666 
MR. H. B. DIXON OX CONDITIONS OF CHEMICAL CHANGE 
as soon as condensation is possible. In the fourth series the maximum tension of 
aqueous vapour at 70° (233 millims.) is just greater than the tension of steam formed 
(219 millims.). Perhaps incipient condensation accounts for the very slight rise in 
the coefficient observed. 
Now by varying the initial pressure the point of possible condensation can be 
altered without varying the temperature of the vessel. For instance, by reducing 
the initial pressure from 1000 millims. to 500 millims. in the two first series, the 
tension of the steam formed in the unimpeded reaction would have been reduced to 
8L millims. and 76 millims. respectively; no condensation could therefore have taken 
place under these conditions until the temperature had been reduced below 50°, and 
so on for other initial pressures. By changing the initial pressure the truth of the 
hypothesis can therefore be tested ; if the condensation of steam interferes with the 
reaction in the manner indicated, the increase of the coefficient will always occur 
when the tension of steam formed is equal to the maximum tension of aqueous 
vapour at the temperature of the eudiometer. Some of the mixture containing 73'4 
parts of carbonic oxide and 26‘6 parts of hydrogen in 100, was exploded under 400 
millims. pressure and at different temperatures with the following results. 
Table XXXIY.—Pressure = 400 millims. 
Reference 
Mo. 
Before the explosion. 
After the explosion. 
Temp. 
a. 
Oxygen. 
Carbonic 
oxide. 
Hydrogen. 
Carbonic 
oxide. 
Carbonic 
acid. 
j Hydrogen. 
Ste mi. 
° c . 
108 
17'6 
73 4 
26-6 
54-0 
19-6 
10-7 
16-0 
40 
44 
109 
55 
59 
53'5 
19-9 
112 
15-3 
50 
3*7 
110 
5^ 
55 
55 
535 
19-9 
11-2 
15 - 5 
100 
37 
In these experiments the tension of steam formed by the unimpeded reaction is only 
62 millims. The maximum tension of aqueous vapour at 50° is 92 millims. and at 40° 
it is 55 millims. If condensation causes the increase in the coefficient, this increase 
should not take place until the temperature is reduced below 50°. The coefficient is 
found to be the same at 50° as at 100°, whereas at 40° a distinct increase is observed. 
When this same mixture was exploded under 1000 millims. pressure the steam pro¬ 
duced had a tension of 153 millims., and at 60° where the maximum tension of aqueous 
vapour is 149, the coefficient began to rise. These experiments also show that the 
coefficient is the same whether the gases are exploded under 400 millims. or 1000 
millims., provided that no condensation takes place. 
Again, the same mixture was exploded under 100 millims. pressure at varying 
temperatures. 
