Adsorbent products possess an extremely porous structure. Their physical properties allow them to capture and to store water and humidity in their microscopic orifice, which are laid out in layers, pores and canals. Adsorbents are insoluble in water. Following the example of clays and zeolites, active carbon is an excellent adsorbent: its ability to absorb gas and organic molecules is remarkable, from which springs its use in protective masks, as a universal antidote or in medicines to counter dyspepsia. Adsorption, not to be confused with absorption, is a process in which gas or liquid molecules attach themselves to the solid surfaces of adsorbents in various ways. Adsorption consists of the fixing of hydrogen onto the surface of the internal cavities of a porous substance, or its being locked into carbon nanotubes. It should also be noted that adsorption constitutes the first stage of reactions which require the use of solid catalysts.