OBSERVATION OF PROPERTIES OF SUBSTANCES 91 



In the same way, things like potatoes, some woods, chalk, bread, 

 blotting paper, and soap can be scratched by the finger-nail, and are 

 therefore softer than the finger-nail. 

 Things like iron, glass, and flint cannot 

 be scratched or cut by the finger-nail, 

 and are therefore harder than it. 



The test of hardness. It is now 

 possible to understand the way to find 

 out which is the harder of two things. 

 What has to be done is to test which 

 will scratch or cut the other. If asked 

 whether glass or flint is the harder, try 

 whether the flint will scratch the glass. 

 It does. Will the glass scratch the flint ? 

 It will not. Which is the harder then ? 

 The flint, of course. 



In the same way, any number of 

 different things may be arranged in the 

 order of their hardness* by taking any 

 one of the substances and finding which 

 of the others it would scratch and which it would not scratch. Then 

 another would be taken, and the same tests made, and so a list like 

 the one below would be made. This is the method always adopted to 

 find out if one thing is harder than another. 



FIG. 69. The things on the table 

 differ in hardness as well as in other 

 ways. 



1. Diamond. 



2. Rock-crystal. 



3. Glass. 



4. Steel. 



5. Iron. 



6. Copper. 



7. Lead. 



8. Wax. 



The hardest substance is first in the list, the next hardest is second, 

 and the softest is last. Any of the substances will scratch a substance 

 lower in the list, and can be scratched by substances higher in the list. 

 Diamond is seen to be the hardest substance ; it will scratch every 

 other thing. Emery and carborundum are also very hard, and there- 

 fore used for polishing many things. 



In actual practice in class it will be sufficient to use a penknife, 

 penny, and the finger-nail as standards. 



Condition. Under this heading a number of properties may be 

 included. If a plate of glass is dropped it breaks into fragments and 

 it is said to be brittle. A biscuit is easily crumbled or powdered and 

 is said to be friable. Solids such as lead which can be beaten out into 

 sheets without breaking are called malleable. Copper, gold and 



