The vapour pressure in equilibrium with substances holding varying amounts of moisture

The knowledge of the quantity of water held under varying circumstances by substances of an absorbent character, such as cotton or woollen material, in an atmosphere of any given humidity, is not only of importance in hygrometry, but is also of general interest in connection with the processes used in drying such materials. No investigations, however, of this subject seem up to the present to have been ever published. Some years ago, while making a series of comparative determinations of the weight of moisture absorbed out of the atmosphere by different kinds of fabrics, two interesting points were noticed. The first was that the weight of water absorbed or held by a given material under different conditions of moisture and temperature of the atmosphere, appeared to depend only on the hygrometric state (i. e. , the ratio of actual vapour pressure to the maximum possible), though of course the actual amount of moisture present in the atmosphere for the same ratio is very different at different temperatures. The second point noticed was that as the atmosphere varied from saturation, the temperature remaining the same, the amount of water held followed some law giving much greater reduction in weight for a given change in vapour pressure when near saturation than is subsequently obtained. As described below, this law ultimately proved to be a simple parabolic one, at least until approaching desiccation.