COLORS.--If the light is passed through a prism, which is a triangularly shaped piece of glass, the rays on emerging will diverge from each other, and when they fall on a wall or screen the colors red, orange, yellow, green, blue, indigo and violet are shown.
The reason for this is that the ray in passing through the prism has the different colors in it refract at different angles, the violet bending more than the red.
THE SPECTROSCOPE.--The ability to make what is thus called a _spectrum_, brought forth one of the most wonderful instruments ever devised by man. If any metal, or material, is fused, or put in such a condition that a ray of light can be obtained from it, and this light is passed through a prism, it will be found that each substance has its own peculiar divisions and arrangements of colors.
In this way substances are determined by what is called _spectrum analysis_, and it is by means of this instrument that the composition of the sun, and the planets and fixed stars are determined.
THE RAINBOW.--The rainbow is one of the effects of refraction, as the light, striking the little globular particles of water suspended in the air, produces a breaking up of the white light into its component colors, and the sky serves as a background for viewing the analysis thus made.
HEAT.--It is now conclusively proven, that heat, like light, magnetism and electricity, is merely a mode of motion.
The _mechanical_ theory of heat may be shown by rubbing together several bodies. Heat expands all substances, except ice, and in expanding develops an enormous force.
EXPANSION.--In like manner liquids expand with heat. The power of mercury in expanding may be understood when it is stated that a pressure of 10,000 pounds would be required to prevent the expansion of mercury, when heated simply 10 degrees.
Gases also expand. While water, and the different solids, all have their particular units of expansion, it is not so with gases, as all have the same coefficient.
The ability to read drawings is a necessary part of the boy's education. To know how to use the tools, is still more important. In conveying an idea about a piece of mechanism, a sketch is given. Now, the sketch may be readable in itself, requiring no explanation, or it may be of such a nature that it will necessitate some written description.