30 



MATTER AND ITS PHYSICAL PROPERTIES. 



in this case, so much dilated by the heat, that it occupies a considerably in- 

 creased space, and fills the bladder, of which it before only occupied a small 

 part. When the bladder is removed from the fire, and allowed to resume its 

 former temperature, the air returns to its former dimensions, and the bladder 

 becomes again flaccid. 



Let a glass tube, with the bulb at the end, have the bulb and a part of the 

 tube filled with any liquid, colored so as to be visible. If the bulb be exposed 

 to heat, by being plunged in hot water, the level of the liquid will rapidly rise. 

 This effect is produced by the dilatation of the liquid in the bulb, which, filling 

 a greater space, a part of it is forced into the tube. This experiment may 

 easily be made with a common glass tube and a little port wine. 



Thermometers are constructed on this principle, the ascent of the liquid in the 

 tube being used as an indication of the degree of heat which causes it. A par- 

 ticular account of these useful instruments will be found in our discourse on 

 them. 



The change of dimensions of solids produced by changes of temperature 

 being much less than that of bodies in the liquid or aeriform state, is not so 

 easily observable. A remarkable instance occurs in the process of shoeing 

 the wheels of carriages. The rim of iron with which the wheel is to be bound 

 is made in the first instance of a diameter somewhat less than that of the 

 wheel ; but being raised by the application of fire to a very high temperature, 

 its volume receives such an increase, that it will be sufficient to embrace and 

 surround the wheel. When placed upon the wheel, it is cooled, and suddenly 

 contracting its dimensions, binds the parts of the wheel firmly together, and 

 becomes securely seated in its place upon the face of the felloes. 



It frequently happens that the stopper of a glass bottle or decanter becomes 

 fixed in its place so firmly, that the exertion of force sufficient to withdraw it 

 would endanger the vessel. In this case, if a cloth wetted with hot water be 

 applied to the neck of the bottle, the glass will expand, and the neck will be 

 enlarged so as to allow the stopper to be easily withdrawn. 



The contraction of metal consequent upon change of temperature has been 

 applied some time ago in Paris to restore the walls of a tottering building to 

 their proper position. In the Conservatoire des Arts et Metiers, the walls of a 

 part of the building were forced out of the perpendicular by the weight of the 

 roof, so that each wall was leaning outward. M. Molard conceived the notion 

 of applying the irresistible force with which metals contract in cooling, to draw 

 the walls together. Bars of iron were placed in parallel directions across the 

 building, and at right angles to the direction of the walls. Being passed through 

 the walls, nuts were screwed on their ends outside the building. Every alter- 

 nate bar was then heated by lamps, and the nuts screwed close to the walls. 

 The bars were then cooled, and the lengths being diminished by contraction, 

 the nuts on their extremities were drawn together, and with them the walls 

 were drawn through an equal space. The same process was repeated with 

 the intermediate bars, and so on alternately, until the walls were brought into 

 a perpendicular position. 



Since there is a continual change of temperature in all bodies on the surface 

 of the globe, it follows that there is also a continual change of magnitude. The 

 substances which surround us are constantly swelling and contracting under 

 the vicissitudes of heat and cold. They grow smaller in winter, and dilate in 

 summer ; they swell their bulk in a warm day, and contract it in a cold one. 

 These curious phenomena are not noticed, only because our ordinary means of 

 observation are not sufficiently accurate to appreciate them. Nevertheless, in 

 some instances, the effect is very obvious. In warm weather, the flesh swells, 

 the vessels appear filled, the hand is plump, and the skin distended. In cold 



