The under surface of the paper is not invariably darker than the top surface. With pigments of less specific gravity the reverse is found to be the case. This is probably to be explained by the fact that some of the colouring matter from the under side is drawn away from the paper by the suction boxes, and the pigment on the top side is not drawn away to any serious extent, because the layer of pulp below it acts as a filter and promotes a retention of colour on the top side. It is interesting to notice that this irregularity sometimes occurs with soluble dyes, as for example in the case of auramine. The decomposition of this dye when heated to the temperature of boiling water is well known, and the contact of a damp sheet of paper coloured by auramine with the surfaces of steam-heated cylinders at a high temperature brings about a partial decomposition of the dye on one side of the paper. Generally speaking, acid dyes are more sensitive to heat than basic dyes. The presence of china clay in a coloured paper is also an explanation of this irregular appearance of the two sides. China clay readily forms an insoluble lake with basic dyes, and when the suction boxes on the machine are worked with a high vacuum the paper is apt to be more deeply coloured one side than another. _The Machine Backwater._--Economy in the use of dyes to avoid a loss of the colouring matter in the "backwater," or waste water from the paper machine, is only obtained by careful attention to details of manufacture on the one hand and by a knowledge of the chemistry of dyeing on the other. The loss is partly avoided by regulating the amount of water used on the machine, so that very little actually goes to waste, and further reduced by ensuring as complete a precipitation of the soluble dye as possible. The _acid_ dyes generally do not give a colourless backwater, and all pulps require to be heavily sized when acid dyes are used. The _basic_ dyes are more readily precipitated than the acid dyes, particularly if a suitable mordant is used, even with heavily coloured papers. The addition of an acid dye to pulp first coloured with a basic dye is frequently resorted to as a means of more complete precipitation.