6 MeldrUM, Development of tJic Atomic TJicory. 



Dalton practically ignores the hypothesis. 



The really important question is, the sense in which 

 Dalton held this hypothesis. Did he perceive and con- 

 sider all its consequences, immediate and remote, and did 

 he, in any \va\', act upon his belief in it ? That he did 

 none of these things is the plain meaning of the passage 

 in which he speaks of his holding the hypothesis as a 

 "confused idea." 



There are four different ways in which Dalton might 

 have applied the hypothesis, or drawn deductions from it : 



1. The hypothesis is to the effect that particles of 

 nitrogen and ox\-gen are of the same size. Dalton's first 

 explanation of diffusion was that particles of oxygen 

 neither attract nor repel those of nitrogen. Between 

 these two opinions there is no necessary connection. He 

 did not hold the diffusion theory as a logical consequence 

 of the hypothesis, and he did not even specify the hypo- 

 thesis in his explanation of the theory. 



2. Dalton did not use the hypothesis as a means of 

 arriving at atomic weights and formula;. He used for 

 that purpose the i : i rule, which led him to the formula 

 OH for water, whilst the hypothesis must have led to the 

 formula H.^O. Thomson, in his first sketch of the theory, 

 says expressly that the i : i rule was " the hypothesis on 

 which the whole of Mr. Dalton's notions ... is founded."^ 



3. What is known as Gay-Lussac's law, regarding 

 the combining volumes of gases, is a necessary con- 

 sequence of the hypothesis. This everyone must admit. 

 Yet Dalton did not at once deduce the law from the 

 hypothesis, and when at length he did so, and endeavoured 

 to test it experimentally, he regarded his results as dis- 



'' " S}'stem of Chemistry,'' 31x1 edition, 1S07, vol. 3, p. 424. 



