UK AT. 



57 



that actually molted. If M weigh exactly a pound, and it bo 

 rallied to the temperature 142 Fahr., the specific heat ia at once 

 known by learning what portion of a pound of water is melted. 

 A quarter of a pound in the vessel would indicate a specific 

 beat of 0*25, and BO on. When the substance haa a different 

 weight, or is raised to a different temperature, allowance moat 

 be made by a sum in proportion. 



There ia another way in which the differences in the specific 

 beats of various substances may bo shown and ascertained ; this 

 is known as the method of mixtures. If wo take a pound of 

 water at 100, and another pound at 150, and mix them, the 

 temperature of the mixture will bo the mean of the two, or 125. 

 If, however, instead of the pound of water at 

 150, we take a pound of mercury at the same 

 temperature, the temperature of the mixture will 

 only bo about 102, showing how much less heat 

 was contained in the mercury than in the water. 

 The mercury has lost 48, while the water has 

 only gained 2, and yet we know that whatever 

 amount of heat the one has lost, the other must 

 have gained. The mode of ascertaining the spe- 

 cific heat of any substance in this way is compa- 

 ratively simple. Suppose, for instance, wo have a 

 piece of copper weigh- 

 ing fifty ounce? ; it is 

 brought to a tempe- 

 rature of 200, and 

 maintained at that 

 for a short time, so 

 that every part may 

 be equally heated. It 

 is then immersed in 

 one hundred ounces 

 of water, at a tempe- 

 rature of 60, and 

 after it has had time 

 to share its heat with 

 the water, which is 

 gently stirred to aid 

 this, the temperature 

 of the whole is found 

 to be 66 4. The water 

 here has gained 100 

 (664 - 60) = 650, 

 while the copper has 

 lost 50 (200- 66 J) = 

 6675, and hence its 

 specific heat is $$, or 

 0-096. The specific 

 heat of liquids may 

 also be learnt by 

 noting the time they 

 take to cool from a 

 high temperature, as 

 those which gain heat 

 most rapidly lose it 

 likewise most rapid- 

 ly. The small speci- 

 fic heat of mercury, 

 it being only about 

 jjth that of water, 

 renders it specially 

 suitable for filling 

 thermometers, since it rapidly acquires the temperature of any 

 liquid in which it is immersed, and does so, too, without greatly 

 lowering its temperature. The annexed table gives the specific 

 heats of a few of the more common substances : 



"Water . . 

 Alcohol 

 Turpentine 

 Charcoal 



I'OOOO Sulphur 

 0-6603 Glass 

 0'4259 ] Iron 

 0-2411 i Zinc 



C'2026 

 0-1977 

 0-1138 

 0-0955 



Copper . 

 Silver . 

 Mercury 

 Gold 



0-0952 

 0-0570 

 0-0333 

 O'032-l 



Now in this table no relation whatever is visible between the 

 different numbers, but if, instead of taking equal weights, we 

 take the substances in the proportion of their atomic weights, 

 wo shall find a simple law. To check this, let us multiply 

 the numbers placed above against the elementary bodies by the 

 atomic weights of those bodies. Sulphur wo multiply by 16, 



iron by 28, zinc by 324, copper by 32, and so on, and wo shall 

 then find that the product* ao obtained correspond in most 

 oaaea very closely. Aa a reanlt of a great number of experi- 

 ment*, it ia found that the ipecijic heat qf equivalent wcightt 

 of moat wimple bodiea variea between 3 and 3*3. Thia is 

 usually accounted for by supposing that the molecule* of all 

 the elements have the same capacity for heat. In those cases 

 whore this does not hold true, the proportion is usually s 

 simple one, as a half, or double. Investigation farther shows 

 that in chemical compounds having similar formulae, the specific 

 heats of equivalent weights are likewise similar , so that evi- 

 dently some hidden link of connection exists between chemical 

 composition and specific heat. 



It now remains for us to inquire into the ways 

 in which heat may be communicated from one 

 body to another, and these may be classed under 

 three different heads conduction, convection, 

 and radiation. The former of these ia most 

 common, and must be spoken of first. If we 

 take a rod of glass, and another of iron, and 

 place one end of each in the flame of a spirit- 

 lamp, these ends will soon become red-hot. After 

 remaining so a few minutes, the iron rod will be 

 too hot to be touched 

 within a consider- 

 able distance of the 

 hot end, whereas the 

 glass rod may be han- 

 dled with impunity 

 almost up to the 

 heated part. In the 

 case of the iron the 

 motion of the mole- 

 cules is transferred 

 from one to another 

 till, in a little time, 

 the whole rod be- 

 comes hot; the glass 

 rod, on the other 

 hand, prevents the 

 passage of these vi- 

 brations, and hence 

 is called a bad con- 

 ductor. 



The apparatus 

 shown in Fig. 21 il- 

 lustrates the differ- 

 ence in the conduct- 

 ing powers of various 

 bodies. A metallic 

 trough has a number 

 of holes made along 

 one side. These are 

 closed by corks, 

 through which rods 

 of various sub- 

 stances as wood, 

 glass, and metal are 

 passed. Melted wax 

 or tallow is now 

 smeared on the rods, 

 and allowed to cool, 

 and the trough is 



then filled with boiling water. The rate at which the heat ia 

 conducted along the different rods is at once seen by observ- 

 ing the distances to which the wax is melted along them. 



Fig. 22 shows a more elaborate plan of ascertaining con- 

 ducting power. A bar of the metal to be tested has cavities 

 made along it at regular distances of three or four inches. 

 Mercury is now poured into these, and a delicate thermo- 

 meter put in each. Heat is then applied at one end, and 

 the rate at which it travels along is shown by observing the 

 readings of the different thermometers. Other experimenters 

 have done away with the cavities, and employed a flat bar, test- 

 ing the temperature at different parts by means of a thermo- 

 electric pile. It is found in this way that the conducting power 

 of different metals varies very greatly, that of silver, which is 

 the greatest, being expressed by 100, while that of German 



