608 



HEAT 



small, and a rough approximation to its value is 

 usually sufficient. 



Glass -00086 



Iron -00122 



Zinc -00294 



Mercury -01803 



Water -0432 



Alcohol -116 



Air -3665 



Hydrogen -3668 



There is one specially remarkable exception to 

 the law that bodies expand by heat viz, that of 

 water under certain circumstances. From ( Centi- 

 grade ), at which it melts, it contracts as the tempera- 

 ture is raised, up to about 4 C., after which it 

 begins to expand like other bodies. We cannot 

 here enter into speculations as to the cause of this 

 very singular phenomenon, but we will say a few 

 words about its practical utility. Water, then, 

 is densest or heaviest at 4 C. Hence, in cold 

 weather, as the surface water of a lake cools to 

 near 4, it becomes heavier than the hotter water 

 below, and sinks to the bottom. This goes on till 

 the whole lake has the temperature 4. As the 

 surface-cooling proceeds further, the water becomes 

 lighter, and therefore remains on the surface till it 

 is frozen. Did water not possess this property, a 

 severe winter might freeze a lake to the bottomland 

 the heat of summer might be insufficient to 

 remelt it all. 



Specific Heat. The thermometer indicates the 

 temperature of a body, but gives us no direct infor- 

 mation as to the amount of heat it contains. Yet 

 this is measurable, for we may take as our UNIT 

 the amount of heat required to raise a pound of 

 water from to 1, which is of course a definite 

 standard. As an instance of the question now 

 raised Is more heat (and if so, how much more) 

 required to heat a pound of water from zero to 10 

 than to heat a pound of mercury between the same 

 limits ? We find by experiment that bodies differ 

 extensively in the amount of heat ( measured in the 

 units before mentioned ) required to produce equal 

 changes of temperature in them. 



It is a result of experiment (sufficiently accurate 

 for all ordinary purposes ) that, if equal weights of 

 water at different temperatures be mixed, the tem- 

 perature of the mixture will be the arithmetic mean 

 of the original temperatures. From this it follows, 

 with the same degree of approximation, that equal 

 successive amounts of heat are required to raise the 

 same mass of water through successive degrees of 

 temperature. As an instance, suppose one pound 

 of water at 50 to be mixed with two pounds at 20, 

 the resulting temperature of the mixture is 30 ; for 

 the pound at 50 lias lost 20 heat units, while each 

 of the other two pounds has gained 10 such units, 

 transferred of course from the hotter water. Gener- 

 ally, if TO pounds of water at t degrees be mixed 

 with M pounds at T degrees ( the latter being the 

 colder), and if 6 be the temperature of the mixture 

 the number of units lost by the first is m(t -6), 

 since one is lost for each pound which cools by one 

 degree ; and that gained by the second is M(0 - T), 

 and these must be equal. Hence m(t -0)- M(0 - T) ; 

 whence, at once, 



= mt + MT 

 m + M 



But if we mix water and mercury at different tem- 

 peratures, the resulting temperature is found not to 

 agree with the above law. Hence it appears that 

 to raise equal weights of different bodies through the 

 same number of degrees of temperature requires 

 different amounts of heat. And we may then define 

 the specific heat of a substance as the number of 

 units of heat (as above defined) required to raise 

 the temperature of one pound of it by one degree. 



From the definition of a unit of heat it is at 

 once seen that our numerical system is such that 

 the specific heat of water is unity ; and, in general, 

 the specific heats of other bodies are less, and are 



therefore to be expressed as proper fractions. For 

 example, if equal weights of water and mercury 

 be mixed, the first at 0, the second at 100, the 

 resulting temperature will not be 50 ( as it would 

 have been had both bodies been water), but 3 -23 

 nearly ; in other words, the amount of heat which 

 raises the temperature of one pound of water 3 '23 is 

 that which would raise that of one pound of mercury 

 96 "77, or the specific heat of mercury is -g^th of 

 that of water. The following may be given as 

 instances of the great differences which experiment 

 has shown to exist among bodies in respect of spe- 

 cific heat : Water, 1 '000 ; turpentine, '426 ; sulphur, 

 203; iron, '114; mercury, -033. 



It is mainly to the great specific heat of water 

 that we are indebted for the comparatively small 

 amount of it required to cool a hot body dropped 

 into it ; for its comparatively small loss of tempera- 

 ture when it is poured into a cold vessel ; and for 

 the enormous effects of the water of the ocean in 

 modifying climate, as by the Gulf Stream. 



It has been found generally that the specific 

 heats of elementary solids are nearly inversely as 

 their Atomic Weights (q.v.). Hence their atoms 

 require the same amount of heat to produce the 

 same change in their temperature. Thus, for simple 

 bodies, we have atomic weight of mercury, 100 ; 

 its specific heat, '033; product, 3'3; atomic weight 

 of iron, 28; its specific heat, '114; product, 3'2. 

 A similar remark may be made, it appears, with 

 reference to compound bodies of any one type ; 

 but, in general, the product of the specific heat 

 and the atomic weight differs from one type to 

 another. 



Latent Heat, Fusion, Solution, and Vaporisation. 

 We are now prepared to consider the somewhat 

 complex effects produced by heat on the molecular 

 constitution of bodies ; and, conversely, the rela- 

 tions of solidity, fluidity, &c. to heat. All solid 

 bodies (except carbon, which has been softened 

 only ) have been melted by exposure to a sufficiently 

 high temperature. The laws of this fusion are : 



( 1 ) Every body has a definite melting-point, assign- 

 able on the thermometric scale, if the pressure to which 

 it is subjected be the same. 



(2) When a body is melting, it retains that fixed 

 temperature, however much heat may be supplied, 

 until the last particle is melted. The last result is 

 most remarkable. The heat supplied does not 

 raise the temperature, but produces the change of 

 state. Hence it seemed to disappear, as far as the 

 thermometer is concerned, and was therefore called 

 latent heat. 



A pound of water at 79 C. added to a pound of 

 water at C. produces, of course, two pounds of 

 water at 39 '5. B'ut a pound of water at 79 C. 

 added to a pound of ice at C. produces two pounds 

 of water at 0. Heat, then, has disappeared in the 

 production of a change from solidity to fluidity. 

 And this we might expect from the conservation 

 of Energy (q.v.), for energy in the shape of heat 

 must be consumed in producing the potential 

 energy of the molecular actions of the separate 

 particles in the fluid. For every pound of ice 

 melted, without change of temperature, 79 units of 

 heat are thus converted into potential energy of 

 molecular separation. 



We give a few instances of latent heat of fusion : 

 W T ater (as above), 79'0; zinc, 28 '1 ; sulphur, 9'4; 

 lead, 5 '4 ; mercury, 2 '8. 



In law 1 it is mentioned that constancy of pres- 

 sure is necessary. In fact, the freezing ( or melt- 

 ing ) point of water is lowered by increase of pres- 

 sure, while those of sulphur or wax are raised; 

 but these effects, though extremely remarkable, 

 are very small. Most bodies contract on solidify- 

 ing; but some, as water, cast-iron, certain alloys, 

 &c., expand. Thus a severe frost, setting in after 



