North of this region the surface of the province is of most fertile soil, the ordinary rainfall sufficing for agriculture.
In the next stage of the process, the glass is raised to a high temperature in order to render it sufficiently fluid to allow of the complete elimination of these bubbles; the actual temperature required varies with the chemical composition of the glass, a bright red heat sufficing for the most fusible glasses, while with others the utmost capacity of the best furnaces is required to attain the necessary temperature.
A preparation sufficing for most purposes is obtained by digesting the commercial article in absolute alcohol, decanting and evaporating the solution to dryness and fusing in silver vessels.
Thus it comes about that the temperature is regulated primarily by adjusting the quantity of silicon in the pig iron treated, of this element usually sufficing.