On Elasticity. 13g 



subtilis, hindered in its attempt to pass, makes an effort 

 to restore the body to its first state. In this theory there is 

 something Hke the effort of a strong mind when first seizin^;- 

 upon a new truth, and endeavouring to reduce it to a de- 

 pendence on and connection with known or admitted facts; 

 but the defects in the reasoning ought to have convinced 

 its supporters, that a more satisfactory way of accountinor 

 for the phaenomenon was still a desideratum. It must be 

 obvious to any person who will take the trouble to ana- 

 lyse the argument, that if the materia subtilis in a spherical 

 cavity could by pressure (being hindered at the same time 

 from passing off) accommodate itself to the same cavity 

 rendered elliptical, that in doing so it has in fact become 

 smaller in volume than it u'as before. The materia subtilis 

 is thus assumed to be compressible ; and, as it makes an 

 effort to restore the body, or, in other words, its cavities, to 

 the first form, it is assumed to be expansive. But what 

 does this amount to ? Merely that a body is elastic because 

 it contains elastic matter. 



Other philosophers, disliking the materia subtilis of the 

 Cartesians, have adopted an ethereal medium. Their mode 

 c'f reasoning, however, is so similar to that of the former, 

 that they leave the mind as unsatisfied as before. Indeed, 

 the mere change of a name can throw no new light upon 

 the subject. 



Some account for elasticity by supposina; that when an 

 elastic body is bent or compressed a number of little vacui- 

 ties are formed in it, and that on removing the force the 

 pressure of the atmosphere, endeavouring to destroy the 

 vacuities thus formed, restores the bodv to its first figure. 

 This doctrine, however plausible, is inadmissible, if for no 

 other, for this one reason : — Many bodies require a greater 

 force to bend them to any given degree than can be found 

 by multiplying the number of square inches in their surface 

 by 14 pounds, the force exerted by the atmosphere on a 

 square inch ; besides, the phtenomcna of elasticity manifest 

 themselves in vacuo. 



Others assume that all bodies contain air in their vacui- 

 ties, and ascribe their elastic property to that of the air in- 

 closed in them. This is little more than a substitution of 

 air for the materia subtilis, and the t'/Acr assumed by others. 

 But whence has air itself the property of elasticity ? This 

 is a part of the general inquiry, and as necessary to be 

 solved as the source whence other bodies derive the same 

 property. 



Others account for elasticity from the lav.- of attraction, 



applied 



