Do not be cast down because you see another term to be explained. A gas is, you may have already guessed, simply a fluid. Matter exists in three states, solid, liquid and gaseous. Everything can exist in these three states under different conditions of heat and pressure.

For instance, ice, water, and steam are precisely the same thing, a mixture of oxygen and hydrogen, though in different states. Hence steam is simply the gaseous form of ice or water. Now some gases are heavier than air, and among them is carbonic acid, a gas given off from the lungs in breathing.

By means of a very simply-constructed balance, you can prove this gas to be heavier than air. Sounds queer, doesn’t it? to talk of weighing something that you cannot handle or see.

It is not difficult to do. Bend some wire, minding that the beams of the balance are curved as in the figure.

For one side of the scales a strong cardboard box will answer admirably; for the other the lid of a round box will serve. Hang the whole on a string and adjust it by putting some grains of sand in the round scale on which the weights are placed, to make each side balance one another and the scales are ready for use.