assumed by Newton that in such a case the rate of cooling is proportional to t-t', or the heat lost per second equals K(t-t'), where K is a constant to be determined by experiment. This law is nearly true if the difference between the two temperatures is not large, and it is often convenient to use. But the true law of cooling is based on the law of exchanges. By the preceding paragraph the heat given out in radiation by a body whose FIG. 252. Curves showing that the wave length of most energetic radiation is shorter in proportion as the temperature of the radiating body is higher. absolute temperature is T is equal to CT where C is a constant. If it receives radiation from a black body at temperature T, it will absorb CT,, consequently the loss of heat per second is equal to C(T-T). 474. Wave Length of Most Energetic Radiation-Wien's Displacement Law. The radiation from a hot body as we shall see later (§897) is complex in its nature and made up of ether waves of different wave lengths. In figure 252 are given curves each of which corresponds to a certain temperature and shows how the intensity of the radiation of a black body at that temperature varies with the wave length. It will be observed that the hotter the radiating body the shorter is the wave length of most energetic radiation (corresponding to the highest points on the curves). Experiment and theory have combined to establish the remarkable law that the wave length of most energetic radiation is inversely proportional to the absolute temperature, or in symbols: = AT a constant, found to be 2940 by Lummer and Pringsheim, where is the wave length in thousandths of a millimeter for the highest point on the energy curve and T is the corresponding temperature measured from the absolute zero. Assuming that the radiation from the sun is sufficiently like that from a black body for the law to apply, we may determine its temperature. For the wave length of maximum energy in the sun's radiation is found by Langley to be .0005 mm., which gives for the sun's temperature 2940 5880° absolute, or 5607° C. Since the longest radiation waves that affect the eye give the sensation of red, the above law shows why a heated body should first become red hot and as the temperature rises the shorter waves become relatively more energetic until finally it appears white hot. 475. Dew.-Leaves and grass are rather good radiators and on clear nights they radiate strongly toward the sky and receive very little radiation in return. What is received comes for the most part from the air, which like all gases is a very poor radiator. Consequently vegetation is cooled and if there is much moisture in the air it condenses in the form of dew. Cloudy nights are unfavorable for the formation of dew since clouds radiate toward the earth. When the temperature of the air is near the freezing point the chilling due to radiation causes ice crystals to form and frost is deposited instead of dew. On windy nights there is usually no dew or frost because the rapid movement of the air over leaves and grass acts by conduction to keep vegetation at the same temperature as the general mass of air, thus the heat lost in radiation is supplied by conduction; but on still nights the layer of air resting next to a cooled leaf soon becomes chilled below the average air temperature. MAGNETISM PROPERTIES OF MAGNETS. 476. Natural Magnets. It was known to the ancients that certain iron ores had the power of attracting iron filings and small fragments of the same ore. The first specimens of this ore were obtained at Magnesia in Asia Minor and were on that account known as magnets. The mineral exhibiting this quality in the highest degree is a compound oxide of iron now known as magnetite. If such a natural magnet or lodestone is dipped into a mass of iron filings they cling to it in tufts especially at certain points called poles. 477. Mariner's Compass.-If a lodestone having a strong pole at each end is balanced on a point or suspended by a cord or placed upon a float in water, it will set itself with one pole toward the north and one toward the south. The mariner's compass, which makes use of this property of the lodestone, was known in Europe in the year 1200 and probably earlier among the Chinese. 478. Artificial Magnets. If a small strip of hardened steel is brought into contact with a lodestone it becomes a magnet, and retains the property even when taken away. Iron filings will cling to it in tufts usually at its ends. If it is balanced on a point, one end will turn toward the north just as in case of the lodestone. Such a piece of steel is said to be magnetized and to exhibit magnetism. When balanced on a point so that it can freely turn it is called a magnetic needle. Very powerful magnets are made by causing a current of electricity to flow around a core of soft iron; such electromagnets, as they are called, will be discussed later (§684). 479. Magnets Have Two Kinds of Poles.-The fact that a magnetic needle will always set itself with the same pole pointing to the north indicates that the two poles are different. If two magnetic needles are brought near each other it will be found that the two north seeking poles repel each other; so also the two poles that turn toward the south repel each other; but if the north pole of one is brought near the south pole of the other decided attraction is observed. Thus like poles repel and unlike attract each other. The pole turning toward the north is usually called the north pole in English books, but the French call it the south pole because its polarity must be like that of the south pole of the earth, considering the earth as a magnet. 480. Number of Poles.-If a thin strip of hardened steel, a piece of clock spring for example, be magnetized by drawing a pole of a lodestone or other magnet over it from one end to the other it will probably be found to have two well-marked poles, one at each end. If we break the magnet in the middle. and try to isolate one pole, it will be found that poles have appeared where it was broken and that each fragment has two opposite poles. However small the magnet may be broken up, each piece shows a north pole and a south pole. No one has ever made a magnet with one pole. It is possible, however, for a magnet to have any other number of poles and a ring may be magnetized and have no poles at all. Long thin bars of steel when magnetized often show more than two poles. 481. Relative Strength of the Poles.-When a magnetic needle is floated in a dish of water it at once sets itself in a north and south direction, but it shows no tendency to be drawn toward the north or toward the south. The north pole of the magnet is urged toward the north with a force equal and opposite to that acting on its south pole. The force between the earth and the north pole of the magnet is equal and opposite to that between the earth and the south pole of the magnet. It is concluded, then, that the two poles of a magnet are equally strong. If the floating magnet has more than two poles, the result is the same, it is not drawn either toward the north or south. This indicates that the combined strength of the north poles in a magnet is equal to that of its south poles. 482. Nature of Magnetism.-The fact that the fragments of a magnet always have two poles indicates that magnetism is a condition which prevails throughout the whole mass of the |