It is not absolutely necessary to apply artificial heat to the mercury to develop the image, for fair proofs have been produced by placing a plate over the bath at the ordinary temperature of the atmosphere.

This plan, however, requires a long time and cannot be adopted in practice, even if it were advisable.

The time more usually required in developing the image over the mercurial vapors, is about two minutes, and the temperature is raised to a point necessary to produce the desired effect in that time.
This point varies as indicated by different scales, but for the ordinary scales it is not far from 90 deg.

cen.
The mercury bath is accompanied with a centigrade thermometer, by which the heat is regulated.

Those furnished by the manufacturers are not always correct, and it requires some experience to find the proper degree on the scale.
I would here remark that it is advisable, when placing the spirit lamp under the bath, to so arrange it that the position of applied heat should always be on the same point, viz., should the heat be directly under the bulb containing the thermometer it would raise the mercury in the tube to the point marked, and the temperature of that in the bath would be far below what it should be; hence it is (where time is followed for developing) that many failures occur.