56 Scientific Proceedings, Royal Dublin Society. 



molecule of hydrogen, and the semi-molecule of chlorine, are, in 

 the present state of science, to be accepted as the chemical atoms 

 of these substances. Hence we may write — 



H, the atom of hydrogen = the semi-gaseous molecule of 



hydrogen, and 

 CI, the atom of chlorine = the semi-gaseous molecule of 

 chlorine, 

 and we see that HCl is the proper formula for hydrochloric 

 acid. We may further deduce from the observed densities of the 

 gases that the masses of the atoms of hydrogen, chlorine, and 

 Hydrochloric acid are to one another in the ratio of 1, 35i, 36i. 



Another experiment shows us that a litre of steam may be 

 resolved into a litre of hydrogen and half a litre of oxygen at the 

 same temperature and pressure ; in other words, that N molecules 



N 

 of steam are formed of N molecules of hydrogen and ^ molecules 



of oxygen. Hence each molecule of steam contains a whole 

 molecule (or two atoms), of hydrogen, and a semi-molecule of 

 oxygen. We thus arrive at the semi-molecule of oxygen as a 

 quantity that enters into combination, and as all other experiments 

 with oxygen concur, the semi-molecule of oxygen is to be received 

 as its atom, and HgO is the proper formula for what is both the 

 gaseous molecule and the atom of water. From the densities we 

 may also deduce that sixteen is the atomic weight of oxygen, 

 i.e. that an atom of oxygen is sixteen times as heavy as an atom 

 of hydrogen. 



Similarly from the densities of ammonia and of its constituents 

 we learn that the atom of nitrogen is the semi-molecule and that 

 the mass of its atom is 14 times that of hydrogen. 



It must not be assumed that the atom is always the semi- 

 molecule. In some cases it is found to be the entire molecule, 

 and in other cases the quarter molecule. Thus the mercuric 

 compounds of mercury give vapours of the same bulk as the 

 vapour of the mercury they contain, and indicate an atom of 

 mercury equal to its molecule, while the other volatile compounds 

 of mercury contain more than one molecule of mercury in each 

 molecule of the compound, and therefore do not disturb this 

 conclusion. Again, a litre of phosphuretted hydrogen yields a 

 quarter of a litre of the vapour of phosphorus and one and a-half 



