Meteors and meteorites produce a very bright light, burning as they do in the oxygen of our air, and though they are very close to us they are so small we cannot feel any heat sent out by them. We have said that when a match is struck the friction produces heat and that when the wax of a candle, the oil of a lamp or the gas of a jet is burned they produce light and heat, and this is also true of the blazing Sun, the fiery comets and the burning meteors. Where there is light there is usually heat and turn about where there is heat there is light if the temperature is high enough. Heat is produced before light, but the two are nearly always found together and they are so much alike they might be called the Siamese twins. =What Heat and Light Are.=—We know that both heat and light are caused by burning gases, but let’s get a little closer and find out just how and why gases which are burning send out heat and light. Now gases are formed of particles of matter called _atoms_ and these atoms are very small but they have a certain size and weight according to the substance they form. When these gases are cold the atoms are comparatively quiet, but when they are heated to a high temperature they are thrown into a violent state of motion and _vibrate_ to and fro a given number of times per second, or _frequency_, as it is called, according to the substances they are made of. The next question in order is what makes the particles, or atoms move, or vibrate and the answer is _heat_, which sometimes produces burning. Combustion, or burning is a chemical action and can be explained by saying that it is the combining of a substance with oxygen, or other substance with the production of heat and light. The only difference between heat and light is that of _wave length_ as we shall see presently, and the length of heat waves and of light waves depends entirely on the rapidity with which the atoms vibrate, or the _frequency of vibration_, as it is called. If the atoms of gas move slowly heat is sent out and if the atoms move rapidly the heat grows more intense and light is radiated. When the atoms of a burning gas vibrate just fast enough to produce light the color of the light is red; when the atoms vibrate still faster the color of the light is green and when the atoms vibrate very fast the light sent out is violet, so we see that not only do the vibrating atoms send out light but that the rapidity, or frequency with which they vibrate makes, or determines, the color as well of the light which is sent out. One thing more: the rapidity of the motion, or vibration, of the atoms of a gas depends entirely on the substance which is being burned, so that certain substances when burning always produce certain colors. (See Chapter XII, _What the Stars Are Made Of_.)