ALBUMINATE of silver yields an image with well-marked contrasts, rather hard even: the chloride, on the contrary, a picture which may be pronounced as too uniform, or flat. Therefore, in varying the proportions of these two elements, by employing simply albumen more or less salted, or a paper more or less porous, the operator can obtain at will any effect be may desire. But this will not be sufficient: the positive proof must also present another quality; it must be in good condition for toning. Now, every photographer has remarked, 1st, that a proof that always remains red or brown, and becomes of a red metallic colour, never tones well; 2nd, a proof tones well when on removal from the pressure-frame, the blacks are of a green bronze metallic colour; the toning being much more rapid and beautiful as the metallisation is more advanced and greener in hue.

This is but a consequence of a fact, which I shall now explain, i.e. the toning consists of a precipitation of gold from the bath by the metallic silver which forms the image, a precipitation similar to that which takes place in the same bath when the image, instead of being formed of molecules of silver, is formed of molecules of copper or of zinc; only the silver, which belongs to the same section as the gold, can only precipitate the latter metal slowly and with great difficulty. This established, we may easily comprehend that the more molecules of metallic silver there are upon a given point of the positive, the more prompt and abundant the gilding (toning) will be. To obtain a good toning consists, therefore, of accumulating as much metallic silver as possible in the reductions which form the image. And this is the part the nitrate of silver plays.

A sheet of paper imbued with nitrate of silver only, will give an image; but it requires two whole days exposure in the sun; while, in the presence of chloride and albuminate of silver, the reduction of the nitrate of silver proceeds more quickly, and shows itself in a bronze-green metallization on the edges of the taper which project beyond the negative The proportion of, nitrate must not, however, be unreasonably increased in the positive baths. Too large a quantity of this salt remaining on the proof either maintains the paper in a constant state of moisture, the image being then of, a uniform blue or grey tone, or in the crystallizing, it will break up the surface and produce a multitude of holes, and destroy the negative also. If the quantity of nitrate cannot he conveniently reduced below 20 per cent., it ought not to exceed 40 per cent.

A proof when taken from the printing-frame is formed of three super-imposed images, the first furnished by the albuminate, the second by the chlorides, and the third by the nitrate of silver. The albuminate and the chloride, in well chosen proportions, give the true relations of light and shade. The nitrate of silver, in reducing itself with facility by the presence of the chloride and the albuminate, accumulates metallic silver everywhere where reduction takes place, and thus powerfully contributes to supply the element necessary to toning.

I shall notice, en passant, a fact which explains why positive paper keeps without deterioration in boxes with chloride of calcium. A sheet of paper, sensitized in the Ordinary manner, was freed from the nitrate of silver by repeated washings in pure water, and put into a tightly closed box. After the lapse of several months it still retained its primitive whiteness. (This single experiment will require to be repeated to be confirmed.) From this it results that the deterioration of positive papers is due to the change in the nitrate alone; and as the reduction of the nitrate can take place only in presence of moisture, there are only two means of preserving the paper, either by removing the nitrate, or by keeping the paper in a state of absolute dryness. In selecting the latter, the best selection has been made, because the nitrate is necessary to the production of a good image.

ON TONING.--No toning bath can be compared with that of chloride of gold, for beauty and variety of hues, and especially with respect to permanence of the proof. En this bath the proof becomes covered with a protective coating of the most unoxydizable of the metals; for a proof properly toned will not completely disappear in boiling nitric acid, and a silver button, which I had every reason to suppose exempt from admixture of foreign metals, immersed in this bath for 48 hours, became covered with a metallic coating presenting the colour and brilliancy of gold, which resisted the oxydizing flame of the blow-pipe.

This bath may be employed before or after fixing. Employed before it may sulphurise the proof, unless the process indicated by Mr. Maxwell Lyte be adopted In fact, the least trace of acid from the decomposition of the nitrate suffices to set the sulphur free I prefer to fix the proof first and afterwards immerse it in a toning bath composed as follows :--

Water 35 ounces,
Hyposulphite of soda
Chloride of sodium
Chloride of gold 30 grains.

If the addition of the chloride of gold causes a precipitate of sulphur to appear, the bath must be left several days to settle, then carefully decanted and filtered.

Every proof after six hours immersion in this bath should have its bronzed metallic blacks of a fine, velvety black colour. To make the light lilac hue disappear which some times covers the half-tones, it is sufficient to pass the proof into a bath of cyanide of potassium of the strength of the per cent., which causes all the proofs to become blue black in a few seconds.