The first is thought to possess perhaps more uniformity in its action than any other combination I have ever used.
The plate once coated should be kept excluded from the light by means of the plate holder for the camera box.
I will notice one of the principal causes having a tendency to prevent the perfect uniformity of chemical action, between the iodine and silver; hydrogen, or the moisture in the atmosphere, makes a very perceptible barrier.

This moisture may arise as the result of the cold, from a want of friction in the buffing of the plate, which, coming in contact with the warmer air, as a writer on this subject says: "It is well known that as often as bodies, when cold, are exposed to a warmer air, the humidity contained in them is condensed.

It is to this effect that we must attribute the difficulty experienced in operating in most cases." This is corroborated by the results experienced by our operators.

So it is seen that the plate should be of a temperature above that of the atmosphere.

Mr.Gurney submits his plates to a gentle heat from a spirit lamp just before exposing them to the vapor of iodine.