Take the force of gravitation. The effect of gravitation is one of the most familiar experiences in everyday life, as we know when we fall down, and when we see anything that is thrown into the air fall back again to the earth. Yet no one knows exactly what this force is Isaac Newton, the great English mathematician, discovered the law of gravitation that is the rule by which this force always seems to work; but even he could not explain what it really is. All he could say was that it seemed to be a force of attraction that tends to draw masses of matter towards each other in a certain way, namely in direct proportion to their mass and in inverse proportion to the square of their distance. Hence the earth on which we live draws all smaller objects near it towards its centre, so that, as we say, everything tends to fall on to the earth; hence the sun draws the earth towards itself, and keeps it revolving round it.
But for gravitation, all the things of the surface of the earth would be flung off, the earth itself would fall to pieces, and rush off in fragments into space. Men have always been familiar with lighting; but it is only in modern times that they have indentified it with electricity, and found ways of producing it and making it useful to them, in such forms as the telegraph, telephone and wireless, and harnessed it to drive engines and tram-cars, and to give them light and heat. In fact it is now supposed that solid matter itself is really a form of electricity, because the atoms of matter are composed of electrons, which are really electrical energy.
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Time fails to speak of light and heat, except to say that without them the most wonderful and mysterious force of all, namely life, would be impossible. Biologists can tell us a lot about living organisms plants and animals but no one can tell us exactly what life is. All we know is that it is the force of life that makes the seed grow into a great tree, and the tiny baby in a strong man; and that enables plants and animals to produce other living plants and animals.