652 
MR. H. B. DIXON ON CONDITIONS OF CHEMICAL CHANGE 
Table XVII.—Length of column = 400 millims. 
Reference 
No. 
Pressure. 
„ • CO, 
Ratio HjU 
Temperature. 
millims. 
° C. 
51 
75 
1-54 
18-8 
52 
100 
1-40 
18-2 
53 
125 
1-29 
16-2 
54 
200 
1T9 
12-6 
55 
400 
1T9 
13-7 
56 
500 
149 
8-8 
For a mixture containing 19 per cent, of oxygen the critical pressure is about 
200 millims. Now since a mixture containing 19 per cent, of oxygen was found to be 
unaffected by changes in the length of column under 250 millims. pressure, it seemed 
likely that in other mixtures changes in the length of column would cease to affect the 
division of the oxygen as soon as the critical pressure was reached. The critical 
pressure for the mixture containing 12 per cent, of oxygen is 400 millims. The 
following experiments with some of this same mixture show that at this and higher 
pressures changes in the length of column have no effect. 
Table XVIII.—Pressure = 400 millims. 
Reference 
No. 
Length of 
Column. 
„ . COo 
Ratio 
Temperature. 
millims. 
0 c. 
57 
200 
•905 
13 
58 
200 
•88 
95 
48 
450 
•91 
15-3 
Table XIX.—Pressure = 1000 millims. 
Reference 
No. 
Length of 
Column. 
„ . CO, 
Ratio q 
Temperature. 
2 
millims. 
° c. 
59 
100 
•89 
10-4 
60 
100 
•92 
12*9 
50 
450 
•90 
15-7 
The critical pressure of a mixture may therefore be defined as the lowest pressure 
at which any further increase of pressure or any change in the length of the column of 
exploded gases ceases to affect the ratio of the products of combustion. The critical 
pressure is lowered as the quantity of oxygen in the mixture increases. 
These facts account for the agreement of Hokstmann’s first series of experiments 
