Page:Elementary Text-book of Physics (Anthony, 1897).djvu/219

§ 183] throughout may, without sensible error, be considered uniform and the same as that at its middle point. Summing up all these quantities, we obtain the quantity $$Q$$ which passes the section $$b$$ in the unit time. Then '''179. Conductivity diminishes as Temperature rises.'''—By the method described above, Forbes determined the conductivity of a bar of iron at points at different distances from the heated end, and found that the conductivity is not the same at all temperatures, but is greater as the temperature is lower.

180. Conductivity of Crystals.—The conductivity of crystals of the isometric system is the same in all directions, but in crystals of the other systems it is not so. In a crystal of Iceland spar the conductivity is greatest in the direction of the axis of symmetry, and equal in all directions in a plane at right angles to that axis.

181. Conductivity of Non-homogeneous Solids.—De la Rive and De Candolle were the first to show that wood conducts heat better in the direction of the fibres than at right angles to them. Tyndall, by experimenting upon cubes cut from wood, has shown that the conductivity has a maximum value parallel to the fibres, a minimum value at right angles to the fibres and parallel to the annual layers. Feathers, fur, and the materials of clothing are poor conductors because of their want of continuity.

182. Conductivity of Liquids.—The conductivity of liquids can be measured, in the same way as that of solids, by noting the fall of temperature at various distances from the source of heat in a column of liquid heated at the top. Great care must be taken in these experiments to avoid errors due to convection currents.

Liquids are generally poor conductors.

183. Radiation.—We have now considered those cases in which there is a transfer of heat between bodies in contact. Heat is also transferred between bodies not in contact. This is effected by a process called radiation, which will be subsequently considered.