SOME CONSEQUENCES OF GRAHAM'S WORK 589 



can excite them electrically and make them glow in various 

 colours, some of them in more than one way ; the spectra 

 they afford are not only very beautiful and characteristic but 

 remarkably complex. 



The picture we form of a gas— mainly in consequence of 

 Graham's researches on gaseous diffusion — is that of a perfect 

 hail-storm of infinitely small shot flying with great velocity in 

 every possible direction, perpetually hitting up against one 

 another and against the walls of the containing vessel — such 

 impacts being the cause of the pressure the gas exerts on the 

 surfaces exposed to bombardment by its molecules, as the 

 flying particles are termed. 



The great generalisation known as Avogadro's theorem — 

 embodied in the axiomatic statement that under the same 

 conditions of temperature and pressure equal volumes of gases 

 contain equal numbers of molecules — enables us to assert that 

 the weights of the molecules of gases are as the weights of 

 equal volumes of the gases at the same temperature and 

 pressure, that is to say, as their relative densities. Thus as 

 oxygen gas is sixteen times as dense as hydrogen gas, molecules 

 of oxygen must be regarded as sixteen times as heavy as 

 molecules of hydrogen, whatever the actual dimensions of the 

 molecules may be. But the molecules even of elements in 

 perhaps most cases are to be regarded as chain shot — not as 

 whole shot or shell — that is to say, as divisible into atoms. 

 Thus a molecule of hydrogen and a molecule of oxygen is 

 each composed of two atoms : moreover, as we assign unit 

 weight to the hydrogen atom and take hydrogen as the unit 

 of relative gaseous density, the conclusion that the hydrogen 

 molecule consists of two hydrogen atoms, each of unit weight, 

 involves the adoption of the rule : 



Relative gaseous density x 2 = molecular weight. 



Obviously, if the relative weight of the atom be known, it 

 is only necessary to divide that of the molecule by that of the 

 atom, to obtain the number of atoms in the molecule — in other 

 words, the complexity of the chain shot. We cannot ascertain 

 the degree of " chain-shottedness " of an elementary molecule, 

 however, unless we know the atomic weight of the element — 

 and this can only be determined from the study of compounds 

 of the element, the atom being the combining unit. 



