The Romance of Modern Chemistry

INVISIBLE SUBSTANCES presence by their smell; coal gas, for example, is invisible, but fortunately our noses soon warn us when it is out of bounds. Other gases, again, are coloured, and we have thus direct evidence of their presence. But there are several indirect ways in which we may convince ourselves that air, oxygen, nitrogen, hydrogen, carbon dioxide, and similar elusive substances have a material existence. Air is indeed invisible, but from the effects which it produces when in motion we may be pretty sure as to its material nature. In every case where mechanical work is done, we shall find on con- sideration that the origin of it lies in the motion of some material body; and the wind, which in its destructive mood can lay low a whole forest, can be regularly har- nessed to work by the sails of our ships and the arms of our windmills. Since work is done by the wind, there must be a material body moving, and the material body, in this case, is the air. A very good reason for regarding air as a material substance is based on the fact, to which every reader will assent, that two different material bodies cannot occupy a given space at the same time. If we are foolish enough to run against a stone wall we learn by experience that one material body resents the attempt of another material body to take its place ; it offers resistance. Now that is exactly how the air and other gases behave. If a tumbler is inverted in a basin of water, the water docs not rise and completely fill the tumbler. There is something inside which takes up room and so offers resistance to the water occupying the same space. Reasoning from our general experience of material bodies, we may conclude that the invisible something inside the tumbler is certainly of a material nature. 40