The demand for an abnormally white paper is indirectly the cause of deterioration in colour, but in this case the ultimate effect is not a fading but a discoloration of white to a more or less distinct yellow or brown colour, due to changes in the fibre which may often be traced to excessive bleaching. In this case the fading of colour is directly due to deterioration of the paper itself, and may occur in celluloses of the best type. With lower-grade papers containing mechanical wood pulp the degradation of colour and fibre is inevitable. _Air and Moisture._--The exact effects produced on paper freely exposed, or in books as ordinarily stored, depend upon the condition of the atmosphere. Pure air has little or no action upon paper, cellulose being a remarkably inert substance, and even in impure mechanical wood pulp, if merely exposed to pure dry air, the signs of decay would be delayed considerably. The combined action of air and moisture is of a more vigorous character in promoting oxidation changes in the fibres, or a dissociation of the sizing and other chemical ingredients of the paper. The presence of moisture is, indeed, absolutely essential for the reaction of some substances upon one another, and it is easy to show that certain chemical compounds can be left in ultimate contact, if absolutely dry, for a lengthened period without reacting, but the addition of a little moisture at once produces chemical union. This may be shown by a simple experiment. Thus a piece of coloured paper which may be bleached immediately if suspended in an atmosphere of ordinary chlorine gas will remain unbleached for several hours if first thoroughly dried in an oven and exposed to dry gas. In the case of books and papers, these conditions which promote slow disintegration are aggravated by the presence of impurities in the air, such as the vapours of burning gas, the traces of acidity in the atmosphere of large manufacturing towns, the excessive dampness and perhaps heat of a climate favouring the growth of organisms. All these factors are of varying degrees in different places, so that the deterioration of papers does not proceed in the same measure and at the same rate everywhere. _Moisture._--It may not be out of place to discuss some important relations between moisture and the physical qualities of a sheet of paper. A paper in its normal condition always contains a certain proportion of water as one of its ingredients, and the presence of this moisture has much to do with the strength, elasticity, and use of the paper, the absence of moisture giving rise to defects and troubles in the use of the paper which to a certain extent lower its commercial value and deteriorate it, though not perhaps in the sense of permanent degradation of quality.