102 DEFINITE PROPORTIONS. SECT. XIV. 



ticity of the air. The definite proportions of chemical 

 compounds afford one of the best proofs that divisibility 

 of matter has a limit. The cohesive force which has 

 been the subject of the preceding considerations, only 

 unites particles of the same kind of matter ; whereas 

 affinity, which is the cause of chemical compounds, is 

 the mutual attraction between particles of different 

 kinds of matter, and is merely a result of the electrical 

 state of the particles, chemical affinity and electricity 

 being only forms of the same powers. 



It is a permanent and universal law in all unorganized 

 bodies hitherto analyzed, that the composition of sub- 

 stances is definite and invariable, the same compound 

 always consisting of the same elements united together 

 in the same proportions. Two substances may indeed 

 be mixed ; but they will not combine to form a third 

 substance different from both, unless their component 

 particles unite in definite proportions, that is to say, one 

 part by weight of one of the substances will unite with 

 one part by weight of the other, or with two parts, or 

 three, or four, &c., so as to form a new substance ; but 

 in any other proportions they will only be mechanically 

 mixed. For example, one part by weight of hydrogen 

 gas will combine with eight parts by weight of oxygen 

 gas and form water ; or it will unite with sixteen parts 

 by weight of oxygen, and form a substance called 

 deutoxide of hydrogen ; but added to any other weight 

 of oxygen, it will produce one or both of these com- 

 pounds mingled with the portion of oxygen or hydrogen 

 in excess. The law of definite proportion established 

 by Dr. Dalton, on the principle that eveiy compound 

 body consists of a combination of the atoms of its con- 

 stituent parts, is of universal application, and is in fact 

 one of the most important discoveries in physical science, 

 furnishing information previously unhoped for with re- 

 gard to the most secret and minute operations of nature, 

 in disclosing the relative weights of the ultimate atoms 

 of matter. Thus an atom of oxygen uniting with an 

 atom of hydrogen forms the compound water ; but as 

 every drop of water, however small, consists of eight 

 parts by weight of oxygen and one part by weight of 

 hydrogen, it follows that an atom of oxygen is eight 



