Another scale till recently much used in Germany is that called Réaumur's, in which the freezing-point is marked 0°, and the boiling-point 80°. The degrees on this scale are thus larger than those of Fahrenheit, or even of the centigrade: 9° F. 4° R. or 5° C; and by means of these proportions, a temperature stated in one scale may be reduced to either of the others, care being taken to allow for Fahrenheit's scale commencing, not at the freezing-point, as the others do, but 32° below it. Examples of converting degrees of one scale into those of another :

Fahrenheit chose the temperature of 32° below freezing as the zero-point of his scale, because it was the lowest that had then been observed, and was considered to indicate the complete absence of heat; but it is now known that there are natural temperatures at least 90° below this; and by artificial mixtures, a cold has been produced of 146° F.

SPECIFIC HEAT.

83. Different substances require different quantities of heat to raise them to the same temperature. This is expressed by saying that each possesses a specific capacity for heat, or, more shortly, a specific heat. The fact can be proved in a variety of ways. Thus, if we cause equal quantities of bodies which have all been raised to the same temperature, to melt ice, we shall find that a much greater weight of it will be melted by one body than by

another. Thus, mercury at 212° will melt much less ice than an equal quantity of water at the same temperature will, for the mercury has much less heat to give out, so as to produce liquefaction, than the water has.

Specific heats are generally stated with reference to equal weights, rather than to equal measures, of bodies. Thus, a pound of water in rising to a given temperature, absorbs thirty times more heat than a pound of mercury in rising to the same temperature; so that the capacity of water for heat exceeds that of mercury thirty times. Water has a greater capacity for heat than any other known substance. If the specific heat of water, then, be taken as unity, that of any other substance will be a proper fraction. Thus: Water, 1.000; turpentine, .426; sulphur, 203; iron, 114; mercury, 033.

84. Importance of Specific Heat of Water.-The great specific heat of water has a most important relation to the welfare of the living creatures on the globe. The sea, and other great beds of water, which spread over so large a portion of the earth, cannot in the hot months of the year become rapidly raised in temperature; in the cold seasons of the year, on the other hand, they cool slowly, and, moreover, in cooling, evolve much heat, which equalises the temperature of the air as well as that of the land.

PROPAGATION OF HEAT.

Heat is transferred from one portion of matter to another in three different ways, which are termed Conduction, Convection, and Radiation.

85. Conduction implies the passage of heat from one particle of matter to another in physical contact with it. It is best seen in solids, and particularly in metals, which are the best conductors. A rod of iron placed

with one end in the fire speedily becomes hot at the opposite end, owing to the conduction of heat from particle to particle along the rod. Dense bodies are generally the best conductors; light and porous ones the worst. Feathers, down, fur, flannel, and most of the fabrics used for winter dresses, owe their so-called warmth to their low-conducting power for heat. Their action is altogether negative, being limited to the prevention of the rapid escape of heat generated by the living beings whose bodies they cover.

The relative conductivities of the principal metals are exhibited in the following table:

On the same scale, the conductivity of marble would be expressed by 8, of porcelain by 6, of brick earth by .55.

86. Convection.-Liquids and gases rapidly rise in temperature when heat is applied; but the heat passes through not by conduction, but is carried from one part to another by the particles being set in motion. If a spirit-lamp is applied to the top of a tube filled with water, the upper portion of the liquid is soon heated to boiling, while hours will elapse before even a slight degree of heat will reach any distance down the column. Water is thus seen to be a bad conductor. But if the lamp is applied at the bottom of the tube, the heat is soon felt at the top, and the whole liquid is made to boil in a few minutes. This remarkable difference is owing to a motion that takes place among the particles of the liquid. The portions resting on the bottom, being

expanded and made lighter by the heat, begin to

Fig. 20.

ascend, and thus circulating currents are established which convey the heat to all parts of the mass. This circulation may be made visible in a glass flask (fig. 20) nearly filled with water, having a few bits of blue litmus swimming in it, and with a spiritlamp below.

Air and other gases are raised in temperature in the same way that liquids are. They conduct heat with extreme slowness, as may be proved by applying heat to the top of an air-tight glass vessel with a thermometer suspended a little below the heated

portion. But when the heat is applied from below, currents of circulation immediately begin, as in the case of a liquid.

87. Why Wool, Down, and Snow are warm Coverings.-Woolly coverings and furs imprison the air within their substance, and prevent it from circulating, while they afford but few points of solid contact for the direct conduction of heat. These two circumstances combined, give them their remarkable power in arresting the escape of heat; and that power is greater the finer and lighter their texture. Swan's-down is said to be the most perfect insulator of heat. From the same causes, snow is an excellent non-conductor, and, like a fleece of wool, protects the earth from any cold much below 32°.

88. Radiation.-When the hand is brought near a hot body, like a mass of iron, heat is felt to be darting

from it across the intervening space. This way of propagating heat is called radiation, because the heat streams off like rays (radii) of light from a luminous body. There is more, however, than an analogy between the two things. Radiant light and radiant heat are only different degrees of one and the same thing-of Radiant Energy.

89. The Spectrum.-When a ray of light enters a dark room through a small hole in a shutter, it forms a round white spot on the opposite wall. If now a glass prism with its edge downwards is interposed in the path of this ray, it is refracted or bent out of its path, and falls on a higher part of the wall; and what is more, it no longer forms a round white spot, but is drawn out vertically into a long rainbow-tinted ribbon, known as the spectrum, having the red colour at the lower end, and the violet at the upper end. This arises from the different kinds of coloured light of which white light is composed, being refracted by the prism in different degrees, the red being least refracted and the violet most. Between these extremes there are innumerable distinct kinds of rays, each having its own degree of refrangibility, and therefore maintaining a fixed place in the spectrum relatively to the others. But besides the visible spectrum, there are invisible rays beyond the red end, and it is in them that the heating power of the beam chiefly resides. There are also invisible rays beyond the violet end; and they are more powerful than the visible in producing chemical changes, as on a photographic plate.

90. The Ether, its Waves or Pulses.-Now, the undulatory theory of light assumes the existence of an invisible, imponderable, elastic medium called ether, pervading all space and filling the interstices between the molecules