PHYSICS OF NINETEENTH CENTURY 171 



But, to return to the case of water, two volumes 

 (and therefore molecules) of hydrogen combine with 

 one of oxygen to form two volumes (or molecules) of 

 steam. It will be seen that the simplest theory which 

 will explain these relations is one which supposes that 

 the physical molecules of hydrogen and oxygen 

 each contain two chemical atoms, and that the 

 molecule of water vapour has the chemical composi- 

 tion represented by H 2 O. Thus Dalton's combining 

 weights need to be brought into line with the facts 

 revealed by the later experiments before we are in 

 a position to assign to the elements their true atomic 

 weights. 



Dalton's theory, when developed by the addition 

 of Avogadro's hypothesis, arose naturally out of de- 

 finite experimental knowledge. Other phenomena 

 were waiting to be interpreted by its light, and still 

 more were discovered and co-ordinated as time went 

 on. Except for a temporary tendency in the last 

 few years of the nineteenth century to replace the 

 theory of chemical atoms by relations based on the 

 theory of energy, it may be said that atomic and 

 molecular conceptions have inspired most of the 

 chemical and physical research which has distinguished 

 the past century. 



One result of Dalton's work was to give a working 

 definition of an element, as a substance possessing 

 a distinct and unchanging atomic weight from 

 whatever source it was obtained. Hence, since it 

 was now possible, it became of high importance 

 to extend the knowledge of the substances which 

 were in truth elements, incapable of further analysis 

 or subdivision. 



