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THE PHOTOGRAPHIC NEWS. [March 6, 1863. Uoxxespoudente. IODIDES AND BROMIDES IN COLLODION. Dear Sib,—The leading article of your issue of the 24th October last somewhat alarms me ; but, I really hope you do not mean to close the discussion. I am aware that I need expect no mercy, for I am contesting a widely received principle, and, thereby, placing myself in direct antagonism to a large section of photographers. But I believe that you will give me one more opportunity to be heard, because of “the earnestness and manifest honesty” which you have handsomely attributed to my exposition of the question of “ iodides and bromo-iodides.” Your claim “ to do full justice to all sides of all photographic questions” cannot be disputed, and its enunciation, particularly, makes me bold to augur well of the opportunity you will now finally afford me of defending myself from misconception on the one hand, and of pointing out error on the other. You pronounce my first error as having “misappreciated the avowed character of Mr. Blanchard’s experiments,” and explained that those experiments were held “ tolerably con clusive of the fact, that bromides and iodides, in combination, were more sensitive than either bromides or iodides alone.” I was well aware of this, provided, that by “iodides alone” is meant ammonium, or potassium, or magnesium, and no other; for, at page 479 of the Photographic News, I remarked as follows :—“ They (Mr. Blanchard’s experiments) were simply illustrative of the circumstance, that, under certain conditions, which nobody should adopt, bromo-iodized collodion is superior under iron development to simply iodized collodion.” You also remark, that, on the “general question,” Mr. Blanchard’s experiments were not put forth as conclusive.” No admission could be franker or fairer, and I am sorry for having understood otherwise. The “ general question ” (the exposition of which has never yet been attempted) I have endeavoured to show, involves the arranging against each other of an iodizer and bromo-iodizer made of one collodion, and constructed of like salts, and like combinations, i.e., if the iodizer is compounded of ammonium and cadmium, or cadmium and potassium, the bromo-iodizer should also be made of the same salts, in the same proportion ; only, that a part or all of one of the salts in the latter case should be a bromide; and that, if the iodizer is made only of ammonium or only potassium, or only cadmium, that the corresponding bromo-iodizer should be made of iodide andbromideofammonium, or iodide and bromide of potassium, or iodide and bromide of cadmium, when, a comparison will show the iodizers invariably superior in every respect to their pair bromo-iodizer. It is possible, nay, certain, that in the several combinations of cadmium and ammonium, or cadmium and potassium tested, that the bromo-iodizer of one experiment or pair will be superior to the iodizer of another pair ; but this bromo-iodizer will in its turn be inferior to its pair iodizer. We can evidently, then, make iodizers and bromo-iodizers of various capacities (the cadmium being the regulator of durability and sensitiveness) alternately inferior and superior, until we come to an iodizer superior to the best bromo-iodizer, which will be at the same time the pair of that iodizer, and superior to all other iodizers and bromo-iodizers. And, above all, the pure cadmium iodizer will rank first. I have nowhere taken ammonium or potassium iodizers into account, because, for practical purposes, they can bear no comparison with the combination salts. I have shown the results of my experiments in a letter dated and posted a month ago to your address, under the head A to D, as also the comparative merits of pyro and iron development. An exami nation of the same will show that iron development stands first. To show that the combinations of salts do possess the different degrees of sensitiveness and durability that I claim for them, I w 11 place the matter in the following light. Very sensitive ammonium and potassium collodion can be made. They are undisputable facts; for instantaneous work has been done with them, as with cadmium collodion, or with collodion salted with combinations of salts. But they (the potas sium and ammonium iodizers) are so capricious as to render them a source of trouble. Hence cadmium collodion ranks first on the score of stability. Next to it we must place com binations of cadmium and ammonium, or cadmium and potas sium, their order being regulated by the proportion of the cadmium salt present in each, until we end with ammonium or potassium only. Now, all are equally sensitive. But, for all practical purposes, how do matters stand? Wo cannot avail ourselves of each in its best condition; we cannot at will give to each the collodion most suitable to it; but we may at all times succeed in getting a collodion fit to be iodized with one or more of the combinations, and we can never better employ a collodion. Any collodion fit to be iodized, with potas sium or ammonium alone, will admit of the introduction of cadmium more or less. Any collodion fit to be iodized with cadmium only will admit of any combinations of salts (which anybody may ascertain for himself in a moment) being used with it, although it will be worthless for ammonium or potassium alone. Any such collodion will therefore be the best to experi ment with. Now, cadmium glutinises collodion and attains maximum sensitiveness slowly, and it deteriorates slowly ; but potassium and ammonium liquefy collodion—they attain sensitiveness quickly, and as rapidly lose it. Under these circumstances the effects of the alkaline salts upon the collodion are not so energetically injurious in the presence of cadmium as when employed alone; but injurious they still are in proportion to the quantity, and are comparatively innocuous when their quantity is small. I should therefore conclude that the smaller the proportion of the alkaline salts, and the greater that of cadmium, the less the injury to the collodion by the greater glutinosity opposed to the liquefaction : the greater the stability by the resistance to the deteriorating effects upon the collodion, and the superior the ultimate sensitiveness by the time given to attain it. Only on one supposition, it appears to me, could there be an equality of sensitiveness in all combinations with any given collodion—if the alkaline and acid salts, apart from the question of stability, sensitize the collodion equally rapidly, and affect it identically ; but this can never bo the case. As respects Mr. Blanchard’s experiment with iodide of potassium on the one hand, and a bromo-iodized collodion on the other, made to test whether a potassium iodizer was the most sensitive, as claimed by some operators, I beg to observe that under no circumstances could the result be reliable, unless the comparative experiments (provided that the collodions and baths most suitable to the salts had been furnished) had ben extended over a period such as would have included the best average condition of such collodion, as insisted upon in my last letter. As regards equality of sensitiveness between all iodizers, I think there should be very little room for doubt if the exact condition of suitability of collodion and availability in the exact stage of sensitiveness—especially as regards potassium or ammonium collodion—are secured. For we read of instantaneous work by Count Montizon, Legray, Maxwell Lyte, &c., with the ammonium and potassium salts. No operator now, however, would so use them because of their notorious capriciousness and instability, and all prefer collodion salted with cadmium, or at least as half of tho total quantity of salts, because stable and comparatively reliable. As respects tho exception I have taken to Mr. Blanchard’s experimenting with one plate, two collodions, and two lenses, which you have thoroughly explained, I scarcely care to wish it to be decided (more emphatically than it has been done by you) whether tho plan I recommend or Mr. Blanchard’s is the legitimate one, and which of them is to bo recommended for simplicity, and, above all, for unquestionable accuracy and certainty of result. For you already concur with me in the use of one lens, and, by implication, ono plate and one collodion. I think you agree with me that one collodion, iodized and bromo-iodized similarly, ono lens, and light exposure are indispensable. I claim also one bath for each pair of iodizers and bromo-iodizers, but you contend that “ a bromo-iodized collodion, for instance, works best with nitric acid in the bath ; to a simply iodized collodion this would be fatal.” To prevent tho possibility of any misapprehension of terms here, I beg to inquire whether by “ simply iodized collodion ” you mean a collodion iodized only with potassium, or ammonium,or magne sium, or with cadmium and combinations including cadmium. If with tho first three salts, I concur in part with you ; but if with cadmium and combinations including it, I differ most emphati cally from you. All my experiments wore made with nitric acid baths only ; and in page 467 of the Photographic News I laid down the following as a radical principle:—“The constitution of th