CRYSTALLISATION OF METALS. 25 



cool slowly ; bismuth is one of the best for this purpose. Melt a 

 few ounces of bismuth in a crucible or ladle, and then let it stand 

 until the surface just begins to become solid, forming a crust. 

 With an iron skewer or thick knitting-needle poke two holes in 

 this crust on opposite sides near the edges, and then tilt the 

 crucible so that the melted metal that has not yet solidified may 

 run out from the interior through one hole, air entering by the 

 other one. When the crucible and what is left in it is cool 

 enough to touch, you will find that the crust can be broken away, 

 and will form a sort of metallic cake, studded on the under surface 

 with well-shaped crystals of bismuth, which often show a beautiful 

 iridescence like the melted lead in Expt. 15, and for the same 

 cause, viz., that they are coated over with a thin film of " oxide 

 of bismuth," owing to the action of the oxygen of the air on the 

 hot metal. Sometimes the bottom and sides of the crucible are 

 similarly coated with crystals. 



The term "crystal" is derived from the Greek K/wcrraAAos, 

 meaning originally ice or what is now called quartz (rock-crystal), 

 and implying something transparent ; whence the phrase " as clear 

 as crystal." But it was early noticed that when water freezes to 

 ice in thin films (for example, when moisture condenses inside the 

 window of a warm room and becomes frozen by intense cold out- 

 side), or when it becomes snow, that the solid formed is bounded 

 by plane surfaces regularly arranged in accordance with definite 

 geometrical laws ; and that the same remark as to shape applies 

 to much of the quartz or rock-crystal occurring in nature ; accord- 

 ingly the terms " crystal " and " crystalline " by and by became 

 extended to mean all substances so shaped, whether clear and 

 transparent or not. 



Cast iron is crystalline in structure; when broken across it 

 shows a "grain." Some kinds are very finegrained, especially 

 when cast in small quantities at a time ; other kinds when frac- 

 tured show a pretty large grain, being made up of crystals ^ inch 

 or more in length. A peculiar variety termed Spiegeleisen, largely 

 used in the manufacture of certain kinds of steel, sometimes shows 

 crystals some inches in length, the broken surface exhibiting large 

 brilliant flat planes; the name is derived from this, being the 

 German for mirror-iron. 



Expt. 19. To Crystallise Sulphur. Melt some roll brimstone 

 at a gentle heat, taking care not to set it on fire (Expt. 8) ; let it 

 cool until a crust forms, and then treat it as the bismuth in the last 

 experiment. The cooled mass finally obtained will be seen to be 

 made up of fine needle-shaped crystals. 



Another way of crystallising sulphur is by dissolving it in 



