HISTORICAL INTRODUCTION. 25 



11 Iron . 3-5 



12 Zinc . 4 



13 Potassium . 5 



14 Barium 8-?5 



He observed that all these atomic weights are 

 multiples of the atomic weight of hydrogen : 

 indeed, that all of them are multiples of twice 

 hydrogen, or 0*25, and most of them of 4 hydro- 

 gen, or 0*5. He observed also, that in general 

 the specific gravity of the body in the gaseous 

 state, may be obtained by multiplying its atomic 

 weight by O5555, or half the specific gravity of 

 oxygen gas ; because the atom of oxygen is re- 

 presented by half a volume, but that of most 

 other substances by a whole volume. 



VIII. It was this admirable paper that 



,.,... tions of 



ned me that new analytical investigations were author, 

 still necessary to determine the atomic weights 

 of bodies with perfect accuracy ; and I formed 

 the project of attempting the investigation my- 

 self by direct experiment. This project it was 

 not in my power to commence till the summer 

 of 1819, because it was not till the end of the 

 preceding summer that the college of Glasgow, 

 to which I had recently been attached, could 

 furnish me with a laboratory. Since that period 

 I have devoted the whole of my spare time to the 

 investigation ; and have resolutely withheld my- 

 self from entering upon several rather inviting 

 fields, which have attracted general attention. My 



