GASEOUS ELEMENTS. 35 



particles of matter of different kinds, the result 

 of which is the formation of new particles 

 possessed of qualities different from those of their 

 components; as quicklime combines with sul- 

 phuric acid, to form gypsum or land plaster. 



This definition will show the difference be- 

 tween chemical attraction or affinity, and the 

 forces of gravitation and cohesion. Gravitation 

 is exerted at all distances between masses of 

 matter without regard to their nature, and differs 

 entirely from affinity. 



' Cohesion differs less widely from affinity, since 

 its attraction is exerted only at minute distances. 

 This force, however, is exerted more frequently, 

 and with greater energy between similar particles 

 of matter, than between particles of different 

 kinds ; and the operation of these forces is not 

 attended with any material alteration in the 

 properties of matter. 



We shall now proceed to the consideration of 

 the different elements found in plants, in the 

 order of their division. 



GASEOUS ELEMENTS. 



Oxygen was discovered by Priestly, in 1774. 

 It is permanent, colorless, tasteless, and odorless, 

 and is a supporter of combustion; any combustible 

 substance burns in oxygen with increased bril- 



