Glass has been defined as a substance which, during its manufacture, passes from the liquid to the solid state so rapidly that no crystals are formed. Usually when solids are melted and then allowed to cool, they do so with the formation of crystals, this may be shown in the case of sulphur by melting a little and then allowing it to cool. After a while, as cooling takes place a solid crust will be formed on the surface of the molten sulphur. If two holes are pierced in the crust and the still liquid sulphur poured out, it will be found that the sulphur which adhered to the vessel in which the melting took place has formed beautiful needle shaped crystals. A great amount of original work has been done on the subject of glass manufacture and especially on the kind used for optical instruments--as a result there are many kinds of glass differing from one another in physical properties and in chemical composition. Although the various chemicals used and their proportions are fairly well standardised, as the result of long experience, it is probable that glass is not a definite chemical compound but a mixture, in which certain of the components act as solvents for the rest. The ordinary glass in use in this country, apart from specially prepared optical glass, may be either English flint glass, plate glass or Bohemian glass. The first named is composed of sand, potassium carbonate and red lead; plate glass is made of sand with the carbonates of sodium and calcium, whilst similar ingredients are used for Bohemian glass except that carbonate of potassium is substituted for carbonate of sodium. It is chiefly owing to the requirements of optical instrument makers that, new kinds of glass containing very many previously untried chemicals, have been produced. As a result glasses are now made with specific gravities varying from 2.5 to 5.0, that is to say the weight of a square inch, or a square foot or a square yard of glass may weigh anything from two and a half to five times more than a square inch, foot or yard of water, according to its composition.