426 SCIENTIFIC RECREATIONS. 



ORDER V. THE NATIVE METALS. 

 ORDER VI. SULPHURETTED METALS. 



Iron pyrites. 



Galena. 



Grey antimony ore. 



Sulphur. 



Diamond. 



Coal. 



INFLAMMABLES. 



Grey copper ore. 



Blende. 



Ruby-blende 



Mineral resins. 

 Combustible salts. 



These are only a portion of the minerals, but it would be scarcely inte- 

 resting to give the list at greater length. In the foregoing we recognize the 

 metals and various combustible and non-combustible substances familiar to 

 us, existing, as people say sometimes, in " lumps." But if any one will take 

 the trouble to examine a " lump," he will find the shape is definite and 

 even. These regular forms of the minerals are called CRYSTALS, from the 

 Greek word krustallos, ice. The term was originally applied to quartz, for 

 in olden times it was thought that quartz was really congealed water. We 

 can define a crystal as " an inorganic solid bounded by plane surfaces 

 arranged round imaginary lines known as axes? It must not be imagined 

 that crystals are small bodies ; they may be of any size. There are crystals 

 of many hundredweight ; and although the usual crystal is comparatively 

 small, it may be^any size. 



Crystallization has occurred by cooling, or by other natural means ; 

 and we can form crystals by evaporation from certain salts deposited in 

 water. So we may conclude also that the evaporation of water in the early 

 periods deposited many forms of crystals. We have crystals in the air, such 

 as snowflakes, which are vapours crystallized. Carbon, when crystallized, is 

 the diamond. Boron is very like it. Oxygen cannot be crystallized. 

 Alumina makes sapphires and ruby with silica. Alumina and earth give 

 us spars, tourmaline, and garnets. Limestone also has beautiful forms, as 

 in Iceland spar. Crystals, therefore, are certain forms of nature, corre- 

 sponding in the inorganic kingdom to the animals and plants of the organic. 



Let us look a little more at these. Here we have a group of crystals 

 of different forms. Earths are metals combined with oxygen, and the 

 principal earths are alumina, lime, and silica. To these three we are chiefly 

 indebted for the ground we live on, and from which we dig so many useful 

 metals and other minerals. Earths are coloured by the substances mixed 

 with them. We can thus find copper, silver, gold, lead, etc., by noting the 

 appearance of the soil. True earths are white. Strontia and baryta arc 

 also earths, and the latter is used in firework manufactories. Our chief 

 assistants are ALUMINA, which furnishes us with bricks and slate ; LlME, 

 which gives us marble or stones for building in a carbonate form. Quick- 

 lime, by which is meant lime freed from the carbonic acid, is well known ; 

 and plaster of Paris is only lime and sulphuric acid in combination. The 



