CHAMBERS'S INFORMATION FOR THE PEOPLE. 



As to the nature of heat, there have been two 

 theories. The prevailing theory at one time was, 

 that it was a distinct material substance, though 

 imponderable ; a subtle fluid stored up in the 

 spaces between the atoms of the grosser kind of 

 matter. The universal view now is, that heat is a 

 mode of motion. When a lump of lead is allowed to 

 drop from a considerable height upon a hard body, 

 it is found to be sensibly heated. In this case the 

 motion of the lead has been arrested, but not 

 destroyed. Motion, like every other form of 

 energy, is as indestructible as matter itself; it is 

 only the form of it that has been changed.* The 

 motion of the mass has been transferred to the 

 atoms or molecules, which are made to move or 

 vibrate to and fro, each by itself; and it is this 

 movement of the atoms and molecules that affects 

 the nerves with the sensation of heat These 

 molecular motions are, like the molecules them- 

 selves, far too minute for actual observation ; so 

 that we cannot say whether they are revolutions in 

 an orbit, or oscillations backwards and forwards ; 

 but the assumption of a motion of some kind 

 seems necessary to the explanation of the observed 

 facts. 



Perhaps no class of natural laws are more in- 

 teresting in themselves, or more practically import- 

 ant, than the laws of heat A knowledge of them 

 is of direct application in every art and handicraft, 

 as well as necessary for domestic economy and 

 personal comfort A full discussion of the subject 

 would require a volume ; it is only the most 

 general facts that the limits of this tract permit 

 to be stated. 



EXPANSION. 



As a rule, all bodies, whether solid, liquid, or 

 gaseous, which are not decomposed by heat, are 

 expanded by it Solids expand least, gases most, 

 and liquids speaking generally are intermediate 

 between them in expansibility. 



To illustrate the expansion of solids, a rod of 

 iron may be taken, and its length and diameter 

 exactly measured at the temperature of the air. 

 If it be now raised to a red heat, it will be found 

 to have suffered an increase in length, and to be 

 too wide to fit an aperture through which it passed 

 before. When allowed to cool to its original 

 temperature, it will exactly recover its previous 

 dimensions. 



The expansion of liquids is familiarly illustrated 

 by heating a glass flask filled with any liquid. 

 The liquid rapidly expands, and manifests its ex- 

 pansion by running over. If a bladder three- 

 fourths filled with air is held near a fire, it becomes 

 quite stretched, and may be made to burst, thus 

 illustrating the expansion of gases. 



Nearly every solid and liquid has an expansi- 

 bility peculiar to itself. Among solids, the metals 

 are the most expansible bodies. Zinc expands 

 most, platinum probably least among bodies of 

 the metallic class. Glass, brick, porcelain, marble, 

 and stone, have small expansibilities. If a rod of 

 iron which measures 819 lines in length when as 

 cold as melting ice, is made as hot as boiling 

 water, it is found to measure 820 lines. Between 

 the freezing and boiling points, then, iron increases 



* This is the_doctrine embodied in the phrase, The Conservation 

 of Energy, which will be further exemplified in treating of Light 

 and Electricity. 

 M 



FT? of its length ; for the same increase of heat, 

 glass expands only rrVr of its length. 



Among liquids, we find those which are most 

 volatile more expansible than others. Gases, 

 unlike solids and liquids, have not specific ex- 

 pansibilities, but each undergoes almost the same 

 amount of expansion for the addition of the same 

 amount of heat. 



When heated from 32 to 212, mercury dilates 

 Js of its bulk, and alcohol i ; air or any other gas 

 about . An increase of i of temperature, there- 

 fore, increases a body of air by ^H of its bulk. 



The mechanical theory of heat, that is, the 

 theory that it is a mode of motion, quite accounts 

 for the phenomena of expansion. If heat consists 

 of vibrations of the particles of matter, to increase it 

 is to increase the rapidity and width of the vibra- 

 tions. Thus the particles are made to urge one 

 i another farther apart, and the volume of the body 

 is increased. Further, as the force of cohesion 

 which opposes this separative tendency differs in 

 different substances, we should expect the expan- 

 sive effect of heat to be greater in one case than 

 another ; and this is what, in fact, happens. The 

 liquid and gaseous forms of matter are readily 

 explained as still further effects of these molecular 

 motions. In solid bodies, we may conceive that 

 the force of cohesion, while allowing to each mole- 

 cule a power of vibration within certain limits, yet 

 holds the orbits or paths of these vibrations to- 

 gether in fixed positions relatively to one another. 

 But as the particles continue to be urged farther 

 apart by fresh accessions of heat, cohesion be- 

 comes too weak to hold them in fixed positions ; 

 they are now at liberty to roll or glide around each 

 other ; in other words, the substance has melted 

 or become liquid. If urged yet farther asunder, 

 they get beyond the sphere of cohesion, and seek 

 to fly away into space, and in this condition they 

 constitute gas or vapour. 



Water presents a singular irregularity in its 

 expansions and contractions. If boiling water is 

 taken, and allowed gradually to cool, it follows the 

 general law, and goes on contracting until it is 

 within a few degrees of freezing (at 39) ; it then 

 begins to dilate, and continues to do so till it come 

 to 32, the freezing-point At the moment of be- 

 coming solid, it undergoes a sudden enlargement. 

 It is this enlargement of freezing water that causes 

 it to burst pipes and vessels in which it is con- 

 fined ; it is also the reason that ice is lighter than 

 water, and floats on the surface. Ten cubic inches 

 of ice weigh as much as nine cubic inches of water. 



In laying the rails on a railway, and in all 

 structures where metal is used, allowance must be 

 made for the expansion and contraction of the 

 metal by change of temperature. The practical 

 mechanic avails himself of the law of expansion 

 in putting on the tires or rings of wagon-wheels 

 and iron hoops on vats. Being made a degree 

 too small at first, the tire or hoop is heated to 

 redness, and in this state driven on ; it is then 

 cooled with water, when it contracts, and draws 

 the parts of the wheel or vessel together with an 

 irresistible force. 



THE THERMOMETER. 



The thermometer is an instrument in which 

 temperature that is, the intensity of heat is 

 measured by the amount of expansion it produces. 



