PHYSICS OF THE EIGHTEENTH CENTURY. 



293 



the ice in the interior cavity carries off the vitiated air, thus deprived 

 of all its heat. 



The two savants whose names occur in the foregoing paragraph 

 were the first who succeeded in making accurate measurements of the 

 amount of expansion which is produced in solids by a given rise of 

 temperature. The increase of bulk which heat occasions in solids is 

 too small to be recognized by the eye ; but by such apparatus as that 

 shown in Fig. 140 it is customary to demonstrate the fact in lectures 



FIG. 140. 



on heat. A B is a cylinder of copper, which at ordinary temperatures 

 exactly fits into the space A B, and passes through the circular opening E 

 in the gauge c D ; but when the cylinder is hot, it proves too large to 

 enter these openings. The method of measuring the expansion which 

 Lavoisier and Laplace adopted is an excellent illustration of one of 

 the artifices often employed in the measurement of very small quan- 

 tities. The bar of metal or other substance, c D, Fig. 141, was placed 

 in a long metallic trough, A B, in which the bar rested on rollers, so 

 that it might experience no resistance to longitudinal movement. One 

 end of the bar abutted at c against an independent immovable stop, 

 and the other end, D, was pressed by the end of a lever which turned 

 on a horizontal axis E, that also carried a telescope. The telescope 

 was provided with a cross-wire, and when the bar was at the tempera- 

 ture of the melting ice filling the trough, a vertical scale placed at 

 a distance of 200 yards was viewed through the telescope, and the 

 division of the scale cut by the line of sight E F was noted. By the 

 application of heat the ice in the trough was melted, and the water 

 raised to the boiling-point (212 F.). The bar by increase of tem- 

 perature expanded and pushed out the end of the lever, thus turning 

 the axis carrying the telescope through a small angle, so that the cross- 



