Dalton's Atomic Theory. 353 



(1) When only one combination of two bodies, A and B, 



can be obtained, it must be supposed to be a binary 

 one, a compound of one atom of A with one atom of 

 B, unless some cause appear to the contrary. 



(2) When two combinations are known, they must be pre- 



sumed to be a binary (A + B) and a ternary (A 2 + B) 

 or (A + B 3 ). 



(3) When three combinations are obtained, we may expect 



one to be a binary and the other two ternary. 



(4) When four combinations are observed, we should ex- 



pect one binary, two ternary, and one quaternary 

 (A 3 + B) or(A + B 3 ). 



(5) A binary compound should always be specifically 



heavier than the mere mixture of its two ingredients. 



(6) A ternary compound should be specifically heavier than 



the mixture of a binary and a simple which would, 

 if combined, constitute it; &c. 



(7) The above rules and observations equally apply when 



two bodies, such as (C + D) and (D + E) are com- 

 bined (N.S. 214). 



The question is : How did Dalton arrive at these principles ? 

 Boscoe and Harden think that the answer to this question is 

 contained in some notes which Dalton had written down for 

 a lecture delivered in London on Jan. 27th, 1810 (R. 13). 



The pertinent passages of the notes are as follows : — 



" As the ensuing lectures on the subject of chemical ele- 

 ments and their combinations will perhaps be thought by 

 many to possess a good deal of novelty as well as importance, 

 it may be proper to give a brief historical sketch of the train 

 of thought and experience which led me to the conclusions 

 about to be detailed. 



" Having been long accustomed to make meteorological 

 observations, and to speculate upon the nature and constitu- 

 tion of the atmosphere, it often struck me with wonder how a 

 compound atmosphere, or a mixture of two or more elastic 

 fluids, should constitute apparently a homogeneous mass, or 

 one in all mechanical relations agreeing with a simple atmo- 

 sphere." 



In explanation of the foregoing remarks it may be men- 

 tioned that the molecules of all gases, therefore the atoms of 

 the constituents of the atmosphere, oxygen and nitrogen, were 

 at the time assumed by Dalton to be of equal volume : in 

 other words, equal volumes of oxygen and nitrogen gas would, 

 under normal conditions, contain an equal number of mole- 

 cules (N.S. 188). Hence the specific gravity of an atom of 



Phil. Mag. 8. 5. Vol. 42. No. 257. Oct. 1896. 2 



