EXPANSION OF SOLIDS. 3 



This is the increase which these bodies sustain in length. 

 Their increase in general bulk is about three times greater. 

 Thus, if glass elongates 1 part in 1248 from the freezing to the 

 boiling point of water., it will dilate in cubic -capacity 3 parts 

 in 1248 or 1 part in 416. The expanded bodies return to their 

 original dimensions on cooling. Wood does not expand much 

 in length ; hence it is occasionally used as a pendulum rod. 

 For the same reason a slip of marble has lately been employed 

 for that purpose, in constructing the clock of the Royal Society 

 of Edinburgh. Flint glass expands by the table i-sWth part,, 

 while the metal platinum expands very little more, (-nVy). 

 Hence the possibility of cementing glass and platinum together, 

 as is done in many chemical instruments. Other metals pushed 

 through the glass when it is red hot and soft, shrink afterwards 

 so much more than the glass on cooling, as to separate from 

 it, and become loose. Lead is the most expansible of the 

 metals ; it expands between three and four times more than 

 platinum from the same heat. 



By far the most important discovery in a theoretical point 

 of view, that has been made on the subject of the dilatation 

 of solids by heat, is the observation of Professor Mitscherlich 

 of Berlin, that the angles of some crystals are affected by 

 changes of temperature. This proves that some solids in the 

 crystalline form do not expand uniformly, but more in one 

 direction than in another. Indeed, Mitscherlich has shewn that 

 while a crystal is expanding in length by heat, it may actually 

 be contracting at the same time in another dimension. An 

 angle of rhomboidal calcareous spar alters eight and a half 

 minutes of a degree between the freezing and boiling points of 

 water. But this unequal expansion does not occur in crystals of 

 which all the sides and angles are alike, as the cube, the regular 

 octohedron, the rhomboidal dodekahedron. In investigating 

 the laws of expansion among solids, it is adviseable, therefore, to 

 make choice of crystallized bodies. For, in a substance not 

 regularly crystallized, the expansion of different specimens may 

 not be precisely the same, as the internal structure may be 

 different. Hence the expansions of the same substance, as 

 given by different experimenters, do not always exactly 

 correspond. The same glass has been observed to dilate 

 more when in the form of a solid rod, than in that of a 



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