2 OF MOTION. SECT. I. 



to have caufed the proje&ion of the planets from the fun, and 

 to prevent their relapfe into one chaotic mafs. Thefe motions 

 are diftin^uifhed by their tendency to or from the centres of the 

 fun or planets. 



id. The chemical clafs of motions includes all the various ap- 

 pearances of chemiftry. Many of the fads, which belong to 

 thefe branches of fcience, are nicely afcertained, and elegantly 

 clafled ; but their laws have not yet been developed from fuch 

 fimple principles as thofe above mentioned ; though it is prob- 

 able, that they depend on the fpecific attractions belonging to 

 the particles of bodies, or to the difference of the quantity of at- 

 traction belonging to the fides and angles of thofe particles, to 

 which (hould perhaps be added the fpecific repulfions belonging 

 to the particles of bodies. When thefe repulfions prevail over 

 the attractions, they may caufe the diffufions of light and of 

 odours, the explofions of fome bodies, and the flower decompo- 

 fition of others, and occafion our ideas of fluidity ; when the 

 attractions prevail over the repulfions, they may caufe the drift- 

 er combinations and cohefions of matter, as in cryftallization or 

 cooling, and give rife to our ideas of folidity ; and when thefe 

 two caufes of motion are in a&ive equilibrium, they may pro- 

 duce the vibrations of the particles of bodies, and occafion our 

 ideas of found. The chemical motions are diftinguifned by their 

 being generally attended with an evident decompofition or new 

 combination of theaftive materials. 



3 d. The third clafs includes all the motions of the animal and 

 vegetable world ; as well thofe of the veflels, which circulate 

 their juices and of the mufcles, which perform their locomotion, 

 as thofe of the organs of fenfe, which conflitute their ideas. 



This laft clafs of motion is the fubjeft of the following pages ; 

 which, though confcious of their many imperfections, I hope 

 may give fome pleafure to the patient reader, and contribute 

 fomething to the knowledge and to the cure of difeafes. 



SECT. 



