of potassium or bromide of ammonia. These two bodies react upon the metallic silver in an unknown way. The proportions between these two salts in making this solution are purely arbitrary and these proportion do, not represent in any way whatever chemical proportions.

The generally accepted or published recipes vary greatly, but taking the one as published in the Photographic Almanac as the standard we find that two parts of ferricyanide of potassium and three parts of potassium bromide are directed to be used, and the majo ity of workers weigh these salts with the most exacting care as though everything depended upon this accuracy of proportion. This is a totally erroneous fallacy. This may appear a bold assertion, but one I want to show is true; and for this reason I made a series of experimental prints, but before dealing with the experiments I would like to reason this point out with you. We have two data to start with: (1) To have chemical action you must of virtue have chemical proportions. (2) If there is no chemical action you have physical mixtures based upon the law of excess proportion in which the chemicals exert no action upon each other. Now with these two statements in front of us, let us see which we are dealing with in this solution. We have a mixture of two solutions of two neutral salts. We may make up separate solutions of each and mix them we like and how we like, and can see no difference except in color, because obviously the colored ferricyanide, being mixed with the colorless bromide solutions, suffers dilution and so in color.

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Now take a different type of solution. Take, for instance, the mixture of iodide of potassium and perchloride of mercury, such as you use for intensification. We mix these, a precipitate occurs; add more iodide of potassium, the precipitate is dissolved; add still more and no more precipitate occurs; why is this? In this case you have a direct chemical action taking place, and therefore require exact proportions, because you are producing a new salt quite different from the one you started with.

In the ferricyanide and potassium solution you are not making a new salt, and it is quite possible to separate these salts from the solutions and obtain again the ferricyanide of potassium and the bromide of potassium.

It would be an easy matter to produce a large series of solutions in which there are no chemical reactions, and the salts are merely mixed upon the basis of the law of excess. It is equally easy to place before you a series of solutions which are chemical solutions and whose proportions are strictly chemical and not arbitrary, and whose efficiency depends upon accuracy. Precept without practice is no more good than capital without interest. I have made a few experimental prints to prove this theory, by taking a graded print which was cut into stripes, and using inverting solutions, one having exactly the reversed proportions of the other. There was no material difference in the sulphided resultant, proving that there is no chemical relationship between these two salts, and they are purely arbitrary proportions, just as the ingredients of a bottle of mixed pickles or the currants in a cake.

The converting solution essentially consists of what is known as an unstable sulphur body of the group called sulphides. All these sulphides give up their sulphur very readily in solution and this liberated sulphur is seized upon by the compound silver salt to form a very insoluble stable salt of silver, known as sulphide of silver. In the present case sodium sulphide is the one employed. When this is dissolved in water, partial dissociation takes place, sodium hydroxide, caustic soda, and sulphuretted hydrogen, commonly called "stink," are formed in solution. This sulphuretted hydrogen is the carrier of the sulphur which reacts on the silver of the print.

Another frequently employed converting solution is ammonium sulphide, which is a solution of a mixture of the various polysulphides of ammonium. It has the disadvantage of being alkaline and some papers blister (some very badly) in an alkaline bath, especially an ammonia alkaline one.

The sulphide of sodium solution has a dangerous side to it, which lies in the fact that this sodium salt has the property of being partially changed into our friend "hypo" by oxidation. If this happens the hypo present dissolves out some of the silver salt and the resulting print is considerably weakened. This can be guarded against by washing the crystals of sodium sulphide with water before dissolving them, because the hyposulphite being a product of oxidation, the "hypo" is formed on the surface only and can be removed quite easily.

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Having briefly reviewed (or shall I say inspected) our materials and refreshed our memories about their nature and substance we must plunge into the discussion of the actual method employed and try and obtain a concrete conception of the whole process. it has been stated, the bromide print is really an undulated layer of metallic silver incased in a gelatin matrix, the particles being both separated and encelled. The first operation consists of inverting this silver into a salt capable of reacting with the sulphur.

You flood your print with the inverting solution and gradually the black image changes into a bleached image. Now what has happened? The solution has first of all had to penetrate the gelatin wall, which it does in two ways, by microdiffusion through the porosity of the surface, or by microfiltration through the interspaces of the gelatin wall itself into the cell, where it reacts upon the metallic silver to form an insoluble complex salt of silver.

If this process is considered in the right way, which is, that the print consists of layers of silver particles, we at once see that time must elapse before the bottom particle of the metallic silver becomes reacted upon. This is demonstrated at once by the fact that the high lights rush away and the deep shadows are the last to be completely changed.

III. CONVERTING OR SULPHIDE SOLUTION

When this process of inversion is considered complete you rinse the paper

and subject it to the sulphide or converting solution; and with a rapidity far and away greater than the "inversion" required, the "conversion" is accomplished. Why? Simply because the gelatin is now well soaked and the solution has a better opportunity of reaching the silver salt by osmotic action, and reacting with the halide and displacing the resulting potassium salts of the reaction. These are very soluble and so are quickly returned to the solution by reversed osmotic action, and you have your toned bromide as your resultant, provided all has gone well.

How often does everything go well? What disappointments are met with! Sometimes nasty, biliously prints, as though we were advertising mustard or margarine; at other times big blotches of a different tone value or blue spots and blisters-all crop either individually or in combination, and our longed-for, hoped for, muchly desired sepia print lights the fire or our pipe of miscontentment. These errors (for after all what are they but errors?) are often, in the language of a schoolmaster, gross, careless errors, and deserve no pity, but severe censure. These errors should not happen if the process is understood. Taking them "seriatim" and studying them briefly, we shall then be able to realize what does happen.

Washed-out Mustard-color
Nondescript Tones

This trouble is generally due to one factor, imperfect, faulty development. If you "overexpose," which is not exceptional or unusual but none the less wrong, and then develop, you find that your print rushes up, and you withdraw it and pass into "hypo" quickly to save your picture. So you do, so far as a black-and-white print is concerned; but if you proceed to sulphide tone it you obtain a miserable failure. Why? The reason is not far to seek. When you are developing, you are acting at first on the outer layers of celled silver bromide, and by removing it to the "hypo" bath, to prevent it becoming too dense, development is arrested when

only these top layers are acted upon, and the "hypo" dissolves the lower strata away, leaving a comparatively thin layer of silver to constitute your print. This when sulphided is too thin to show proper depth of tone, and the result is a washed-out, biliously looking affair. The remedy, and in fact the correct procedure, is to employ a fully but not over-exposed print and then develop it to the very fullest extent, in fact until the high lights show a tendency to fog. In this case you have the maximum amount of silver deposit of such a thickness that your picture when sulphided is full of tone value. This is the ideal print to work with under all conditions for this process.

Blotches

Frequently these arise even with a perfectly exposed print. I have gone into this matter pretty carefully myself, by making trial prints and then making one go wrong. It is all very well for any worker to say what is wrong and how to avoid it, but unless he can produce the trouble he has not become master of the situation. What I mean is this: that it is all very well to theorize about any trouble, and to dogmatize that it is due to this factor or to that; but to prove it I maintain that you must be able to deliberately produce a print with these failures in the same way as you would set about producing one without. Until you can do this I do not consider you are master of the subject. It is a simple matter to say that the blotches are due to uneven development; but that this is so, I purposely developed a correctly exposed print unevenly and then toned it. The black-andwhite print showed no apparent faults, but the sulphided print did so at once. The reason is an obvious one: that the portion of the print which had, say, fifteen seconds more development than the rest, by flowing the developer on unevenly or by using too little, had an advantage of fifteen seconds' development over the rest of the print. This dose should not show if you fully develop the print, but only in those prints in which exposure was slightly excessive

and development arrested before full penetration had been allowed.

You then get that portion which had this increase of action having a greater thickness of silver particles which when altered by the sulphide becomes optically apparent.

So then this fault is due to using a slightly over-exposed print, unequal, careless, and arrested development.

Blue Spots

These are due to circumstances not so easily controlled. The local water supply comes through iron pipes, is stored in iron cisterns, and small inobservable particles of iron get detached and enter into our solutions, and directly the ferricyanide comes into chemical contact with them a blue patch is formed, and the retouching knife is the only remedy, unless of course you can afford distilled water stored in glass. One remedy suggested, but which I have never tried, is to tie a piece of flannel over the tap from which you get your water, but I am skeptical.

Blister

These bugbears can be eliminated quite easily. They are due to the fact that the sodium sulphide dissociates in solution into hydrogen sulphide and caustic soda. If your solution is too strong you have an alkaline bath acting upon a sodden gelatin matrix. Most probably, if the solution is made at the time it is required to be used, it will have a lower temperature than the washing bath, and the sudden contraction and expansion of the film occurs, and at the point of least cohesion with the paper the blister is formed. papers are more prone to the formation of blisters than others, and it is difficult to say precisely what the remedy is, except that your solution must be fresh and not too strong.

Control

Some

I have heard this question asked in this room: How long do you keep the print in the inverting solution? and the

answer was: "Oh, until it is completely bleached!" Certainly you can do so if you like, but if you really grasp the nature of this process there is no reason why you should. When discussing the action of the developer on the encased silver bromide, it was seen that the depth of action was in direct proportion to the time of development. This is a most essential point to bear in mind in this as in all photographic processes: that the longer the action is allowed to take place the deeper it will be, until of course complete, and if I might be allowed here to say it, the action is not complete until the last particle of silver halide is decomposed. By this I mean the unexposed as well as the exposed, and in all cases of development, whether plates or paper, it is all a matter of judgment when development is complete, because if you have sufficient developer to act upon the encased silver salt you will ultimately reduce the whole of the silver salt to the metallic state, and it is the exposed portions which develop first. Ulti mately every particle will be acted upon; so therefore this axiom, that depth of action is in direct proportion to the time, is true.

So much for this theory and fact, but apply it in practice. The bromide print is one in which you have an incomplete area of silver, while in the undeveloped paper you have a complete area; otherwise your conditions are relatively comparable. When the inverting solution is applied it starts to work on the outer surface and gradually penetrates by microdiffusion to the lower layers. This action is considerably slower than in the case of development because of blockage which occurs, due to the fact that instead of

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making a smaller substance you are increasing the quantity. You end by making a very complex molecule by adding to the silver the ferricyanide and bromide, thus, so to speak, choking the upper surface and retarding the rate of diffusion to the lower strata. Having studied that proposition, why should we invert all the particles of silver? Is it reasonable or logical? I would advise all workers to cut a print into four to six strips, give them different times in the inverting solution, then sulphide, and see for themselves and prove to their own satisfaction what a ridiculous dogma it is that it is essential to invert the whole of the image, or, as it is called, "bleach right through." Withdraw your print when you think fit, and, believe me, it is entirely a matter of judgment and personal predilection how far this inversion is to be allowed to proceed, and there is no scientific basis for complete inversion. All sorts of very delightful or hideous brown tones can be obtained by partial inversion of the silver, because you have the black silver deposit lying underneath, and this, associated with the sulphide of silver, gives a richness to the tone which although pleasing to some may be distasteful to others, for, after all, tone colors are all a matter of taste and not science.

This system of control is capable of a great development and only requires a little practice and judgment. Decide beforehand what tint you are to aim at, and then be determined to get it. Make your print to fit your process, and your process to meet your tastes; realize that you and you only are to be the master of the situation and not the process, and that the process is merely a tool in the operator's hand.