But what is, perhaps, most curious about lavas is that when the surface of a freshly broken piece of cold lava is carefully examined, it is found to contain a number of small crystals of such mineral substances as quartz, feldspar, hornblende, mica, magnetite, etc. The best way to study the different forms of lava crystals is to prepare a thin transparent slice of hardened lava and then examine it with a good magnifying glass. It will be found that the slice consists of a mass of a glass-like material through which the crystals are irregularly distributed, not unlike the raisins and currants in a slice of not over rich plumcake. When examined by a more powerful glass, such as a microscope, cloudy patches can be seen distributed irregularly through the glass-like mass. When these patches are examined by a higher power of the microscope they are seen to consist of small solid particles of definite forms known as _microliths_ and _crystallites_. It has been shown by a careful study of these minute objects that they form the exceedingly small particles of which crystals are built up. If we fuse a small quantity of lava and then let it slowly cool, the glassy mass will be found to contain numerous crystallites. On the other hand, when fused lava is permitted to cool quickly, it takes on the form of a black, glass-like mass known as _obsidian_ or _volcanic_ glass, a very common form of lava in some parts of the world. In some lavas there are found larger crystals that appear to have been separated from the glassy mass, under the great pressure that exists in the subterranean reservoirs at great depths below the volcanic crater, and then floated to the surface surrounded by the glass-like material. Now when we examine these crystals with a higher power of the microscope, we frequently find in them minute cavities containing a small quantity of liquid and a bubble of gas, thus causing them to resemble small spirit levels. The liquid in such cavities has been examined chemically and in most cases has been found to consist of water containing several salts in solution. Sometimes, however, the liquid consists of liquefied carbonic acid gas. These wonderful things will be discussed at greater length in the Wonder Book of Light. When the mass of molten rock or lava that comes out of the crater of a volcano is thrown upwards in the air the condition it assumes by the time it falls back again to the earth depends on the height it reaches. If this height is great the lava chills or hardens before reaching the earth, and assumes various forms according to the size of the fragments. The largest of these fragments are called _cinders_; the finer particles _volcanic dust_; while most of those of intermediate particles are known among other things as _volcanic ashes_.