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making it less sensitive; reverse the experiment, and, instead of acid, drop into the silver 1 drop of caustic potass, 1 grain to an ounce of distilled water ; do this five or six times, compare the iodides in appearance and working ; the perfectly neutral giving by far the most certain as well as the most truthful results, the acid iodide giving thin pictures, and the alkaline giving unstable patchy ones that decompose the development. An iodide made from a concentrated silver solution will give a stronger picture than one made from a weak solution. When a bath is made, the water-bottles, &c., should be of an equal temperature, as near 60° Fahr, as possible. To prove this, take a rather acid silver, let the water be from 65° to 70 Q , iodize with iodide of potassium, dilute to requisite strength, and you are almost certain to have slightly streaked plates with indications of fogging. Such a bath will soon give hard pictures, especially if the precipitate of iodide has not been great; if it lias, they will be evener but rather more foggy; this is caused by the tempera ture separating the iodine from its base, free iodine causing tine streaks, the base the fogging. Long rest or sunning will cure it of its fogging propensities. Should you add nitric acid to cure it your difficulties begin, because of it acting upon and decom posing the iodides in the collodion ; for according to the state of the bath or collodion will you have one or other of the iodides described. Alkaline plates are often obtained from an acid bath, because of the collodion being made of strong alcohol and ether ; the iodizer not being of the most soluble kind under such circumstances, decomposition or disturbance takes place in the plate while the iodide is forming; with an acid bath this is always so more or less. This tells me that for certainty and perfection of results, both bathand collodion should be neutral. By the common system of working this is not possible (at any rate for daily practice), but my way of preparing the bath allows this, as it gives the necessary firmness to the picture, as well as having the power to rectify any alkalinity in the collodion. Every different kind of silver makes baths of different degrees of quality and stability, and this is not caused by any impurity, but is entirely owing to the state the crystals are in. Condition of chemicals is everything, and not—what might be supposed— actual purity. A clear, transparent, neutral, dull, and blue-look ing crystal is the best of all; the semi-opaque, slightly alkaline, and white-looking, being the worst. Baths from this silver are not so lasting, nor do they generally give such clean pictures. This kind, even when perfectly neutral, is what I should call weak in constitution, and no addition of acid, however carefully added, will ever give it the same stability as the clear, deep coloured, neutral crystal. The addition of acid helps to give it stability, but it does not answer so well as the proper crystals. The acid seems to act by itself, and to affect the collodion much more injuriously. I will now examine the collodion, and see if it is likely to be in a proper condition for forming an iodide or bromo-iodide of silver in equivalent proportions when it is placed in the bath. For this to be possible it must be perfectly homo geneous ; by that 1 mean there must be an atom of iodine in combination with an atom of base. This, I think, is very rarely if ever obtained, because of the action of the pyroxyline and the solvents upon the iodizer, for as much as we depart from equi valent proportions do we fail in obtaining, not only stability, but, more than all, truthfulness. Excess or separation of base gives hardness and a corresponding want of the power of rendering perfect anatomy. The free iodine, although serving to give clear pictures and reduce intensity, is constantly acting injuriously, upon the bath. Neither the free iodine nor the separated base play any direct part in the formation of the image, but they are highly injurious in the nitrate bath, causing hardness and greasiness, and speedily destroying its good work ing qualities. Why cannot we obtain the iodizing solution un decomposed in the collodion ? There are various reasons why this cannot be obtained: first, strong ether and alcohol will de compose the iodides more or less, according to their solubility ; the pyroxyline that is generally used will decompose the iodides more or less, and so will the acidity or alkalinity of the solvents cause a precipitate of the iodides or the bromides, as the case may be. I shall, no doubt, be told that pyroxyline can be made that will not decompose the sensitizer. Of this I am aware; but it will often do worse in many ways not possible to be explained in this article. Very soluble iodides can also be used, but they give thin pictures; for remember, it is the state of solution of the iodide that is everything, and not the kind of iodide. Condition is everything; the more acid collodion is, the more unstable it is as a rule. Now, the effect of introducing into the ordinary nitrate bath a collodion with a part of the base of the iodide free is to produce an iodide of silver in the wrong condition. According to the state of solution in which the iodide is in the collodion, so will the picture be flat, hard, or round ; but it cannot be truthful, although it may be sometimes more pleasing than if it was so, hard features being favoured by an iodide that is easily soluble. Pyroxyline I will say nothing of, as the varieties are so numerous that I should only waste time by alluding to any ; yet it is of more importance to photographers to thoroughly understand their different actions in collodion than is generally imagined. There is a revolution that will come from pyroxyline as great as any that I claim for my bath. When this is accomplished, as it will be, failures will no more be known but through carelessness, and never, until it is, can we have an iodide of silver in a perfect state. There is now the iron solution, or developer. Nothing I have ever read has given me an insight into its different states or conditions ; it has always been so many grains protosulphide, as if it was always of the same kind, any more than collodion is; I refer to the protosulphide alone, without the many additions it has from time to time been subjected to. There are just as many kinds of iron as there are of silver or collodion, and 411 guaranteed to be pure; and I have no doubt they are; for although there ought to be nothing easier than to obtain a definite pure crystal of iron, yet it is not so easy : for unless you purchase a lot at once, every different sample will produce a difference in the negatives. I will describe some different kinds :—No. 1 is a deep green crystal, dissolves quite clear, and keeps so for a whole day and night without the addition of any alcohol or acid. No. 2 is less deep in colour, dissolves clear, and keeps clear perhaps for a half-day. No. 3 is much lighter in colour; this is said to be absolutely pure ; after it is dissolved it gets slightly muddy in about ten minutes. No. 4, common green coperas, impure, at Id. per lb., and such as is used by dyers, goes quite muddy even while it is dissolving, There are many other kinds of iron ; but these are sufficient for illustration, every one wanting different treatment in its pre paration ; for every one of them is invaluable when its special qualities are known. Take No. 1, 15-gr. iron to 1 oz; water, to 16 ozs. of which add 5 drms. alcohol, pure spirit, and 3-2 drms. glacial acid; try to develop, and you will find that it will not flow evenly over the plate ; No. 2, and the same until you come to the slightly muddy one, the pale-blue crystal pure iron ; this will go evenly over. But see what a difference in their action: the iron that dissolves clear will give you a clear picture, but thin, because of the silver being washed away; the iron next in clear ness, when dissolved, will give a more intense picture, principally because of its remaining more even on the plate; and those that dissolve milky, or become so after a short time, will give thin foggy pictures. Now try an experiment: take any that keeps clear, dissolve in the same amount of water, add pure spirit, place the bottle, in a saucepan on the fire with something to keep it from the bottom of the pan, fix the stopper loose, boil as long as you please (say for an hour or more) ; do this only for an experi ment. Now, if you do not turn the neck of the bottle up the chimney before you take ont the stopper, you will have the contents in your face. The iton that dissolves next in clearness will be less liable to do this, and so on until you come to the kind that dissolves milky. With them there is no expansion of the alcohol of any consequence. How is this? Just because of the first samples of iron having more acid in them than the others, the alcohol being neutral, or perhaps slightly acid from age ; in this state it will not amalgamate with an iron salt containing a trace of free acid. Take the weak potash solution which is used for the collodion, drop two or three drops into the alcohol, add this to plain iron solution ; now boil as much as you like; the alcohol, being homogeneous with the water, will not be sepa rated ; before they were merely mechanically combined, and, when placed upon the plate, naturally parted; now they will lay even. The iron salts that dissolve clear do not need to bo decomposed by heat to prepare them for developing; and they will give the mostdenso pictures. Aboutten minutes’boiling will make the picture develop still more densely. Very long boiling (say one hour) will give you a very thin picture, but particularly clear. In a word, the irons that dissolve cleat have more developing power ; the muddy ones loss, and so give much more half-tone. All the irons that dissolve the least