5$ SENSORIAL ACTIONS. SECT. XT. *J 



Many of thefe external properties of bodies, which 

 ftimulate our organs of fenfe, do not feem to effect 

 this by a fingle impulfe, but by repeated impulfes ; 

 as the nerve of the ear is probably not excitable by 

 a fingle vibration of air, nor the optic nerve by a 

 lingle particle of light ; which circumftance pro- 

 duces fome analogy between thofe two fenfes, at 

 the fame time the fblidity of bodies is perceived by 

 a fingle application of a folid body to the nerves of 

 touch, and that even through the-cuticle; and we 

 are probably poffeiTed of a peculiar fenfe to diftin- 

 guifli the nice degrees of heat and cold. 



The fenfes of touch and of hearing acquaint us 

 with the mechanical impact and vibration of bodies, 

 thofe of fmell and tafle feem to acquaint us with 

 fome of their chemical properties, while the fenfe of 

 vifion and of heat acquaint us with the exigence of 

 their peculiar fluids. 



Senfation and Volition. 



II. Many motions are produced by pleafure or 

 pain, and that even in contradiction to the power 

 of volition, as in laughing, or in the ftrangury ; 

 but as no name has been given to pleafure or pain, 

 at the time it is exerted fo as to caufe fibrous mo- 

 tions, we have ufed the term fenfation for this pur- 

 pofe ; and mean it to bear the fame analogy to plea- 

 lure and pain,vthat the word volition does to defire 

 and averfion. 



i. It was mentioned in the fifth Section, that, 

 what we have termed fenfation is a motion of the 

 central parts, or of the whole fenforium, beginning 

 at fome extremities of it. This appears firit, be- 

 caufe our pains and pleafures are always caufed by 

 our ideas or mufcular motions, which are the mo- 

 tions of the extremities of the fenforium. And, 

 fccondly, becaufe the fenfation of pleafure or pain 



frequently 



