vessel, just as in the case of a metal bar heated at one end. When the thermometers indicated that a constant state had been acquired (which was generally after thirty-six to forty hours), the temperatures indicated by the various thermometers were noted.

Various liquids were treated in this manner, the hot water in A being in each case at the same temperature. The respective conductivities were proportional to the squares of the distances downwards from the copper vessel corresponding to a given fall of temperature. (Compare with the result of Ingen-Hausz's experiment.)

Bottomley's Method. The above arrangement was modified by Bottomley, two sensitive thermometers placed horizontally one above the other at a small distance apart being employed to determine the fall of temperature per centimetre length near the top of the vessel, whilst the average temperature of the water below that point was indicated by a thermometer with a sufficiently long bulb. From successive readings of the latter thermometer the quantity of heat which passed through the section near the top of the cylinder in one second could be calculated. The vessel A was dispensed with, hot water being poured in a slow stream on to a small wooden float, and withdrawn at an aperture suitably placed after it had spread over the surface of the water B, without, however, mixing with it. The conductivity C was calculated from the formula :—

C

Heat passing through unit area in one second

Fall of temperature per cm. at section.

For water, the value found was C = 0'002.

Numerous other experiments have been performed, for a description of which the student is referred to Preston's Heat, Chap. VII. Section II. As a general rule, the results obtained

cannot be considered to be so accurate as those obtained in the case of solids.

Conductivity of Gases.-The difficulty of determining the coefficient of conductivity of a gas is enormously greater than in the case of a liquid. In the case of a gas, we have not only convection currents to consider, but errors due to radiation must be provided against. Energy may be propagated through a gas by any or all of the following methods :—

1. Conduction, i.e., transfer of energy from molecule to molecule without the production of convection currents. Thermal changes produced in this manner are very small in magnitude. 2. Convection, i.e., transfer of heat by the bodily motion of large quantities of heated gas. On the earth such transfers constitute winds, and the rapid variations of temperature often experienced in England when the wind changes, give a sufficiently good idea of the magnitude of the results so produced.

3. Radiation, i.e., transfer of energy in the form of waves in the luminiferous ether. In this case the gas molecules, among which the radiation passes, are themselves unaffected, just as the radiation from the sun passes through the atmosphere, without appreciably altering the temperature of the latter. If, however, the radiation falls on the bulb of a thermometer, the latter will be heated, the effect being the same as if the heat had been communicated by the surrounding gas at a high temperature.

As a consequence the most stringent precautions are necessary in experiments on the conductivity of gases. The method which has, up to the present, proved most satisfactory is to determine the rate of cooling of a thermometer bulb, first in a vacuum, and then in the gas in question. In order to eliminate convec tion currents, the pressure of the gas is reduced. The conductivity is independent of the pressure of the gas, unless this becomes so small that the mean free path of the molecules is comparable with the dimensions of the containing vessel. When the latter stage of exhaustion is reached, a sudden fall in conductivity ensues. It is for this reason that Prof. Dewar's vacuum vessels have proved so valuable in preserving liquefied gases.

The value deduced by Stefan for the conductivity of air, is 0'000056, which is less than one ten-thousandth of the conductivity of copper.

The conductivity of hydrogen is seven times as great as that of air. This is due to the fact that, at a given temperature, a hydrogen molecule is moving more quickly than the average velocity of the molecules composing air. (See Chap. XIII.)

Effects of Conduction in Gases.-If a piece of fine platinum wire, through which an electric current is passed, is placed in a glass tube, as in Fig. 202, when the tube is exhausted of air the wire glows brightly. On admitting air the wire becomes dull, owing to the fact that heat is rapidly carried away from it by the air. If the tube is again exhausted, and

F F

then filled with hydrogen, the effect is still greater; it is then extremely difficult to make the wire luminous.

FIG. 202.-Method

of showing the

effects of the

conductivity of a gas. (P.)

Electric glow lamps are exhausted as perfectly as possible, in order that energy should not be carried away from the filament by any inclosed gas. If a small fracture is made in a glow lamp surrounded by an explosive mixture of oxygen and hydrogen, no explosion will The gas conducts the heat away from the filament so quickly that the temperature of the latter falls below the temperature of ignition of the mixture of gases. For this reason glow lamps can be safely used in mines where firedamp is prevalent.

Occur.

SUMMARY.

Convection of Heat.-When heat is carried from one place to another by the motion of finite parts of a substance, the process is termed convection.

Conduction of Heat.-When heat is propagated from one part of a body to another without the occurrence of motion in any finite part or parts of the body, intermediate points being thereby warmed, the process is termed conduction.

Coefficient of Thermal Conduction of a Substance. This is defined as the quantity of heat which passes through unit area in one second, divided by the fall of temperature per centimetre length normal to that surface.

Forbes determined the conductivity of metals by heating a bar at one end, and determining the temperature at different points when these acquired constant values, and subsequently observing the rate of cooling of the bar when uniformly heated.

If different bars of similar sectional areas are coated with wax, and have their ends maintained at uniformly high temperatures, the conductivities of the bars are proportional to the squares of the distances through which the wax is ultimately melted.

The Conductivities of Liquids have been determined by heating the surface of the liquids and using a modification of Forbes's method. Water is a very bad Conductor of Heat.

The Conductivities of Gases have been determined by observing the rate of cooling of a body when surrounded by different gases in a rarefied condition.

QUESTIONS ON CHAPTER XX.

Show how to

(1) A rod heated at one end has reached a steady state of temperature, and the curve of temperature is known. The rate of loss of heat of the surface for different temperatures is also known. determine the conductivity of the rod from these data. (E.) & Hon., 1899.)

(S. & A. Adv.

(2) Give an account of experiments on the conduction of heat in crystals, and discuss the results obtained. (S. & A. Hon. I., 1899.)

(3) Describe a method of measuring the thermal conductivity of a bar of iron, and indicate clearly how to calculate the conductivity of the metal from your observations. (S. & A. Adv., 1897.)

(4) Give an experiment which shows that metals are good conductors, and that wood is a bad conductor of heat.

How many gram-degrees of heat will be conducted in an hour through each square centimetre of an iron plate 3 centimetres thick, its two sides being kept at the respective temperatures of 50° C. and 200° C., the mean specific thermal conductivity of iron between these temperatures being 12? (S. & A. Adv., 1896.)

(5) Describe a method of determining the thermal conductivity of a metal bar. (S. & A. Hon., 1892.)

(6) Define thermal conductivity. A metal vessel, I square metre in area, and whose sides are 5 cm. thick, is filled with melting ice, and is kept surrounded by water at 100° C. How much ice will be melted in an hour? The conductivity of the metal is 02, and the latent heat of fusion of ice 80. (Inter. Sci. Pass, July, 1895.)

(7) Describe experiments which have been made to determine the conductivity of iron bars.

A quantity of water is maintained at 100° in a closed iron tank by passing steam into it. If the quantity of steam is 100 grams per second, and if the area of the tank is 6 metres, the thickness of the iron 4 cm., and its conductivity 2, find the temperature difference between the inside and the outside of the iron. (Inter. Sci. Hon., July, 1895.)

(8) Some ice is to be kept as long as possible in a warm room. Describe, and give reason for, the construction of a suitable box. (Pre. Sc. Pass, Jan., 1896.)

(9) Describe the Davy safety lamp. What thermal principles are applied in its construction? (Sen. Camb. Local, 1897.)

What is the co

(10) Define the coefficient of conductivity for heat. efficient of conductivity of a badly conducting substance upon which the following experiment was made? A tin cylinder, 40 cm. in diameter and 50 cm. in length, is covered all over by a layer of the material 33 cm. in thickness. Steam is passed through the cylinder at a temperature of 100°C., and the external temperature being 20° C. water is found to accumulate at the rate of 3 grams per minute. The latent heat of steam at 100° may be taken as 537 [gram-] calories per gram. (Lond. Univ. B. Sc. Pass, 1897.)

CHAPTER XXI

RADIATION

EVERY one is familiar with the fact, that when the surface of a body is illuminated for some time by a ray of sunlight, the temperature of the body is raised. It may be shown that the sun is the only ultimate source of heat which is of much importance to us on the earth. It has often been pointed out that the heat obtained from burning coal is derived from energy originally stored up under the action of sunlight by the plants from which coal was formed. With the exception of the heat transmitted from the hot interior of the earth, and the small amount of heat which might be obtained by burning the metals which occur in an uncombined state in the earth's crust, we are entirely dependent on the heat which is derived, either directly or indirectly, from the sun.

How then is this heat communicated to the earth at a distance of 90,000,000 miles from the sun? It does not travel from the sun to the earth in the form of heat (i.e., energy possessed by vibrating material molecules), since the space intervening between the sun and the earth is free from matter. Nevertheless our knowledge of mechanics renders it necessary for us to consider that energy can only be transmitted from place to place by the motion of something. Consequently, we assume that all space is filled with a medium, which possesses no appreciable weight, but which is capable of transmitting energy. This hypothetical medium is termed the Luminiferous Ether. By its agency light is transmitted in the form of waves; and we shall see that these waves are capable of setting material molecules in motion, and thus generating heat.