637 



HEAT. 



stop-cock, a body is plunged at any temperature, and remains until it 

 ceases to melt the ice, when the quantity of melted water conducted 

 through the stop-cock is taken as a measure of the quantity of heat 

 given out by the body. This instrument is of use in determining the 

 upecijic heats of substances, and the calculation of latent heat. For the 

 measurement of high temperatures, see PYROMETER. 



The dilatation of substances by heat is, in general, nearly propor- 

 tional to the increase of temperature, except when they are about to 

 change their physical or chemical states; thus water near the freezing- 

 point expands when the temperature is diminished, which is probably 

 owing to the different arrangement assumed by its constituent particles 

 preparatory to crystallisation. 



The value of the thermometer mainly depends on the assumption 

 that equal increments of heat produce equal amounts of expansion. 

 Still, however, this is not strictly the case ; mercury, for example, in 

 the 10 between 30 and 40, expands less than in the 10 between 200 

 and 210', but it fortunately happens, that the increase in the capacity 

 of the glass bulb, especially if the thermometer be made of crown- 

 glass, very nearly compensates for the increasing rate of expansion in 

 mercury. Between the freezing and boiling points of water, the ther- 

 mometer may therefore be relied on ; but above 212 the instrument is 

 not so exact. In general, in all bodies, there is an increasing rate of 

 expansion for increasing temperatures. In the case of mercury, Reg- 

 nault found that between 32 and 212, its expansion was 1 in 55'08; 

 between 212 and 392, making an equal interval, it was 1 in 54'61, 

 and between 392 and 5/2, also an equal interval, it was 1 in 54-01. 

 Platinum expands more equably than any other of the metals ; but gives 

 a similar increase in the rate of expansion as the temperature rises. 

 According to Dulong and Petit the increase in dilatation o*f the following 

 substances is the result of experiment : 



HEAT. 



EXPANSION OF LIQUIDS. 



033 



Regnault further found that the dilatation of mercury between 32 

 and 662 was 1 in- 95827. At high temperatures air is more reliable 

 than mercury. The temperature of 572, as measured by an air- 

 thermometer would, if measured by a mercurial thermometer be 686. 

 In gases, and also in vapours considerably above their points of con- 

 densation, the expansion is the same in all under similar variations of 

 air and pressure, and in general it may be stated that from the freezing 

 to the boiling point of water they increase in bulk more than one-third, 

 1000 parts at 32 becoming 1366 at 212'. Rigid inquiry, however, by 

 such observers as Hegnault and Magnus, show that the co-efficient of 

 expansion is not strictly uniform for all aeriform bodies : the expan- 

 sion u -greater for those which are most readily condensable, but most 

 uniform for gases which have not been liquefied. Practically, however, 

 there is but little chance of error in the statement that gases and 

 vapours between 32 and 212 expand by heat JJths of the volume at 

 32, or about 775 for each degree of Fahrenheit. 



It was noticed by Mitscherlich that doubly refracting crystals under 

 the influence of heat expand unequally in different directions. A 

 crystal of calcareous spar, for example, when raised from 32 to 212, 

 elongates most in the direction of the optic axis, and contracts in 

 directions at right angles to this. As in the case of liquids, lollds 

 expand unequally for equal additions of heat ; zinc expanding much 

 more than iron, and iron more than glass. The total expansion of a 

 body may be obtained very nearly by multiplying the linear expansion 

 by 3. The following table gives the expansion in length and in bulk 

 of certain solids between 32 and 212 : 



It will be seen from this last table, that liquids expand much more 

 rapidly than solids, and differ in that property to a much greater 

 extent, the most volatile being the most expansible. 



The amount of force exerted by the expanding or contracting of a 

 body tinder varying changes of temperature is equal to that which 

 would be required to elongate or compress it to the same extent by 

 mechanical means. According to Barlow, a bar of malleable iron, 



1 square inch in section, is stretched fijTJooth of its length by 1 ton 

 weight ; a similar elongation is produced by a rise in temperature 

 equal to about 16 Fahr. The difference between the summer and 

 winter temperatures in this climate is sufficient to cause an iron bar, 

 fastened by its extremities to exert a strain of many tons on the 

 square inch. In engineering "and other works it is necessary to make 

 some provision for expansion and contraction. Instances of it are 

 familiar in the cracking of glass and cast iron : the sudden application 

 of heat produces a sudden dilatation on the surface, which is torn away 

 as it were from the interior and colder portions. The unequal con- 

 traction due to sudden cooling may produce a similar effect. In the 

 application of solders and cements, regard must be had to the relative 

 expansions of the solders and the bodies soldered. Iron, platinum, 

 and glass do not greatly differ in their rates of expansibility, and hence 

 those two metals may be soldered into glass, whereas silver, gold, 

 copper, and some other metals differ greatly in their rates of expansion 

 from glass, so that when soldered into it, they separate as the joint 

 cools. 



Reference has been already made to the three methods concerned in 

 the equilibrium of temperature. But we may be allowed in this place 

 to state a few of the results of modern scientific enquiry on those 

 important processes. 



And first, as to conduction. Several distinct sets qf investigations 

 have been made as to the relative conducting power of different solids. 

 Wiedemann and Franz (' Pogg. Annal.' Ixxxix. ) employed equal bars 

 of the substances, and exposed one extremity of each to a uniform 

 source of heat : the progress of the temperature along each bar 

 was measured at intervals of 2 inches by means of a thermo-electric 

 arrangement. Their conclusion was, that the conducting power of 

 metals for heat proceeds in the same order as their electrical con- 

 ducting power. Calvert and Johnson (' Phil. Trans.,' 1858) employed 

 two vessels of vulcanised India rubber on account of its low conducting 

 power, and passed in succession bars of metal (each 2'36 inches long, 

 and 0'393 inch square), through an opening in one of the sides of each 

 vessel, into which it projected one-sixth of its length, the portion out- 

 side and betweenthe two vessels being covered with vulcanised India 

 rubber. A given weight of cold water, sufficient to cover the bar, was 

 poured into one of the vessels and the temperature carefully noted, 

 while into the other vessel was poured a given weight of water at 194, 

 that .temperature being maintained for 15 minutes by the occasional 

 injection of steam. The temperature of the colder vessel was then 

 taken, and a comparison of its rise in temperature when bars of different 

 metals were employed, gave the relative conducting power, correction 

 being made for the loss of heat by radiation and transfer from one 

 vessel to the other. The following table, based on these experiments, 

 represents rather the order of conductability than the conducting 

 power of the metals, for in order to obtain this, it would be necessary 

 to repeat the experiments with bars of the same metals of different 

 lengths : 



Metals Employed. 

 Silrer . 

 Gold 



Gold alloy tffa . 

 Rolled copper . 

 Cast copper . 

 Mercury . 

 Aluminum . . 

 Rolled zinc 

 Cadmium . . 

 Bar iron . 

 Tin ... 

 Steel 

 Platinum 

 Sodium 

 Cast iron 

 Lead 



Antimony . . 

 Bismuth . 



Rise in Temp. 

 Centigrade Scale. 

 . .11-90 

 31-30 

 26-80 

 26-95 

 55-87 

 21 -CO 

 21-20 

 20-45 

 18-40 

 13-92 

 13-45 

 12-05 

 1S-15 

 11-65 

 11-45 



9-17 



6-85 



1-95 



Mean 



Conductability. 



Silver = 1000. 



1000 



981 



840 



845 



811 



677 



C65 



041 



677 



436 



412 



397 



380 



Ifl 



359 



287 



215 

 61 



Of the metals employed in obtaining the above results the platinum, 

 aluminum, iron and sodium were commercial samples: the other 



