10 ELEMENTARY GENERAL SCIENCE CHAP. 



It is found that a wire of twice the cross sectional area of 

 another will be just twice as tenacious. Evidently, then, if we 

 wish to compare the tenacity of two wires of different materials, 

 it will make the experiment much simpler if wires of the same 

 cross section are selected. Cast steel is the most tenacious of 

 all metals, being about twice as much so as copper and forty 

 times as tenacious as lead. But the tenacity of steel itself is 

 exceeded by that of unspun silk, while single fibres of cotton 

 can support millions of times their own weight without breaking. 



2. Ductility is the property by virtue of which solids can be made 

 into wires. A ductile material is thus one which can be drawn 

 out. The change of form in this case is produced by pulling. 

 Malleability is a similar property to ductility, but the change of 

 form is brought about by the application of pressure ; gold, 

 copper, and lead, for instance, can be beaten out into thin plates, 

 and are therefore malleable substances. Lead is an example of 

 a malleable material which is not ductile it can be beaten out 

 but cannot be drawn into wires. 



Platinum is the most ductile and gold the most malleable 

 metal known. Platinum has been drawn out into wire so fine 

 that a mile of it weighs only one and a quarter grains. Gold 

 has been beaten into plates so thin that it would require three 

 hundred thousand of them placed one above the other to make 

 a layer an inch thick. 



3. Hardness is the property by virtue of which solids offer resist- 

 ance to being scratched or worn by others. This is a property of 

 great importance in the study of minerals, as it often affords a 

 ready means of distinguishing them. The method of measuring 

 hardness consists in selecting a series of solids, each one of the 

 series being harder than the one above it, and softer than the 

 one below it. At one end of the series, therefore, the hardest 

 solid known is placed ; at the other end, the softest which we 

 may wish to measure. 



DISTINCTIVE CHARACTERS OF LIQUIDS. 



We have now to learn the leading properties which liquids 

 I x ).sscss, which distinguish them from solids on the one hand and 

 from gases on the other. We have already learned that, being 

 forms of matter, they have certain general characters in common 

 witli all other material things ; but what is there about a liquid 

 which makes us give it a name of its own ? A liquid adapts 



