16 Britain's Heritage of Science 



of multiple proportions, but he did not base his results 

 so much on accurate measurements, as on the logical 

 coherence of the system he advocated. In its simplest 

 form, this principle means that if one atom of an element 

 can combine with one, two, or more atoms of another, the 

 weight of the compound molecules formed must increase 

 by equal steps. But in the " New System of Chemical 

 Philosophy " (first published in 1810), though examples 

 are given in illustration, no systematic attempt is made to 

 reach an accuracy sufficient to establish a proof. To Dalton 

 the principle was obvious, and he was mainly interested in 

 determining the relative atomic weights and showing, for 

 a number of simple substances, how many atoms of each 

 element are combined to form the compound molecule. 

 The most important portion of the work deals with sub- 

 stances in which one or all of the combined elements are 

 gaseous, and he depends a good deal on the measurement 

 of volumes before and after combination. As the methods 

 of drying and otherwise purifying gases were imperfectly 

 understood at the time, the figures which he obtained were, 

 according to our standard, very inaccurate; nevertheless, 

 the power and success with which he treated the subject 

 very soon convinced other chemists that the foundations 

 of his system were correct. 



Dalton's evidence was cumulative rather than indi- 

 vidually decisive, and it may be said that he convinced 

 the scientific world more by the strength of his own con- 

 victions than by the experimental proofs he supplied. 



The total number of elements known in Dalton's time 

 was twenty-three, but others were soon added, until, towards 

 the middle of last century, over 'sixty elementary sub- 

 stances were recognized. At present -we have reason to 

 believe that the number' is strictly limited. 1 Whatever 

 opposition there was to Dalton's views it. died. out quickly, 

 though some philosophers found much that was distasteful 

 in the immediate result of his teaching. There is, indeed, 

 at first sight, something repellent in the idea that there 

 should be one number, whether it be sixty-three or ninety- 

 two, raised in importance so far above all others that it 



1 See the result of Moseley's researches, page 185, 



