We can never...

We can never, merely by looking at two wires, one of which is electrified, determine which of them has an electric current. We can only discover the existence of this current from the effect of the electricity, e.g. a lamp being lit. Electricity is something, which exists although our eyes cannot, directly, see it. b. Another example is gravitation. If you let go off the book, which you now have in your hand, it will fall to the ground, i.e. the ground will pull the book towards itself.

This power is something, which we do not directly perceive through our senses. Gravitation is again one of those things which is not visible, but of whose existence we feel sure through observing the falling of bodies - which is its result. c. One more example is magnetism. We place a magnet beside a piece of iron. Externally there is nothing except what we see, but when the iron is pulled towards the magnet we understand that in the space around the magnet the phenomenon of magnetism exists. d.

Invisible radiation is another case. If we shine white sunlight through a prism we see on the other side of the crystal six colors (the spectrum), which are: red, orange, yellow, green, blue and violet. On this side of red and beyond violet no more colors are visible. However scientists have discovered that in the place where the eyes see no more light, further 'colors' exist which have heating and chemical properties. These 'lights’ are called infrared and ultra-violet. In the year 1800 AD.

a physicist and astronomer called Herschel thought of the idea of researching into whether radiation existed beyond what the eye can see. He gradually altered the position of a thermometer placed on a screen on which the six-coloured spectrum (from red to violet) was projected, and exposed it to the various colours, measuring the heating effect of each of them. When he got beyond the red, he discovered that the thermometer indicated greater heat from this.

He then became certain of the existence of invisible radiation - called infrared radiation - which produced more heat than visible light, and this same heat, which is a result of the radiation, convinced people of the existence of infrared radiation. At the same time, another scientist called Wollaston placed a quantity of the chemical compound silver chloride beyond the violet end of the spectrum.