The United States Department of Agriculture have devised certain methods for ascertaining the suitability of stamping ink used by the Government and suggest the qualities desirable in such an ink. The ink, first of all, must produce an indelible cancellation; that is, it must be relatively indelible as compared with the ink used for printing the postage stamps. The post-mark made with the ink must dry quickly in order that the mail matter may be handled immediately without any blurring or smearing of the post-mark. Both this property and the property of the indelibility involve the question of the rate at which the ink penetrates or is absorbed by the fibre of the paper. A satisfactory ink does not harden or form a crust on the ink-pad on exposure to air. There must be no deposition of solid matter on the bottom of the vessel in which the ink is stored, and the pigments on which the indelibility of the ink depends, if insoluble, must not settle out in such a way as to make it possible to pour off from the top of the container a portion of the ink which contains little or none of the insoluble pigment or pigments. _Colour._--If the subject of deterioration of paper is to be considered in its broadest sense as including changes of any kind, the fading of colour must be taken into account. The use of aniline dyes which are not fast to light results in a loss of colour in paper just as with textiles, and the fading may be regarded as a function of the dye and not as arising from its combination with the paper. The gradual fading of some dyes, however, and of many water-colour pigments may be traced to the presence of residual chemicals in the paper and to the presence of moisture in an atmosphere impregnated with gaseous or suspended impurities. In fact the latter is a greater enemy to permanence of colour than light, since it has been proved by experiment that most colours do not fade when exposed to light in a vacuum. The oxygen of the air in combination with the moisture present is the principal agent in bringing about such changes. The dulling of bronze, or imitation gold leaf, on cover papers is a practical illustration of this, though this can hardly be quoted as an instance of actual deterioration of the paper. The maintenance of the original colour can only be assured by the careful selection of pure fibrous material, the use of fast dyes, and the preservation of the book or painting from the conditions which favour the fading as described above. For common papers such precautions become impossible, but for water-colour drawings and valuable papers they are essential.