HK AT 



609 



copious ruin, splits mckH, &c., by the expansion of 

 freezing water ; and tlm- also we obtain in iron the 

 ilelicate and faithful copy of a mould, and in 

 tin- fusihlu alloy a clear-cut copy of a type. The 

 iiKtilrrn dynamical theory of heat (thermo-dynamics ) 

 enables UH to nee that a per|xtual motion would be 



I >i<>< -in-able if bodies which contract on solidifying 

 i;ul not their melting-point raised by pressure, and 

 'rsA, 



Analogous to the fusion of a solid is its solution 

 in a liquid, or the mutual conversion into liquids of 

 two solids which are intimately mixed in powder. 

 Here, also, we should expect kinetic enemy, in the 

 shape of heat, to be used up in producing the poten- 

 tial energy of the liquid state ; and, indeed, such 

 is always the case. Such changes of arrangement 

 destroy heat or produce cold ; but this in many 

 cases is not the effect observed, as there is gener- 

 ally heat developed by the loss of potential energy 

 if there be chemical action between the two sub- 

 stances. Hence, in general, the observed effect 

 will be due to the difference of the heat generated 

 by chemical action and that absorbed in change of 

 state. 



If a quantity of pounded nitrate of ammonia ( a 

 very soluble salt ) i>e placed in a vessel, an equal 

 weight of water added, and the whole stirred for a 

 minute or two with a test-tube containing water, 

 the heat required for the solution of the salt will 

 be abstracted from all bodies in contact with the 

 solution, and the water in the test-tube will be 

 frozen. In this sense the arrangement is called a 

 freezing mixture. For additional illustrations of 

 heat becoming latent, see FREEZING MIXTURES. 



Of course the converse of this may be expected 

 to hold, and latent heat to become sensible when 

 a liquid becomes solid. As an example, when a 

 supersaturated solution of sulphate of soda begins 

 to deposit crystals of the salt with great rapidity 

 the temperature rises very considerably ; and it is 

 the disengagement of latent heat that renders the 

 freezing of a pond a slow process, even after the 

 whole of the water has been reduced nearly to the 

 free/ing-point. 



Vaporisation. Almost all that has been said on 

 the subject of fusion is true of vaporisation, with 

 the change of a word or two. Thus, however 

 much heat we supply to a liquid, the temperature 

 does not rise above tlie boiling-point. Heat, then, 

 becomes latent in the act of vaporisation, or rather 

 is converted into the potential energy involved 

 in the change of state. It is found by experi- 

 ment that 540 units of heat (each sufficient to 

 heat a pound of water 1 C. ) disappear in the con- 

 version of a pound of water into steam. Hence 

 a pound of steam at 100 C. is sufficient to 

 raise 5'4 pounds of water from zero to the boiling- 

 point. 



COMMUNICATION OF HEAT. There are at least 

 three distinct ways in which this occurs, and these 

 we will take in onl.-r. 



Conduction. Why is it that, if one end of a 

 poker and of a glass or wooden rod be put into 

 a fire, we can keep hold of the other ena of the 

 latter much longer than we can of the former? 

 The reason is that heat is more readily transmitted 

 in- the iron from particle to particle than it is in 

 glass or wood. This is conduction. It is to be 

 noticed, however, that in this experiment a great 

 portion of the heat which passes along each rod is 

 given off into the air by tlie surface. The mathe- 

 matical theory of conduction has l>cen most ex- 

 quisitely investigated by Fourier, but on the 

 supposition that the rate at which heat pa-M-s from 

 a wanuor to a colder portion of a body is proj>or- 

 tional to the difference of temperature. As most 

 of the experiments which have lieen made with 

 the object of ascertaining the conductitnty (not 

 247 



conduct ihility, the erroneous word too commonly 

 in use) of different bodies have been made in thin 

 way, it is not surprising that our knowledge on 

 this point is very meagre indeed. We know that 

 silver ami copjier conduct better than most other 

 metals, and that the metals in general conduct 

 better tluwi other solids ; but our further informa- 

 tion is neither very extensive nor very definite. 

 The first determinations of conductivity which are 

 at all trustworthy are those of Forbes, lli- 

 method was immensely superior to those of hi* 

 predecessors. Before we give one or two numer- 

 ical data, we must explain what the numbers 

 mean. The following definition is virtually that 

 of Fourier : 



The thermal conductivity of a sulmtance in 

 the number of units of heat which pass per unit of 

 surface per unit of time, through a slab of unit 

 thickness, whose sides are kept at temperatures 

 differing by 1 C. Taking the unit of heat as above 

 descrilied, a foot as unit of length and a minute as 

 unit of time, the conductivity of iron is about 

 0'8, while that of copper varies from 4 to little 

 more than 2. (Very slight impurities affect to a 

 great extent both the thermal and the electric 

 conductivity of copper.) Contrasted with these 

 we find that the conductivity of rocks is very 

 small, ranging from 0'015 to 0'04. 



In conjunction with their radiating power (see 

 next section), the conductivity of bodies is most 

 important as regards their suitableness as articles 

 of clothing for hot or cold climates, or as materials 

 for building or furnishing dwelling-houses. We 

 need but refer to the difference between linen and 

 woollen clothing, or to the difference (in cold 

 weather) of sensation between a carpet and a bare 

 floor, in order to show how essential the greater or 

 less conducting power of bodies is to our everyday 

 comfort. 



Radiation. By this is understood the passage 

 of heat, not from particle to particle of one body, 

 but through air or vacuum, and even through solid 

 bodies (in a manner and with a velocity quite 

 different from those of conduction ) from one body 

 to another. There can be no doubt whatever as 

 to radiant heat being identical with light, differ- 

 ing from red light, for instance, as red light differs 

 from blue i.e. having (see LIGHT) longer waves 

 than those corresponding to red light. This idea 

 might easily have arisen during the contemplation 

 of a body gradually heated. At first it remains 

 dark, giving off only rays of heat; as its tempera- 

 ture increases it gives us, along with the heat, a low 

 red light, which, by the increase of the temperature, 

 is gradually accompanied by yellow, blue, &c. rays, 

 and the incandescent body (a lime-ball, for in- 

 stance) finally gives off a light as white as that 

 of the sun, and which therefore contains all the 

 colours of sunlight in their usual proportions. In 

 fact there is great reason to believe that the sun 

 is merely a mass of incandescent matter, probably 

 in the main gaseous, and that the radiations it 

 emits, whether called heat or light, merely differ 

 in quality, not in kind. Taking this view of the 

 subject at the outset, it will oe instructive to 

 compare the properties of radiant heat with those 

 of light throughout. It must be understood wlien 

 we make this comparison that the term heat is 

 improperly used in this connection. Radiant heat 

 is not heat in the ordinary sense of the word. It 

 is a form of energy, a transformation of the heat 

 of a hot body, ami can be transformed into heat 

 again when it is ataorhed, but on its passage it is 

 not what we ordinarily understand by the word 

 heat. 



Light, then, moves (generally) in straight linet. 

 This is easily verified in the case of heat by the 

 use of the thermo-electric pile and its galvano- 



