216 LECTURES. 



Elasticity. 



(111.) By this term we understand that property of bodies by which 

 they return to their original form and dimensions when an extraneous 

 force, to which they have been submitted, is withdrawn. 



The term ehisticity is also used to express the force with which any 

 body resists a change of density or of form. In tliis sense the elas- 

 ticity of water is greater than that of air. The ambiguity may be 

 avoided by employing the expression elastic force lor the latter. 



All bodies in mechanics are sometimes divided into two classes, 

 elastic and non-elastic, and sometimes into perfectly and imperfectly 

 elastic. Examples. 



But in realit}' all bodies are perfectly elastic within certain limits 

 which differ widely in different bodies. The late experiments pre- 

 sented to the British Association do not, I think, establish the con- 

 trary. 



(112.) Elasticity of gases. — The molecules of gases being entirely 

 within the region of repulsion, they tend constantly to separate from 

 each other, and are only confined within a given volume by the sides 

 of the vessel which contains them. The range of elasticity in these is 

 much greater than in liquids and solids. 



The laws of Boyle and Mariotte. 



1. The elastic force and density of a gas are cliredly as the pressure. 



2. The hidk of a gas is inversely as the iiressure. 



(113.) Experimental proof of these laws. Precautions to be ob- 

 served. The second has been found to hold true in the case of com- 

 mon air, to the extent of a pressure of twenty-four atmospheres. 



It is probable, however, that these laws are true for all gases only 

 within certain limits ; several gases have been condensed into liquids, 

 and analogy would lead us to infer that all of them might be reduced 

 to the same state if sufficient pressure could be applied. In those 

 which have been liquefied, the laws fail as the point of liquefaction 

 is approached. On the other hand, if the gases were sufficiently 

 expanded, we cannot doubt that the molecular repulsion' would finally 

 pass into the attraction of gravitation. These facts are in accordance 

 with the theory of Boscovich. 



Experiment to illustrate this. Several gases submitted at the same 

 time to the same intense pressure ; condensation finally becomes 

 unequal. 



To account for the laws of elasticity, we may suppose, with Newton, 

 that the force between the atoms is inversely as their distances ; but 

 if we adopt this hypothesis we are obliged to admit that the action of 

 each atom does not extend beyond the atoms nearest to it, however 

 greatly they may be crowded together. The explanation of Dr. Kobi- 

 son is more probable ; according to this, the repulsion remains the 

 same for a certain range of distance, and the law of elasticity is the 

 result of the greater number of atoms forced into the same space ; 

 the repulsion being in proportion to the number of tile repelling 

 centres. 



Illustration of this by a diagram of atoms, and also by the curve of 

 Boscovich. 



