The sole merit of this substance is the facility with which half-tone process blocks can be utilised for the purpose of picture production. Beyond this, nothing can be said. The paper is brittle, susceptible to the least suspicion of dampness, with a high polish which in artificial light produces fatigue of the reader's eye very quickly, heavy to handle, and liable to fall to pieces when bound up in book form. As the fibrous material is completely covered by mineral substances, it is frequently considered of secondary importance, with the result that the "value" of the paper is judged entirely by the surface coating, with little regard to the nature of the body paper. In such cases, with an inferior body paper, the pages of a book very quickly discolour, and the letterpress becomes blurred. ANALYSIS OF A TYPICAL ART PAPER. ---------+-----------+-----------+------------ | Per Cent. | | Volume -- | by | -- | Composition | Weight. | | per Cent. ---------+-----------+-----------+------------ Fibre | 77·5 | Fibre | 68·3 Ash, etc.| 22·5 | Ash | 12·0 | | Air space | 19·7 | ----- | | ----- | 100·0 | | 100·0 ---------+-----------+-----------+------------ _Rosin._--The presence of an excess of rosin is a well-known factor in the disintegration of the paper, even when the fibrous composition is of the highest order. The decomposition is largely due to the action of light, many experiments having been made by Herzberg and others to determine the nature of the reactions taking place. One of the chief alterations is the change brought about in the ink-resisting qualities of the paper. The actual character of the chemical reactions as far as the effect on the fibre is concerned is not accurately known. The degradation of a hard-sized rosin paper by exposure to strong sunlight, for example, is probably due to the alteration in the rosin size, and not to any material change in the cellulose. It is hardly conceivable that in a pure rag paper sized with rosin and yielding readily to ink penetration, after about one year's exposure to light, the cellulose itself had undergone any chemical changes capable of detection. _Gelatine._--Papers properly sized with gelatine are preferable to those sized with rosin for the majority of books and documents preserved under normal circumstances. But the nature of a tub-sized paper may be, and often is, greatly altered by unusual climatic conditions. In hot, damp countries papers are quickly ruined, and high-class drawing papers sized with gelatine often rendered useless. The change is scarcely visible on the clean paper, and is only observed when the paper is used for water-colour work, the colour appearing blotchy in various parts of the sheet where the gelatine has been decomposed by the united action of heat and damp.