54 ExlraB of a Memoir on Elajtici/j'. 



placement of the particles of the body, v.'hich is then faid to 

 be duftile. If it cannot withdraw itfelf, It yields or refifts. 

 When it yields, the body is loft; when it refifts compref- 

 (ion, it experiences the effects of it as long as the molecula; 

 are compreifed 3 it then tends to recover its former ftale ; and 

 this i* what renders bodies elaftlc. 



There is no body pcrfe6lly foft, ductile, or ekftic. Nature 

 prefents none which under compreffion does not fuffer a 

 portion of caloric to efcape. Thus a body is never perfe6Uy 

 elaftic, becaufe the quantity of caloric compreOed, being lefs 

 than the total quantity, cannot reftore itfelf with the fame 

 force as if this fluid had remained in its entire ftate, and 

 cannot keep the nioleculse of the body feparated at the fame 

 tlljknce as before compreffion. Bcfides, the velocity with 

 which it reftores itfelf is alfo lefs than that which produced 

 the compreffion, for a part of this velocity has been deftroyed 

 by the entire mafs of the comprefled bodv. 



A body is more flexible according as it contains more ca- 

 loric between its moleculse. This very eompreffible fluid 

 permits concave molecuhe to approach each other without 

 the convex moleculae being obliged to feparate from each 

 other, as if there were not^caloric interpofed between them. 



The preceding obfervationsmay ferve to throw fomc light 

 "dn various phenomena of elaflicity. A blade of copper not 

 hammered evidently remains in that ftate into which it is put 

 by bending, becaufe the moleculaa of the Concave part ex- 

 prefs, by approaching each other, that portion of caloric 

 which adheres leaft to them. The other portion, which 

 does not efcape, is indeed comprclTed ; but the excefs of the 

 fpring is compenfated by the excefs of the adherence of the 

 moleculfe brought together : the body remains in that fl;ate 

 in which it is placed. 



If ti^-e, blade has been hflmmcred, it lofes by that operation 

 a portion of opJoric; the other portion remains comprelTed ; 

 and, when this blade is bent, 'the compreffion of the fluid is 

 incrcafed. The exeefs of fpring which it acquires is not 

 counterbalanced by the excefs' of the adherence of the mo- 

 leculae : it tends to reftore itivlf ; and the body pafl[es to that 

 R^ite which is called elaJUc. 



Tlie 



