10 



UNDULATORY FORCES. HEAT. 



expansion of a metal, both in reapcct to its length and 

 biv.idth, may bo thus tried : 



Efptrimcttt 6. Fit a brass ring very accurately on an 

 iron rod ; having removed the ring, heat tlio rod nearly 

 to a red-heat, and attempt to replace the ring. It will 

 be found, that owing to the expansion of the metal, the 

 ring cannot be passed on to the rod until the latter has 

 cooled down. 



I'.speriment 6. Fix an iron rod, whilst cold, so as that 

 it may exactly rest between two points touch ing each of 

 its ends. On heating the rod, and attempting to replace 

 it, its expansion lengthways will be found to prevent its 

 resting between the two points, because they will bo at 

 less distance from each other than are the two ends of the 

 heated metal. The rod, on cooling, will again fit between 

 them, owing to the loss of heat and subsequent contraction 

 which it undergoes. 



Having thus illustrated the linear and general expansion 

 of a solid by the agency of heat, we remark that differ- 

 ent bodies expand at unequal rates; and perhaps- the 

 simplest illustration of this is afforded by the following 

 experiment : 



/.' .'/<. , iinent 7. A piece of brass bar, an eighth of an 

 inch thick, one inch wide, and eighteen inches long, is to 

 be firmly rivctted to an iron one of the same size, care 

 being taken that the two shall form a straight bar when 

 completed. On heating such an arrangement, it will 

 immediately take a curved form, because the metals ex- 

 pand unequally. The brass, expanding more than the 

 iron, will form an arch", enclosing the iron. The bar will 

 regain its straight form on again becoming cool. 



A vast variety of interesting applications of this law of 

 expansion by heat are made in daily life. The tires of 

 railway wheels are made to embrace the spokes tightly, 

 by making them a little too small when cold. On heating 

 the tire, it expands, and can then be passed on to the 

 ring of metal in wliich the spokes end. On cooling, the 

 tire contracts with great force, and so remains completely 

 fixed in its desired position. The tires of carriages and 

 cart-wheels are affixed in a similar manner; and some 

 parts of machinery are thus easily made to retain their 

 position, such as in the case of rings or pulley-wheels, 

 often secured on shafting. In constructing railway and 

 other iron bridges, allowance is always made for expan- 

 sion by heat ; and in laying down the rails on the track 

 of a railroad, sufficient space is left between each metal 

 to permit free extension. On a railway where sufficient 

 allowance had not been made, the rails were lifted, during 

 a hot summer day, several inches from the ground. The 

 rails on a line of railway of two hundred miles' nominal 

 length, will be some hundred feet longer in summer than 

 in winter time, presuming the absolute length of iron 

 surface is carefully measured. 



It has been noticed that a rod of metal will contract to 

 its previous bulk, when the heat which had caused its 

 expansion had been lost. An interesting experiment 

 may be adduced in illustration of this fact. 



Experiment 8. In an arm of cast-iron (a), represented 

 in Fig. 2, have two holes made at 6, so that a rod of 

 wrought- iron (c) may be screwed tightly, by means of a 

 thumbscrew, as at d. Remove this rod, and heat it red- 

 hot, afterwards replacing it in this state, and screwing it 

 firmly in the arms of the cast-iron frame. On pouring 

 cold water on the hot iron, the metal will rapidly con- 

 tract, and in so doing, will act so powerfully on the cast- 

 iron frame as to break it into pieces, with a loud report. 



Fig. 2. 



An adaptation of this experiment has been occasionally 

 employed for the purpose of restoring walla wlu'ch had 

 bulged outwards. Iron plates are fixed on the out 

 of the wall, and a stout iron rod is stretched between 



them, and firmly screwed at its extreme ends, beyond the 

 plates. On the rod being heated, it of course expands, 

 ami is again screwed up tight. On contracting, it draws 

 together the plates and walls with great force. A repe- 

 tition of the process generally succeeds in restoring the 

 perpendicularity of the walls of a building, and is easy of 

 application. 



Srrit nix accidents have happened in cases when builders, 

 on fastening iron girders resting between stone walls, 

 had neglected to make allowance for the expansion of the 

 metal. In many instances, extensive erections have been 

 tlirown down : for as the iron expands in length, it forces 

 the walls outwards ; and should no immediate apparent 

 mischief result, still the alternate expansion and contrac- 

 tion has the tendency of weakening the brickwork, and 

 thus of endangering the stability of the building. 



The annexed table, which lias been arranged from 

 various sources, gives the linear expansion of the sub- 

 stances named, on their being raised from a temperature 

 of 32 F. to 212 ; and is to.be understood as expressing 

 the ratio of their individual expansion, in the order in 

 wliich that increases. 



Glass Tube 0.000861 



1'latina 0.000884 



, 0.000092 



Iron 0.001220 



Cold . . 0.0014B6 



0.001555 



Copper 0.001 V.'2 



Silver 0.001909 



Lead 0.002M8 



Zinc 0.003011 



The above numbers are expressed in decimals, as giving 

 a greater amount of accuracy than would have been 

 arrived at by employing vulgar fractions ; and the stan- 

 dard is unity. Hence the length of a bar of metal will be 

 increased to the extent of its original length, multiplied 

 by the factor in the table, on the temperature of the 

 metal being raised the number of degrees above indicated. 



Various interesting applications of the law of the ex- 

 pansions of solids will be mentioned in the section on 

 Mechanical Philosophy: such as the regulation of pendulu, 

 watch-springs, and other similar matters. Their intro- 

 duction in this place would tend to distract attention 

 from the laws to wliich we prefer for the present to con- 

 fine our remarks. 



EXPANSION OF LIQUIDS. As a general rule, liquids 

 expand far more in bidk, by the same increase of tempe- 

 rature, than solids ; and hence these changes become more 

 evident to the senses. They have been chosen as the 

 fittest media for the purpose of measuring the tempera- 

 ture of bodies within a limited range. Hence the con- 

 struction of thermometers of various kinds, to wliich we 

 shall now devote our consideration. 



That liquids expand considerably on being heated, is 

 often observed in daily life. If a kettle, completely filled 

 with cold water, is heated, the liquid it contains gradually 

 expands, and soon passes out of the vessel. A more 

 philosophical mode of illustrating this effect, is that of fixing 

 into a glass flask a long, narrow glass tube, and filling the 

 vessel with coloured water, so that a portion of the liquid 

 shall also rest in the tube. On heating the flask, the 

 water will gradually expand, and will do so according to 

 certain laws, indicating its expansion by rising in the tube 

 until the boiling point is reached. On this principle the 

 thermometer is constructed ; mercury, however, being 

 generally employed in preference to other liquids, except 

 for special purposes. , 



Liquids, like solids, vary much in their relative expan- 

 sion on receiving equal increments of heat ; and without 

 giving a tabular form of these, which could not be done 

 with accuracy, we may state that mercury expands least, 

 and alcohol (pure spirits of wine) expands most, on being 

 exposed to the same increase of temperature. The total 

 expansion of a solid by heat is very nearly ascertained by 

 multiplying its linear extension by three. In the case of 

 liquids, they naturally accommodate themselves to the 

 shape of the vessel which contains them, and as a 

 solid that will expand also. It is therefore necessary, 

 in every instance, to make an allowance for this in 

 constructing instruments for measuring changes of 



