ELASTICITY (Germ. Springkrtft, Fe derkruf t) is that property of natural bodies in virtue of which they admit of change either of size or form from the application of external force, resuming, upon the suspension of that force, their proper shape or volume.
Though elasticity is a purely physical pro perty, its investigation is scarcely less interest ing in physiological than in mechanical science. The most cursory examination of a living body is sufficient to convince us, that nature, in regulating its varied functions, has availed herself no less of physical than of vital laws. As it is the province of the physiologist to explain and analyze the several actions whose aggregate is life, to trace each to its proper source, and to distinguish tho'ie which are truly vital from those which are merely mecha nical, it is plain that an acquaintance with the physical properties of the material elements of living bodies becomes one of the foundations of his knowledge. Ilence, in a publication, the design of which is to present a complete view of the structure and functions of living beings, it would be improper to omit some notice of those properties of matter which are so frequently and so admirably employed in fitting them for their uses. In this article we shall offer, in the first place, some remarks upon elasticity generally, upon its laws, and upon the distinction between it and other forces ; we shall next advert to its existence in the organized tissues of the animal machine; and, lastly, we shall point out some important actions in the living body where elasticity plays a principal part.
I. General remarks on elasticity—its laws, degree of elasticity possessed by un organized bodies is extremely variable; in some it is so great that they have obtained the name of perfectly elastic; while in others this property is so extremely small, that its very existence has been overlooked. Air is the most perfectly elastic substance with which we are acquainted; in experiments made upon atmospheric air a portion of it has been left for years subjected to a continued pressure, upon the removal of which under the same temperature and barometric altitude, it forth with resumed its original volume. Amongst
solid bodies, the most conspicuously elastic are certain metals and metallic alloys, glass, ivory, &c.; while other solids, such as moist clay, butter, wax, and many similar substances, possess elasticity in an almost imperceptible degree. Fluids have long been considered as completely inelastic ; but though it is ex tremely difficult to demonstrate this property, yet the experiments of Canton would seem to indicate its existence ; they place at least be yond all doubt their possession of another property, namely, compressibility, — a pro perty somewhat allied to that we are now con sidering.
The laws which regulate the elastic force are not exactly the same in these three classes of natural bodies. In the gaseous or perfectly elastic bodies elasticity may be said to deter mine their volume: their particles having an incessant tendency to expand into a greater space are controuled merely by the surround ing pressure, and hence the bulk of gases is always inversely proportional to the compres sing force. This law, at least in the case of atmospheric air, applies within all known de grees of condensation and rarefaction. By means of accumulated pressure, air may be so reduced in volume, that upon suddenly libe rating it, as in the air-gun, it expands with amazing force; and in the receiver of the air pump, even when reduced to one-thousandth Dart its original quantity, it has still elasticity enough to raise the valve. Another important law of elasticity in gases is that its power is increased by heat and diminished by cold, and this applies not only to the permanently elastic gases but to those likewise of another kind, such as the vapours of alcohol, mer cury, nitric and muriatic acids, and water ; the elastic vapours of the nitric and muriatie acids not unfrequently burst the vessels containing them ; the vapours of mercury have broken through an iron box ; and the vapours of al cohol have sometimes occasioned in distil leries the most terrible explosions : the elas ticity of steam, and the filet that we can in crease its power to any extent by means of heat, has enabled us to construct the steam engine, and thus armed mankind with a phy sical power superior to every obstacle.