lo Mkldrum, DevelopJiient of the Atomic Theory. 



been offered. They have already been mentioned in the 

 second paper of this series. The first of these, coming 

 direct from Thomas Thomson, is that Dalton discovered 

 the composition of marsh gas and olefiant gas and was led 

 thereupon to perceive the law of multiple proportions, 

 and to devise his chemical theory as an explanation of 

 the law. This may be called the inductive account. 



Again, Roscoe and Harden accept an account, offered 

 by Dalton, which may be called the deductive one. Dalton 

 had formed his diffusion hypothesis without considering the 

 " effect of difference of size in the particles of elastic fluids." 

 On consideration he found that " the sizes must be 

 different," and thereupon he revised his diffusion theory. 

 He then introduces the subject of the chemical theory : — 

 " The different si::es of the particles of elastic fluids .... 

 being once established, it became an object to determine 

 the relative si.zes and weights, together with the relative 

 number of atoms in a given volume. This led the way to 

 the combination of gases," etc. 



Objections to the purely inductive and ded^ictive accounts. 



There being these two accounts, the inductive one 

 and the deductive, of the origin of the theory, there arises 

 the question, which comes nearer the truth? The Board 

 of Education has recently committed itself to an opinion 

 on this topic, in the course of its criticisms on the answers 

 of students to its questions on chemistry. The particular 

 question was : — " Give a short account of Dalton's atomic 

 theory, and discuss its value in explaining the laws of 

 chemical combination." 



Teachers of chemistry, to judge from the reference 

 made to them, have been adopting Roscoe and 



