Much of the hyposulphite of soda in market contains a sulphuret, which, coming in contact with the silver surface, immediately causes oxidation.

Such spots, as well also as most all others found on the plate after it has been exposed in the camera, can be removed by the following, solution: To one ounce of water add a piece of cyanide of potassium the size of a pea; filter the solution and apply by pouring it on the surface of the plate.

In all cases the plate should first be wet with water.

Apply a gentle heat, and soon the spots disappear, leaving the impression clear and free from all organic matter.
In the absence of cyanide of potassium, a solution of pure hyposulphite of soda will answer as a fair substitute.
To Redeem, a Solarized Impression .-- The Daguerreotype plate, prepared in the ordinary manner, should be exposed in the camera a sufficient time to solarize the impression.

Then, before it be exposed to the vapor of mercury, expose it for a very brief period to the vapor of either chlorine, bromine or iodine.