NOTES ON THE THEORY OF DISSOCIATION OF GASES. 215 
practice, the co-efficient of expansion of steam is to be measured 
at these "usin and the excess fatto the normal value of 
t 
compared with the amount of Sieuiitigctiess at each temperature. 
the two variables are plotted together, the curve would be a 
straight line if the hypothesis of dissociation is correct, and this 
would moreover require to pass through the origin of co-ordina 
To settle the question of the origin, a sepa arate ig tates: would 
ave to made at some selected temperature. Wes have 
to compare the — from dissociated steam, with the diffusion 
oxygen or hydrogen under similar conditions. We 
therefore, require ee separate experiments to place the theory 
0: for a gas, which 
however, where the supposed change is simply molecular, we are 
unable to apply any analytical process, and so far the theory must 
for them remain a simple hypothesis. The oe given for- 
mule for nitrogen tetroxide and acetic acid are N, Pn: A 
respectively ; but the vapour densities of these panes do 
conform to this statement, for the former at high, and for the latter 
at low temperatures. This is explained by supposing a Ang cia- 
tion takes place. According to the view of Clausius and 
Williamson, and of chemists generally—and those liaaniink with 
the vortex atom theory in particular—a g gas is not a simple thing 
consisting entirely of molecules of one sort. Vapour density 
processes g give usthe mean molecular weights, that is they will 
re as ma 
more simple on the other, than the average molecules. Clifford, 
ong ago, pointed out that our diffusion experiments only point 
to a similarity between molecule and molecule within certain limits 
and we may add that caak experiments would not tell us whether 
a gas was dissociated or not a the products of dissociation 
to be chemically indistinguisha Now although in the case of 
steam and ammonium chloride pean we have a proof based 
measured the oo of heat absorbed per degree of temperature 
at different temperatures for acetic acid vapour, and nitrogen 
