The air you breathe, simple as it seems, is composed of three gases, and is besides full of what Huxley calls "a stirabout" of millions of seeds of animalculae and motes of dust visible in the sunbeam.

That hydrant water you are about to swallow is a rich aquarium full of all manner of monsters, which the oxy-hydrogen microscope will exhibit to your terrified gaze, devouring each other alive.

Should you get rid of them by evaporating your water, your chemist will tell you that still your pure water must be a compound of oxygen and hydrogen.

There is no help for it.
Many years ago some astronomers fancied they had found clouds, or nebulae, of gas, quite simple and uncompounded with anything else, a great many millions of miles away in the sky.

They were so very far away that they thought nobody would ever be able to fly so far to bottle up a specimen of that gas and bring it back here to earth and analyze it, to find out whether it was pure and simple, or compound.