Silver 100 Copper 74 Aluminum 35 Brass 27 Zinc 26 Iron 15 Tin 14.7 German silver 8.4 Mercury 1.7 Granite 0.53 Limestone 0.52 Ice 0.5 Glass 0.2 Water 0.124 Pine, with grain 0.03 Pine, across grain 0.01 Felt 0.008 Air 0.005 To test the conductivity of _liquids_, take a test-tube nearly full of cold water, hold the lower end in the hand while the tube is inclined so that the upper end is heated by a gas flame until the water boils. The lower end will be found to remain cold. (See Fig. 131.) Careful measurements of the conductivity of water show that heat is transmitted through it only {1/800} as rapidly as in silver, while air conducts but {1/25} as rapidly as water. [Illustration: FIG. 132.--Wall construction of a refrigerator. 1, Porcelain enamel lining lock joint; 2, inside wood lining; 3, 3-ply red rope waterproof paper; 4, wool felt deafening paper; 5, flaxlinum insulation; 6, dead air space; 7, flaxlinum insulation; 8, wool felt deafening paper; 9, 3-ply red rope waterproof paper; 10, outside wood case.] [Illustration: FIG. 133.--Sectional view of a Thermos bottle.] =152. Non-conductors and Their Uses.=--Many solids, however, are poor conductors, as leather, fur, felt, and woolen cloth. These substances owe their non-conductivity mainly to the fact that they are porous. The air which fills the minute spaces of these substances is one of the poorest conductors known and hinders the transfer of heat through these solids. For the same reason loosely packed snow is a protection to vegetation covered by it during a period of severe cold in winter. The efficiency of storm sash or double windows, and of the double and triple walls of ice-houses and refrigerators (see Fig. 132) in preventing the conduction of heat is also largely due to the poor conductivity of the air confined in the spaces between the walls. To prevent the circulation of the air, sawdust, charcoal, and other porous material is often loosely packed into the space between the walls of such structure. Other illustrations of effective non-conductors will occur to every one; such as _woolen_ clothing, _wooden_ handles for hot objects, and the _packing_ used in fireless cookers. A _Thermos_ bottle is effective as a non-conductor of heat because the space between the double walls has the air exhausted from it (Figs. 133 and 134).