PHYSICS OF NINETEENTH CENTURY 169 



best known in any case in one of the phases, but in 

 suitable conditions can exist in either of the three. 

 This advance in knowledge was followed by the study 

 of the laws of chemical combination, since gases, in 

 which the laws can be traced most simply, had been 

 now brought into a correct relation with other bodies. 



As the result of careful analysis, it was found that a 

 chemical compound was always made up of precisely 

 the same amount of the constituent parts, and this 

 fact of the fixity of composition played an essential 

 part in the scheme of the new chemistry. Water, 

 however obtained, always consists of hydrogen and 

 oxygen combined in the ratio of one to eight. Thus 

 the conception of combining weight was reached, the 

 combining weight of oxygen being eight, if that of 

 hydrogen be taken as unity. Once again, if two 

 elements combine in more than one way, to form more 

 than one compound, the proportion of the constituents 

 in one compound is simply related to the proportion 

 in the other : fourteen parts of nitrogen combine 

 with eight of oxygen in one compound, and with 

 sixteen parts in another, exactly double the first 

 proportion. 



John Dalton (1766-1844), a colour-blind Quaker 

 chemist, who was born in Westmoreland and worked 

 and died at Manchester, saw that these phenomena 

 pointed to the conclusion that matter was not in- 

 finitely divisible, and that combination took place 

 between discrete particles with definite weights char- 

 acteristic of each substance. Led by these con- 

 siderations, he revived the old atomic theory in modern 

 guise, the essence of the new form of it being that it 

 was possible to arrive at a knowledge of the relative 



