In many of the vapours of volatile liquids, the preceding law only prevails to a certain amount of pressure differing in each case, beyond which increase of tension produces diminished effects. In sulphuric ether the change begins at the eleventh term.
In bisulphide of carbon the law changes after the sixth measure, &c.
In order to adapt the apparatus for experiments on coloured gases, a glass experimental tube 2 ft. 9 in. long, and 2 ft. 4 in. in diameter, was substituted for the brass tube, and, instead of boiling water, sources of radiant heat having a constant temperature of 270° Cent. were adopted.
The following table shows the absorption of a number of gases at a common pressure or tension of one atmosphere.
| Dry air | = 1 | |
| Oxygen | 1 | |
| Nitrogen | 1 | |
| Hydrogen | 1 | |
| Chlorine | 39 | |
| Hydrochloric acid | 62 | |
| Carbonic oxide | 90 | |
| Carbonic acid | 90 | |
| Nitrous oxide | 35·5 | |
| Sulphuretted hydrogen | 390 | |
| Marsh gas | 403 | |
| Sulphurous acid | 710 | |
| Olefiant gas | 970 | |
| Ammonia | 1195 |
The absorptive power of ammonia is so great, that although as transparent in the glass tube as if it had been a vacuum, a length of three feet of it would be perfectly impervious or black to the heat here employed, yet even this does not express the energy which it exhibits under one inch of pressure.
When the relative absorptive actions of gases and vapours is compared, it must be under the same amount of pressure. Hence, for one inch of tension, the absorptive action of
| Dry air | = 1 |
| Oxygen | 1 |
| Nitrogen | 1 |
| Hydrogen | 1 |
| Chlorine | 60 |
| Bromine | 160 |
| Hydrochloric acid | 1005 |
| Carbonic oxide | 750 |
| Nitric oxide | 1590 |
| Nitrous oxide | 1860 |
| Sulphide of hydrogen | 2100 |
| Ammonia | 7260 |
| Olefiant gas | 7950 |
| Sulphurous acid | 8800 |
Thus, for a tension of an inch of mercury, the absorption of ammonia exceeds that of air more than 7000 times; the action of olefiant gas is 7950 times, and that of sulphurous acid 8800 times, greater than the absorption of air.
The effect produced by 1⁄30th of an inch of tension of air and the elementary gases is equivalent to that produced by one inch in the others, so the unit representing the absorption of these four gases is only the 1⁄30th part of the unit in the preceding table.