30 



MECHANICS. 



fluid by heat is used as an indication, 

 or measure, of the degree of heat to 

 which the instrument is exposed, (see 

 HEAT, Chap. IV.), and the fluid which 

 is used in thermometers may be either 

 liquid or aeriform, although most fre- 

 quently the former. 



(59.) Bodies whether solid, liquid, or 

 aeriform, exert a certain degree of me- 

 chanical force, in the process of enlarg- 

 ing their dimensions, on receiving an ac- 

 cession of heat ; and any obstacle which 

 opposes this enlargement sustains an 

 equivalent pressure. This force is fre- 

 quently used as a mechanical agent, and 

 has this to recommend it, that it may 

 be produced to almost any degree of 

 intensity, without the expenditure of 

 any other mechanical force in its pro- 

 duction. In this respect it has the 

 advantage over the mechanical agency 

 of air, arising from its pressure and con- 

 densation, ( 53, 54.) 



A remarkable instance of the use of 

 the power with which solids expand by 

 heat, occurred in Paris some years since, 

 in a method which was used to force 

 together the walls of a gallery in the 

 Abbey of St. Martin, now the Conser- 

 vatoire des Arts et Metiers. The weight 

 of the roof was forcing the walls of this 

 building asunder, and they were re- 

 stored to their perpendicular position 

 by the following method : Holes were 

 made at opposite points, in several parts 

 of the walls, through which strong iron 

 bars were introduced, so as to extend 

 across the building, and so that their 

 extremities should extend beyond the 

 walls. Large nuts were placed upon 

 their ends, and screwed up so as to 

 press upon the walls. Every alternate 

 bar was then heated by powerful lamps, 

 so that its length increased by expan- 

 sion, and the nuts, before in close con- 

 tact with the walls, retired to some dis- 

 tance from them. The nuts were then 

 screwed up to the walls, and the bars 

 cooled. The process of cooling re- 

 stored the length of the bars to what it 

 had been before the heat had been ap- 

 plied, and the nuts were drawn together 

 by an irresistible force, and consequently 

 the walls drawn towards each other. The 

 same process being repeated with the 

 intermediate bars, and this being con- 

 tinued, the walls of the building were 

 gradually restored to their perpendicular 

 position. 



In the processes of shoeing wheels, 

 and hooping barrels, the same force is 

 used mechanically. The iron hoop, or 



rim, is put on hot, and made to fit the 

 wheel or the barrel exactly, and being 

 then cooled, it contracts and binds the 

 parts of the wheel or the ban-el together 

 with immense force. 



It is evident, however, that these 

 forces of expansion and contraction of 

 bodies by heat and cold, act through 

 spaces so limited that they can be 

 used as mechanical agents but very 

 rarely, and under peculiar circum- 

 stances. 



(60.) Heat is productive of mechanical 

 agents of much greater power, by the 

 influence which it has upon the form of 

 bodies, than by its power of enlarging 

 their dimensions. We have stated that, 

 in a solid body, the cohesive force of 

 the particles predominates over the re- 

 pulsive influence of the caloric which 

 pervades its dimensions. Supposing 

 the cohesive force to continue unin- 

 creased, what will be the effect if we 

 transfuse through its dimensions, by the 

 application of fire, such an abundant por- 

 tion of caloric that the repulsive force 

 of it will become equal, or nearly equal, 

 to the cohesive force of the particles ? 

 We should evidently anticipate that the 

 particles, having no tendency, or very 

 little, to cohere, they would move freely 

 among [each other, and fall asunder by 

 their own weight, unless they were pre- 

 vented by the sides of the vessel which 

 might contain them ; in fact, we should 

 predict that, by the application of such 

 a quantity of heat, as we have supposed, 

 the solid would become a liquid. And 

 such we find to be the case : solids 

 liquefy by exposure for a sufficient 

 time to the action of fire. 



It would appear, therefore, that the 

 solid and liquid forms in which we find 

 bodies, are maintained by the propor- 

 tion which subsists between the force of 

 cohesion peculiar to the particles, and 

 the repulsive force of the caloric which 

 pervades them, the former greatly pre- 

 dominating in solids, and these force.s 

 being nearly in equilibrium in liquids. 



We accordingly find, by experience, 

 that if a sufficient portion of heat be 

 withdrawn from a liquid it becomes 

 solid, the cohesive force of its particles 

 receiving a sufficient predominance over 

 the repulsive force of the caloric, by di- 

 minishing the quantity of the latter. 



(61.) This, however, is not the only, 

 nor the most important, change of form 

 in bodies, which depends on the propor- 

 tion which subsists between these two 

 forces. 



