but hydrogen will 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° 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