The discontinuity of the hydration process

Abstract
At the Linen Industry Research Institute, Belfast, where the work to be described was undertaken, the study of the absorption of moisture by textile materials has naturally occupied attention. It is clear from an examination of the literature that, in the past, far greater attention has been paid to the development and application of technical methods of drying and of conditioning than to the study of the nature of the changes which are involved. Herzog has recorded certain physical changes in textile materials during hydration, but interest has been focussed more particularly upon the question of the regain of moisture when equilibrium is attained on exposing materials in a humid atmosphere. Orme Masson and Richards and Schloessing have studied the behaviour of cotton and other textile materials from this point of view, and owing, no doubt, to the importance precise information of this kind has in its commercial application, considerable attention has been given to questions of this kind. It has been thought desirable to examine carefully, not only the conditions which determine the equilibrium between the humidity of the atmosphere and that of the substance, but also and more particularly the rate of attainment of such equilibrium, this being a matter of special interest in the Linen Industry. Apart from the work of B. A. Keen§ on the evaporation of water from soil no attempt appears to have been made to follow accurately the rate at which hydration changes of this kind proceed. Our early observations, which were made last year, showed very clearly that a characteristic feature of both hydration and dehydration changes of cellulosic materials is a discontinuity of the process, and evidence in support of this view seemed so convincing that we were led to ascertain whether such discontinuity were in any way characteristic of these materials. From this point of view, we have more recently made a study of the hydration of salts, choosing calcium sulphate as a sparingly soluble salt, copper sulphate as a freely soluble salt, and isinglass as a typical colloid characterised by the readiness with which it absorbs water and the peculiar manner in which it swells. The process of hydration has been studied in both directions. In many cases observations with a single substance have extended over several, weeks, and when hydration was proceeding rapidly they were continued without interruption night and day, by working in shifts. It was found that the behaviour of substances generally, whether crystalline or amorphous, was similar in principle, and that hydration took place in a series of discontinuous steps, the change in rate occurring at frequent intervals. Moreover, action was found to occur along a series of simple curves, generally either parabolic or linear, which were of such a nature that the whole course of action, whether of hydration or dehydration, and covering either minutes, hours or days, could be accurately expressed by a series of simple equations in which time was reckoned from the original starting point.

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