gelatine in which they are produced, most of pigments used, and all of the prettiest ones, being unable to withstand the bleaching action of the light for more than a few weeks.  Carbon pictures are therefore only permanent according to the degree in which the coloring matter employed is capable of resisting the decolorizing action of light.  But there is no pigment in an argentic print, nothing but the silver reduced by the developer after the action of light; and that has been shown by, I think, Captain Abney, to be of a very stable and not easily decomposed nature; while if the pictures are passed through a solution of alum after washing and fixing, the gelatine also is so acted upon as to be rendered in a great degree impervious to the action of damp, and the pictures are then somewhat similar to carbon pictures without carbon.

I may now say a few words on the defects and failures sometimes met with in working this process; and first in regard to the yellowing of the whites.  I hear frequent complaints of this want of purity in the whites, especially in vignetted enlargements, and I believe that this almost always arises from one or other of the two following causes: 

First.  An excess of the ferrous salt in the ferrous oxalate developer; and when this is the case, the yellow compound salt is more in suspension than solution, and in the course of development it is deposited upon, and at the same time formed in, the gelatinous film.

The proportions of saturated solution of oxalate to saturated solution of iron, to form the oxalate of iron developer, that has been recommended by the highest and almost only scientific authority on the subject—­Dr. Eder—­are from 4 to 6 parts of potassic oxalate to 1 part of ferrous sulphate.

Now while these proportions may be the best for the development of a negative, they are not, according to my experience, the best for gelatine bromide positive enlargements; I find, indeed, that potassic oxalate should not have more than one-eighth of the ferrous sulphate solution added to it, otherwise it will not hold in proper solution for any length of time the compound salt formed when the two are mixed.

The other cause is the fixing bath.  This, for opals and vignetted enlargements especially, should always be fresh and pretty strong, so that the picture will clear rapidly before any deposit has time to take place, as it will be observed that very shortly after even one iron developed print has been fixed in it a deposit of some kind begins to take place, so that although it may be used a number of times for fixing prints that are meant to be colored afterward it is best to take a small quantity of fresh hypo for every enlargement meant to be finished in black and white.  The proportions I use are 8 ounces to the pint of water.  Almost the only other complaints I now hear are traceable to over-exposure or lack of intelligent cleanliness in the handling of the paper.  The