144 On Elasticity. * 



themselves of them in explaining those phasnoniena into 

 which they enter. 



In our present inquiry, then, I mean to derive from at- 

 traction the help it offers in explaining the cause of elasti- 

 city; and I hope to make it evident, by a due considera- 

 tion of attraction as common to all matter, and of the laws 

 by which ca/o/7'c constantly endeavours to maintain an equi- 

 librium, not only among systems of bodies, but throughout 

 each individual mass, that the efforts of these two, to main- 

 tain their respective powers over matter, and, in doing so, 

 acting according to known and invariable laws, produce all 

 those phaenomeua to which the term elastic has been applied, 

 whether in solid or in aeriform substances. 



I would say, then, thac attraction, which pervades all mat- 

 ter, and caloric, which also pervades all matter, by their pre- 

 sence, and bv an action in which both participate, occasion 

 elasticity wherever it exists. Not that elasticity must follow 

 as a necessary consequence of their presence, for then every 

 substance in nature would be elastic, which many are not 

 in the common sense of the word. Certain other conditions 

 are necessary to elasticity ; but without these two it could 

 not exist. 



It is admitted by all that attraction is the cause of the 

 aggregation of the moleculie of bodies. When the state of 

 aap-rep-ation is such, that on the apnhcation of a 2;iven desrree 

 of mechanical force to the body the attraction of the mole- 

 culai is overcome (or the body broken), it is called brittle: 

 if the attraction is only partially deranged, the body will be 

 found to have changed its form, and is then called flexible : 

 when the body springs back, or, after certain vibrations, 

 recovers its form, it is called elastic. 



Many metallic bodies which are flexible in tlie sense just 

 mentioned, may however by hammering be rendered elastic; 

 that is, by merely bringing their moleculce into more inti- 

 mate union, or by bringing them reciprocally more within 

 the spheres of each other's attraction. This is a circum- 

 stance which ought not to be lost sight of. Let us for a 

 moment then inquire what takes place in the process, be- 

 sides bringing the particles of the body more nearly into 

 contact, or more of them into actual contact than were in 

 that state before ? When a bar of metal is hammered thin- 

 ner than it v.aj before, a quantity of caloric equal in volume 

 to the diminution of volume imposed upon the bar has been 

 expelled or driven out of it. I need not, however, insist 

 on its being exactly equal in volume, as my present argu- 

 ment only requires that a certain quantity of caloric should 



be 



