It is the coldest substance known. It takes an intense cold to produce it, and it has to remain cold much as ice is cold, only very much more so, as long as it is liquid air. For this reason it is carried about in vessels constructed so as to exclude the heat. Mercury dropped into it becomes a solid block, and meat quickly freezes so hard that it is brittle as glass and may be broken into a thousand pieces. The liquid oxygen left after exposure of liquid air may be placed in a hollow in a cake of ice. Dip into it a watch spring and touch a lighted match to it and you will see the steel spring burn as if it were full of pitch. Eight hundred gallons of common air are compressed into one gallon of the liquid. The liquid is unattractive and very common-looking. You would not suspect its great powers by merely looking at it in a dish. But when it expands into common air it has tremendous energy. A few drops confined in a closed iron pipe will explode and blow the metal to atoms. When first produced it was so expensive a product that its value was above that of rubies. Now it is cheap and becoming more so. We expect it to become an ordinary article of commerce. One company is capitalized at $10,000,000 to push its use in place of steam and electricity. Probably some of the companies advertising shares to sell are putting its powers far too high. One company's agents are representing that a very little of it in a cup will keep an icebox cold all day, and that a pound of it will reduce the heat in a large house on a warm summer day so that it may be kept cool at very small expense. These extravagant claims are probably made for the purpose of deceiving people so they will buy shares. The facts seem to show that a pint of liquid air will not cool an ice box much more than will a pound of ice. The effect of a gallon of it in a large house would scarcely be felt in July, except for a short time in one or two rooms. COMMON MINERALS AND VALUABLE ORES. II.--QUARTZ AND THE SILICATES. THEO. F. BROOKINS, B.S. Principal Au Sable Academy, N. Y. Comparatively few persons associate the gem opal, with its brilliant internal colored reflections, with that material forming so large a part of the soil, sand. Yet the two are almost identical in composition. The mineral constituent of sand and of opal is quartz, though the latter often contains in addition some water.