108 SENSATION AND ITS OBJECTS. 



beef from venison, madeira from claret; to 

 ascertain what are the constituent materials 

 which, flowing from different bodies, excite on 

 the olfactory sense, flavors so various and op- 

 posite; to ascertain the quality of bodies 

 through which impulses are propagated, and 

 which excite on the auditory nerves the sensa- 

 tion of sound, or why a bed of roses and a 

 bed of thorns, excite the sensations of pleasure 

 and of pain ; to ascertain the nature of the 

 atmosphere in general, as well as of the differ- 

 ent bodies which excite upon the nerves of 

 sense, the sensation of dryness and of moisture, 

 of heat and of cold ; to separate the matter 

 which excites the sensation of heat and of cold, 

 from such as excites the sensation of color ; 

 to prove what fire is, as well as ice. It is the 

 duty of the chemist to analyse the materials, 

 which, flowing from different bodies, excite 

 upon the eye the sensation of illumination in 

 general, and of color in particular ; not only to 

 segregate a beam of light into rays, but to ana- 

 lyse each ray into its constituent parts; to se- 

 parate the matter of color from the matter of 

 light; and finally, to present the solar ray pure 

 and unmixed, as it subsists in its elementary 

 and uncombined state. Until ready and most 

 satisfactory answ r ers can be given to these points, 

 I shall consider chemistry most defective and 



