CONTRACTION, in physics, the dimi nishin5 the extent or dimensions of a body, or the causing its parts to approach nearer to each other, in which sense it stands opposed to dilatation or expansion. See EXPANSION.
Water and all aqueous fluids are gra dually'Contracted by a diminution of tem perature, until they arrive at a certain point, which is about 8° above the freez. ing point ; but below that point they be gin to expand, and continue to do so ac cording as the temperature is lowered. Similar effects have been observed with regard to some metals. Speaking of con traction, a remarkable phenomenon, of considerable importance in manufactures, obtrudes itself on our notice. It is the hardness which certain bodies acquire in consequence of a sudden contraction, and this is particularly the case with glass and some of the metals. Thus glass vessels, suddenly cooled after having been formed, are so very brittle, that they hardly hear to be touched with any hard body. The cause of this effect is thus properly explained by Dr. Young. " When glass in fusion is very suddenly cooled, its external parts become solid first, and determine the magnitude of the whole piece, while it still remains fluid within. The internal part, as it cools, is
disposed to contract still further, but its contraction is prevented by the resis tance of the external parts, which form an arch or vault round it, so that the whole is left in a state of constraint ; and as soon as the equilibrium is disturbed in any one part, the whole aggregate is destroyed. Hence it becomes necessary to anneal all glass, by placing it in an oven, where it is left to cool slowly; for, without this pre caution, a very slight cause would de stroy it. The Bologna jars, sometimes called proofs, are small thick vessels, made for the purpose of exhibiting this effect; they are usually destroyed by the impulse of a small and sharp body ; for instance, a single grain of sand, dropped into them ; and a small body appears to be often more effectual than a larger one, perhaps because the larger one is more liable to strike the glass with an obtuse part of its surface."