extinguish an inflamed one, though it will itself burn.  You generally think of water as the great antithesis of, the universal antidote for, fire.  The truth is here again only of an ex parte character, as I will show you.  If I can, by means of a substance having a more intense affinity for oxygen than hydrogen has, rob water of its oxygen, I necessarily set the hydrogen that was combined with that oxygen free.  If the heat caused by the chemical struggle, so to say, is great, that hydrogen will be inflamed and burn.  Thus we are destroying that antithesis, we are causing the water to yield us fire.  I will do this by putting potassium on water, and even in the cold this potassium will seize upon the oxygen of the water, and the hydrogen will take fire.

Specific Gravity.—­We must now hasten to other considerations of importance.  Water is generally taken as the unit in specific gravities assigned to liquids and solids.  This simply means that when we desire to express how heavy a thing is, we are compelled to say it is so many times heavier or lighter than something.  That something is generally water, which is regarded, consequently, as unit or figure 1.  A body of specific gravity 1.5, or 1-1/2, means that that body is 1-1/2 or 1.5 times as heavy as water.  As hat manufacturers, you will have mostly to do with the specific gravities of liquids, aqueous solutions, and you will hear more of Twaddell degrees.  The Twaddell hydrometer, or instrument for measuring the specific gravities of liquids, is so constructed that when it stands in water, the water is just level with its zero or 0 deg. mark.  Well, since in your reading of methods and new processes, you will often meet with specific gravity numbers and desire to convert these into Twaddell degrees, I will give you a simple means of doing this.  Add cyphers so as to make into a number of four figures, then strike out the unit and decimal point farthest to the left, and divide the residue by 5, and you get the corresponding Twaddell degrees.  If you have Twaddell degrees, simply multiply by 5, and add 1000 to the result, and you get the specific gravity as usually taken, with water as the unit, or in this case as 1000.  An instrument much used on the Continent is the Beaume hydrometer.  The degrees (n) indicated by this instrument can be converted into specific gravity (d) by the

    formula:  d = 144.3/(144.3 — n)

Ebullition or Boiling of Water, Steam.—­The atmosphere around us is composed of a mixture of nitrogen and oxygen gases; not a compound of these gases, as water is of hydrogen and oxygen, but a mixture more like sand and water or smoke and air.  This mass of gases has weight, and presses upon objects at the surface of the earth to the extent of 15 lb. on the square inch.  Now some liquids, such as water, were it not for this atmospheric pressure, would not remain liquids at all, but would become gases.  The pressure thus tends to squeeze gases together and