Rapidity of printing is governed first by the density of deposit on the plate; secondly, by the colour of the deposit. This difference in the printing rapidity of negatives exercises a great influence on the tone or contrast of a finished print. A yellow negative gives a much harder result, whilst it is impossible to get a rich purple tone from a thin bluish negative. A bluish negative, or freedom from stain, should be aimed at. Yellow stain is due to the oxidation of the pyro, and may be removed immediately after fixing by placing for a few minutes in a 5-per-cent. solution of hydrochloric acid; afterwards, as this has a tendency to cause frilling, passing through the alum bath. This treatment is, however, useless after the negative has once dried. In this case thiocarbamid may be tried, or the negative may be intensified with mercury and soda sulphite. The negative will, however, with the latter treatment take almost as long as before. If plenty of soda sulphite ia used in the developer (that is, mixed with the pyro and not with the soda, as often recommended),there should be no fear of yellow stain.