22 



UNDULATORY FORCES. HEAT. 



[CONDUCTION. 



conductor than platina. The annexed figure illustrates 

 the mode of carrying out the experiment. 

 Fig. s. 



1000 

 973 

 8P8 

 381 

 374 

 363 

 304 

 180 



found, that the conducting power of gold, 

 per are nearly alike ; as is that of platina, 



a, the nource of best, a * pirK-Ump ; 66, pieces of phosphorus ; e, the cop- 

 per wire ; ft, the twiMcd part where the metals join ; p, the platina wire. 



The wires may be held in the flame by a pair of nippers 

 pressing the twisted part, and each wire should be of the 

 same thickness, and, at the ends, of the same distance 

 from the centre of the flame. 



The following table illustrates the comparative conduct- 

 ing power of some of the inetals, of which gold stands the 

 highest, and lead the lowest, in the list : 

 If Gold conducts heat at the rate of 



Silver 



Copper 



Platina 



Iron 



Zinc 



Tin 



Lead 



It will thus be 

 silver, and copper 

 iron, and zinc. 



Why one solid body should conduct heat better than 

 another, is by no means evident. It'may depend on some 

 peculiar molecular constitution of which we are ignorant. 

 It is plain that solidity or specific gravity is not the sole 

 cause, as, in that case, lead should be a better conductor 

 than iron, etc. We shall have to remark on this subject 

 when we deal with Specific Heat. 



Other solids besides metals, which, as we have observed, 

 . are generally speaking good conductors, vary greatly in 

 their powers of carrying off heat, and, for that reason, are 

 used in daily life as economical arrangements for prevent- 

 ing its escape. Of these may be named earths of various 

 sorts, charcoal, (bo. Whilst gold conducts heat at a rate 

 of 1,000, clay, in the form of brick, has but the one- 

 hundredth part of that power, compared to the metal ; 

 hence the employment of fire-brick in the sides of 

 furnaces, and of clay " backs" in ordinary domestic fire- 

 grates, &c. The same principle has lately been employed 

 in constructing fire-proof safes, which are often made of 

 plates enclosing a comparatively non-conducting material, 

 such as charcoal, by means of which heat is prevented 

 from passing through and injuring the contents of such 

 arrangements. Steam-boilers, and the cylinders and 

 pipes of steam-engines, are covered with wood or felt, to 

 prevent the escape of heat, and the condensation of the 

 steam which they contain. 



A very general idea of the relative conducting power of 

 various materials, may be obtained by placing the hand 

 consecutively on paper, wood, stone, and a metal. Each 

 of these, although of the same temperature, will seem 

 colder than the other, in the order we have arranged 

 them, because they are successively better conductors of 

 heat, and, therefore, more rapidly abstract caloric from 

 the body. A more accurate mode of ascertaining the 

 differences of their conducting powers, is by intro- 

 ducing thermometers into closed vessels coated with 

 each material, and noticing the time required for water 

 of the same temperature in each, in attaining one of a 

 lower degree. Or if rods of various metals, of equal 

 length, have one end immersed in a liquid of high tem- 

 perature, and if the other end is coated with wax, the 

 time elapsing between the melting of the wax on each 

 end will give an approximate ratio. 



The latter mode is subject to error ; because, as differ- 

 ent bodies have different capacities for heat, the length 

 of time they require to be heated to identical tempera- 

 tures will be influenced thereby. 



Either method should be so tried as to avoid the 

 effects of absorption of heat from other bodies ; and un- 

 less each niotal is exactly in the same state as to its 

 external surface, error would be occasioned by its po\\c r 

 of radiation, <tc. 



Ono of the most interesting series of facts in connec- 

 tion with the conducting power of solid bodies, is that 

 which relates to the value of different sorts of clothing 

 employed to retain animal heat; respecting which, in- 

 structive experiments may easily be tried by immersing 

 thermometers in closed tin vessels containing water nf 

 equal temperature in each, and noticing the length of 

 time required for each vessel, clothed externally with 

 different materials, to come down to one standard. 



It is thus found, that the following articles conduct 

 heat in the order we have arranged them ; in which the 

 best conductor is first placed, and those of an inferior 

 kind will be found in succession. The number indicates 

 the time, in seconds, required for a heated body to cool 

 to an even temperature, when coated with the substances 

 named. 



Comparative condiicting power of dothing materials. 



Silk twist 81* 



Cotton wool 104 



Wool nnspun 112 



Silk from the cocoon . , . 128 



Beaver, eider, and hare fur, about . 130 



It is thus found, that the substances with which 

 animals are generally clothed, are the best adapted for the 

 preservation of vital heat ; and, generally speaking, the 

 thickness of the coating is proportioned to the climate in 

 which the animal naturally exists. The reader will not 

 fail to notice, that the animals, birds, <fcc., found in 

 northern regions, are liberally supplied with hair, won], 

 &c., such as the polar bear and the wild goose; whilst 

 those existing in tropical climates, as the giraffe, <tc., 

 are supplied with but a thin coating of external non- 

 conducting material. Man, whose intelligence enables 

 him to adapt himself to circumstances, takes advantage 

 of various materials for clothing purposes; and hence 

 the employment of linen and cotton garments in the hot 

 regions of the tropics, and the use of woollen and fur 

 garments in colder countries. 



CONVECTION OP HEAT. Liquids are, generally speak- 

 ing, very bad conductors of heat, and the process of its 

 propagation through them has been termed convection, 

 because the particles move, and thereby convey heat with 

 them. This is true both with respect to liquids and 

 aeriform bodies ; and we shall therefore consider them 

 together in reference to this property. 



If a solid body is heated in any part, the caloric so 

 imparted at once travels in every direction ; but that 

 such does not occur in liquids, may at once be perceived 

 on trying the following experiment : 



Experiment 14. Place a metal dish, containing spirits 

 of wine, on the surface of some water, and immerse a 

 thermometer in the latter, so that its bulb shall rest three 

 inches below the bottom of the metal vessel. On 

 igniting the alcohol, the water immediately touching the 

 copper may be made to boil, but the thermometer will 

 not show any increase of temperature ; which proves that 

 heat scarcely passes downwards tlirough the liquid. If 

 heat, however, is applied at the bottom of the vessel 

 holding the water, it will be observed that the tempera- 

 ture will rapidly increase, owing to the convection or 

 conveyance of caloric by the particles of the expanded 

 liquid rising to the surface. 



In arranging steam or hot-water apparatus for heating 

 purposes, this principle must always be observed. Steam 

 and water-pipes are, therefore, placed at the lower part 

 of a room, because the hot air they produce, being 

 expanded, rises, and conveys with it the caloric from the 

 heating apparatus. Ventilation is generally effected by 

 adaptations of this principle ; and, as hot air always rises, 

 an apartment may easily be ventilated by introducing, 



