56 



ELASTICITY. 



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, 

 ffc. The 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 muriatic acids 

 not unfrcquently 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 fact 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. 



Solid bodies are never perfectly elastic ; for 

 although some, when acted upon by forces 

 within a certain range, are as completely elastic 

 as the gases themselves, yet if the disturbing 

 force be carried beyond a certain degree, they 

 will never resume their original condition. 

 Thus, a harp-string gently drawn by the finger 

 is thrown by its elasticity into vibratory mo- 

 tions, returning when these have ceased to its 

 exact original state: this may be frequently 

 repeated and always with the same effect, as 

 proved by the same note being repeatedly ob- 

 tained. If, however, it be once drawn with too 

 great a force, it no longer returns to its original 

 condition, a different tone is now produced by it: 

 in other words, the solid substance of which 

 it is composed exhibits a perfect elasticity, 

 not, as the gases, under every degree of force, 

 but only within a certain limit. Heat pro- 

 duces very different effects upon the elasticity 

 of gaseous and solid bodies; we have just 

 seen that we can increase the elastic power of 

 the former to any extent by means of heat, 

 but the elasticity of solids is, on the contrary, 

 usually diminished by it ; very high tempera- 

 tures completely destroy it even in the most 

 elastic metals. The design of this article does 

 not permit us to enter more fully into the con- 

 sideration of those laws, or of the experiments 

 by which they are demonstrated. We must 

 refer for the further investigation of this sub- 

 ject to works which treat expressly upon 

 physics. 



The various hypotheses which have been 

 put forth to explain the nature of elasticity, 

 though many of them extremely ingenious, 

 do not however properly come within the pro- 

 vince of the physical much less of the phy- 

 siological enquirer. Indeed, while men di- 

 rected their attention to such speculations little 

 or no progress was made in real knowledge. 

 The cause of elasticity, like that of life, is 

 probably beyond the sphere of human un- 

 derstanding ; and hence, in both sciences, the 

 method of investigation should be the same 

 to study the laws or conditions under which 

 the phenomena present themselves, and to lay 

 aside all speculations as to their causes. But 

 in abandoning these inquiries into the nature 

 of elasticity we must particularly advert to the 

 necessity of the physiologist possessing a clear 

 and definite idea of this property of matter, 

 so as to be enabled to recognize it under every 

 circumstance, and to distinguish it from other 

 physical and vital forces. Ignorance upon this 

 point has been at all times a fruitful source of 

 error in physiological investigations. The pro- 

 perty with which it is especially liable to be 

 confounded is contractility : when it is re- 

 membered that at one period of medical his- 



