Silica or silex chemically considered is oxide of silicon and represented by the formula: Sio2. When either is pure, they are insoluble in any other but hydroflouric acid, which readily dissolves them, especially on the addition of a trifle of sulphuric acid. Strong acids have no other action upon the pigment, except that of dissolving out such impurities as lime, oxide of iron, etc. Strong caustic solutions of potash or soda dissolve silica, when boiled, producing water glass, known as silicate of potash or silicate of soda according to the nature of the alkali used. Sand, flint, quartz are silicic anhydride and simply known as silica, but when obtained from flint it is known to paint men as silex, while when derived from sand or quartz it is called silica. Under the microscope the latter shows a spherical form, while silex is more prismatic in the formation of its particles. This feature has been made use of in the manufacture of paste wood fillers for hard open pored wood, it being claimed that the structure of pure silex causes the particles to interlock and fill the pores or grains of wood far better than the round particles of silica. However this may be, so much is certain, that good silex in a filler will show up the natural beauty of the grain in wood far better than when the filler is made from silica or clay, the latter obscuring the effect somewhat by lack of transparency. On the other hand, when it comes to paint, the use of silica as a reinforcing extender is preferable to silex by far, for the reason that silex is much more refractory in grinding than silica and to purchase silex of the required fineness is prohibitive on account of cost. Very fine floated silica can be purchased at one-half, even at one-third of what silex of similar fineness would cost. The use of silica in paint has been condemned by many, simply because its function was not as well understood as it now is. There is no question but that silica adds porosity to paint, still if the paint is otherwise well balanced with opaque pigments, such as lead and zinc in a white and dense black, red or brown in a colored paint, the addition of a reasonable percentage of silica will prove far more beneficial than barytes, as it carries so much more oil, or than whiting or clay, because it is not so apt to peel or scale in the presence of moisture in the surface, while it is, like barytes, unaffected by sulphur gases. As the result of researches during the past decade it has been discovered that contrary to the older authorities, a moderate portion of silica in combination with solid pigments, as lead, lampblack or carbon black, chrome greens, oxide of iron and even red lead will produce good non-abrasive, rust-preventative paint for iron or steel. Of course, the percentage of such addition should be figured on the opacity of the pigments it is to be combined with. For instance, 15 pounds of carbon black and 85 pounds of silica would not make as porous a paint as would 75 pounds of native red oxide and 25 pounds of silica.