Glasses are silicates, and t he more silica there is in them the more perfect and hard they �re, The most perfect glass is found in a state of nature-tbe rock crystal-but as this is nearly infusible, it is not possible to manufacture it. To repder the silica fusible, certain fluxes are therefore added to it, sucb Nitroge n, Laughing Gas. as potassa, soda, lime, and the oxide of lead. Our atmosphere is compnsed ot two gases, Silica fuses well with the alkalies, but the nitrogen and oxygen.