332. =Why the Freezing Point is at 32°.=--If the freezing point of water were higher than 32°, freezing would occur so early in the autumn, and the ice and snow would last so late in the spring, that the season would be too short to raise our supplies of fruits and grains. If, on the other hand, it were at a lower point, the earth would not have the protection of its light coat of snow, but instead would be chilled by rains so cold that barrenness would be the result. The multitudes of animals, too, that now live so securely in the water, some of them even with the ice above them, would all perish with the cold. 333. =Force of Expansion in Ice.=--As ice occupies one seventh more room than the water from which it is formed, it exerts in its formation an expansive force which under various circumstances produces varied and often remarkable results. Of the many experiments which have been tried to show the force of this expansion I will mention but one. A bomb-shell was filled with water, at Montreal, and closed with an iron plug which was driven in with great force. The plug was thrown a distance of 400 feet by the expansion when the water froze. This expansion is sometimes an inconvenience to us, as in bursting water-pipes; but besides the great service which it does in the earth, already noticed, it is of service also in loosening the soil, and in supplying it with requisite ingredients from the rocks by breaking them up and pulverizing them in small quantities from year to year. CHAPTER XIV. LIGHT. 334. =Nature of Light.=--We do not know what light is. There are two suppositions in regard to it. One is that of Sir Isaac Newton, called the theory of _emission_. According to this light is a substance, but so ethereal that it has no weight, and is capable of passing through various substances of even great density. The other supposition is what is called the _undulatory_ theory. The advocates of this, which is now quite generally received, believe light to consist of undulations, waves, or vibrations in an ether which is supposed to exist every where, pervading all space and every substance. You perceive an analogy here to sound, the vibrating medium in the case of sound, however, being always some palpable substance--solid, fluid, or aeriform. Heat is supposed, as stated in § 271, to be a vibration of the ethereal substance, as light is, though the two vibrations must of course be somewhat different in character. Any body that is capable of communicating the light-vibration to this ether is said to be _luminous_. 335. =Sources of Light.=--The chief source of light to our earth is the sun, which is a permanently luminous body. Then we have the light of combustion in its various forms. Electricity is another source of light. Light is sometimes emitted during decay or putrefaction of some substances. Some animals--as fire-flies, glow-worms, and phosphorescent animals in the sea--have the power of emitting light.