386 Dr. G. Johnstone Stoney on the 



Let, then, a litre of hydrogen and a litre of chlorine be 

 mixed and exploded, and let the resulting hydrochloric-acid 

 gas be brought back to the original temperature and pressure. 

 It is then found to measure two litres. Hence, if N be the 

 number of molecules in a litre of gas at that temperature and 

 pressure, we learn by this experiment that N molecules of 

 hydrogen and N molecules of chlorine produce 2N molecules 

 of hydrochloric acid. Hence, and since the molecules within 

 each gas are alike, each molecule of hydrochloric acid must 

 contain the quantity of hydrogen represented by a semi-mole- 

 cule of hydrogen gas and the quantity of chlorine represented 

 by a semi-molecule of chlorine gas. We are thus introduced 

 to the semi-molecule of each of these gases as a quantity 

 which enters into combination ; and as no other experiments 

 suggest a smaller quantity, the semi-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 sub- 

 stances. 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 HC1 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, 

 351 36*. 



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 of steam are formed of N molecules of hydrogen 



and -7T molecules of oxygen. Hence each molecule of steam 

 2 J6 



contains a whole molecule (or two atoms) of hydrogen and a 

 semi-molecule of oxygen. We thus arrive at the semi-mole- 

 cule of oxygen as a quantity that enters into combination; and 

 as all other experiments with oxygen concur, the semi-mole- 

 cule of oxygen is to be received as its atom, and H 2 is the 

 proper formula for what is both the gaseous molecule and 

 the atom of water. From the densities we may also deduce 

 that 16 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 consti- 

 tuents, we learn that the atom of nitrogen is the semi-mole- 

 cule, and that the mass of its atom is fourteen times that of 

 hydrogen. 



