Firstly, the fundamental laws for the permeation, diffusion and penetration of fluids into porous material are explained and the available information on the porosity of paper is discussed. Using the classification in which neutral fluids like air, oil and grease are distinguished from those that interact with the pore walls like water and water vapour, the permeability to air and liquids like oil and grease is treated in greater detail. In addition to the description of the process of penetration, which is of special interest for printing inks, the main part of the paper deals with the behaviour of paper in relation to water and water vapour. Although the effect of swelling on porous structure has not yet been investigated sufficiently, some definite statements can be made on water and water vapour permeability . Furthermore, it is known how the moisture and the water content affect many properties of the paper, especially the strength.
Results of recent studies have been incorporated into the information already available. These investigations dealt with the penetration of water vapour into paper and the associated humidity and temperature effects. The paper closes with an account of some little-known studies on the effect of water vapour on paper and a general survey of dimensional stability and the tendency to curl.