The law of distribution in the case in which one of the phases possesses mechanical rigidity: Adsorption and occlusion

The term “occlusion” has been applied somewhat indiscriminately to denote the state in which a gas exists in a solid, by which it has been absorbed, or from which it is evolved on heating, and no doubt the term includes phenomena of a totally different character. Similarly, the term “adsorption” is commonly used to differentiate between simple cases of solution and the removal of substances from solutions by solids in contact with them. It is the object of this paper to point out similarities which exist between certain cases, which may be included in these two groups, and to make some suggestions as to the classification of the phenomena. In the first place we may dispose of certain instances in which solids evolve gases when they are heated, the change being irreversible. In 1898 I succeeded in showing that the evolution of such gases as hydrogen and carbon monoxide from minerals on heating did not require the assumption of their previous existence in the minerals at all; hence, there was no need to introduce the term occlusion to explain it. I proved that the gases were produced by the interaction of water vapour and carbon dioxide with such substances as ferrous oxide, the change being followed quantitatively. Later, I put forward an explanation of the evolution of helium from minerals, in which it cannot be supposed to be present in a state of chemical combination. Ramsay and Soddy had shown that helium was one of the products of radio-active change, and had suggested that its presence in the minerals was due to the decay of some radio-active substance which they had formerly contained. This explanation did not, however, account for the retention of the gas in the mineral, and the clue to the solution of this part of the problem was given by Jaquerod’s discovery that helium would pass through the walls of a quartz bulb at a comparatively low temperature, though this material is quite impermeable to it in the cold. It follows that the gas may be present in the mineral in a state of supersaturation, and may remain in that state for an indefinite time, if the temperature is moderately low. When, however, the mineral is heated, the gas can diffuse freely through it, and there is a tendency to establish equilibrium between the two phases.