Mucilage is found in many plants, especially in the outer skin of seeds, also in many roots, barks, stems and leaves, but its solubility and consistency are very much varied. Generally the mucilage of plants is classed among the gums; but there is a large difference from them in this, that it will not give a perfect, diaphanous and homogeneous solution, but only swells in it to a thick, massy and viscous fluid, which is similar to starch paste. It is composed of small quantities of gum, starch, sugar and consistent plant-mucilage. If prepared from certain plants, for example the algae of the sea, it also contains small quantities of iodine. Starch swells in hot water, while with mucilage the same result is achieved by using cold water. Smaller quantities of mucilage are found in every plant, but larger in gum tragacanth, carrageen or Iceland moss, salep, the leaves of mallows, or of colts foot or tussilage, the roots of marsh mallow, seeds of plantago-psyllium, (flea-bane) linseed and quinces. Of this number but few are of interest to us, which I shall mention further on. The density and proper consistency of the size depends on the bodies, from which the viscous or glutinous substance is derived and also from the quantity of water, in which it is dissolved. The consistency of the size is also materially influenced by the temperature, the warmer, the less substantial and the more pliable; the colder, the more consistent and stiff it is. A good marbling size should be an entirely homogeneous mucilage, i. e., a solution, which should be free of any vestiges of seeds or strings undissolved. A less viscous size is more adapted to the production of comb-edges, while a more viscous size to the production of marbled or veined edges. The consistency of the mucilage of every kind of size will soon experience an essential change, which becomes apparent by the fact, that the size becomes less consistent, until finally an acid reacting fluid remains, which has no more value for marbling purposes. This chemical process is produced by the transformation of the sugar parts of the dissolved starch contained in the size into acid and lactic-acid. The more starch and sugar that is contained in the size, the sooner the acid will be generated, while pure plant mucilage containing but little sugar and starch will resist much longer disintegration. All plant-mucilages, which are soluble in cold water, i. e., swell, will keep much longer than those dissolved in boiling water, because the soluble parts remain inclosed in one cell which is only broken by the boiling heat or by the formation of acid. This is the reason why they only swell up in cold solution.