecules together and prevents disintegration. 

 The tenacity of a substance is generally ascertained by shaping it in 

 the form of a rod or wire, the area of whose cross-section may be 

 accurately measured. Held by one end in a vertical position, the 

 greatest weight which the rod will support is the measure of its 

 tenacity. For any given material, it has been found that the tenacity 

 is proportioned to the area of the cross-section ; e. g., a rod with a sec- 

 tional area of a square inch will carry twice as great a load as a 

 rod of the same material with a sectional area of a half square inch; 

 a rod one decimeter in diameter will carry four times as great a load as 

 a similar rod five centimeters in diameter. The explanation of this is 

 simple ; imagine these rods to be cut across, and it will be evident 

 that, on each side of the cut, the first rod will expose the surfaces 

 of twice as many molecules as will the second, and that the third 

 will expose four times as many molecular surfaces as the fourth. 

 But for the same material, each molecule has the same attractive 

 force. Doubling the number of these attractive molecules, which 

 is done by doubling the sectional area, doubles the total attractive 

 or cohesive force, which, in this case, is called tenacity ; quadru- 

 pling the sectional area quadruples the tenacity. Hence the law : 

 Tenacity is proportioned to the sectional area. 



49. What is Brittleness t Brittleness is that 

 property of matter by virtue of which some bodies 

 may be easily broken, as by a blow. 



(a.) Glass furnishes a familiar example of this property. The 

 idea that brittleness is the opposite of hardness, elasticity or tenac- 

 ity, should be guarded against, Glass is harder than wood, but 



