Yet it has been proved that its presence in a small quantity upon the silver surface has the effect of reducing the time of exposure in the camera from two-thirds to three-fourths.

An application may be made as follows: Pour sweet oil, or rub beef or mutton fat, on a common buff, which is free from all polishing powders.

With this, buff a well-cleaned plate, and it will leave a scum, which should be mostly removed by using another buff, which should be clean.

Coat the plate in the usual manner, and the result will be a great reduction in the time of exposure in the camera.

The impression produced upon a plate so prepared presents, when coming from the vapor of mercury, a grey, scummy appearance, which, on the application of heat in gilding, does not improve; hence its use is not generally adopted.
We have instituted some investigations upon this subject, and in the present volume, we shall not refer to it further.