432 SCIENTIFIC RECREATIONS. 



degrees of hardness being put as extremes, we can in a manner relatively 

 estimate the hardness of all other minerals. We can test this by scratching 

 one against another ; whichever scratches the other is the harder of the two, 

 and thus by taking up and discarding alternately, we can at length arrive 

 at a comparative estimate of the hardness of all. Such a scale was arrived at 

 by Mohs, and arranged in the following order. The softest mineral comes 

 first : 



1. Talc. 6. Felspar. 



2. Gypsum (rock-salt). 7. Quartz. 



3. Calcareous spar. 8. Topaz. 



4. Fluor-spar. 9. Corundum. 



5. Apatite-spar. 10. Diamond. 



Talo, we see, is the softest, and diamond the hardest. Thus " diamond cut 

 diamond" has passed into a proverb expressive of the difficulties one "sharp" 

 person has to circumvent or " cut out" another. Diamond is used by glass- 

 cutters. When geologists wish to express the degree of hardness of any 

 substance, they mention it with reference to the foregoing list ; and if the 

 substance be harder than fluor-spar, but not so hard as felspar, they determine 

 its hardness five, or perhaps between five and six, or between four and five, 

 according as it is harder or less hard than apatite. Thus hardness, or power 

 of cohesion, resistance to exterior force and pressure, is a prime characteristic 

 of the mineral kingdom. The file is the best test. 



We now come to another phase of the physical character of our 

 minerals cleavage. This is the term employed to express the facility of 

 cutting in a certain direction which in the mineral is its direction of cleavage. 

 Take mica, for instance. There is no difficulty in separating mica into 

 thin layers ; we can do so with our fingers. The layers, or flakes, or 

 laminae are so arranged that they exhibit less cohesion in one direction than 

 when tried in other ways. We cut with the grain, as it were in the direction 

 of the fibre when wood is concerned. Here we have another popular saying 

 expressive of this, " against the grain," which signifies an act performed 

 unwillingly and unpleasantly. Cleavability, therefore, means cutting with 

 the grain, as it were, and various minerals are possessed of different degrees 

 of cleavage. It sometimes happens that electric excitement is observed 

 when cleavage takes place. One place will become positive, and the other 

 negative. Mica, arragonite, and calcareous spar will exhibit this action after 

 cleavage or pressure. When a crystal of tourmaline is heated, it will 

 develop positive electricity at one end of its principal axis, and negative at 

 the other. Even if it be broken, the extremities of the fragments will 

 exhibit similar phenomena, and so far like a magnet, which, as we have 

 seen, possesses this attribute of " polarity." But a curious fact in connection 

 with this is that, if the heating cease the polarity ceases for a second or two, 

 and yet as cooling goes on the polarity is restored, with the difference that 

 the positive end has become negative, and the end previously negative has 

 come over to the opposite pole. Electricity, therefore, must be hidden away 



