4 EXPANSION OF SOLIDS. 



tube ; and the numerous experiments on uncrystallized bodies, 

 which we possess have afforded no ground for general deduc- 

 tions. 



It has been farther observed, that the same solid is more ex- 

 pansible at high than at low temperatures, although the increase 

 in the rate of expansion is in general not considerable. Thus, 

 if we mark the progress of the dilatation of a bar of iron under 

 a graduated heat, we find that the increase in dimension is 

 greater for one degree of heat near the boiling point of water 

 than for one degree near its freezing point. Solids are observed 

 to expand at an accelerated rate, in particular, when heated up 

 to near their fusing points. The cohesion or attraction which 

 subsists between the particles of a solid is supposed to resist 

 the expansive power of heat. But many solids become less 

 tenacious, or soften before melting, which may account for their 

 increasing expansibility. Platinum is the most uniform in its 

 expansions of the metals. 



Such changes in bulk, from variations in temperature, take 

 place with irresistible force. This was well illustrated in an 

 experiment, which was successfully made upon a gallery in the 

 Museum of Arts and Manufactures in Paris, in order to preserve 

 it. The opposite walls of this edifice were bulging outwards, 

 from the pressure of the floors and roof, which endangered its 

 stability. By the directions of an ingenious mechanic, stout 

 iron rods were laid across the building, with their extremities 

 projecting through the opposite walls. The rods were then 

 strongly heated by a number of lamps, and when in an expanded 

 condition, a disc on either extremity of each rod was screwed 

 firmly up against the external surface of the wall. On after- 

 wards allowing the rods to cool, they contracted, and drew the 

 walls to which they were attached somewhat nearer together. 

 The process was several times repeated, till the walls were 

 restored to a perpendicular position. 



The force of expansion always requires to be attended to in 

 the arts, when iron is combined in any structure with less ex- 

 pansible materials. The cope-stones of walls are sometimes 

 held together with clamps, or bars of iron : such bars, if of cast 

 iron, vvhich is brittle, often break on the first frost, from a 

 tendency to contract more than the stone will pejmit ; if of 

 malleable iron, they generally crush the stone, and loosen them- 



