PHOTOGRAPHIC CORRESPONDENCE.
A Hint to the Photographic Society.—It has been objected to this Society, that beyond the establishment of its Journal, and the forming of an Exhibition, it has done very little to promote the improvement of the beautiful art it was specially intended to advance. Such objections are very easily urged; but those who make them should at least propose a remedy. It is in no unfriendly spirit that we allude to these complaints; and we well know how difficult it is for a body like the Photographic Society to take any important step which shall not be liable to misconstruction. We would however suggest, that among those endeavours which it would become the Society to make, there is one which might at once be taken, namely, to secure for the photographic public a good paper. The want of such an article is hourly felt. If the Photographic Society, following the example of the Society of Arts, should appoint a Committee to take this matter into consideration, to define clearly and unmistakeably the essentials of a good negative paper for calotypes (for perhaps it would be well to keep to a good negative paper), and offer a premium for its production, a very short time would elapse before specimens of such an article would be submitted for examination. It is clear that the premium need be one only of small pecuniary value; for the fact of a maker having produced such an article as should gain the prize, would secure him an ample recompense in the enormous demand which would instantly arise for a paper which should be stamped with the public approval of a body entitled to speak with so much authority on such a subject as the Council of the Photographic Society.
Test for Nitrate of Silver.—The Reader of Photographic Works, who in Vol. ix., p. 111., asked for information as to how he might know whether nitrate of silver was pure, can detect any impurities with which that salt is likely to be contaminated, by applying a few simple tests to an aqueous solution of it. The impurities which nitrate of silver most frequently contains are nitrate of copper, nitrate of potash, and free nitric acid. It is also sometimes intentionally adulterated with nitrate of lead. The presence of a salt of copper is detected by the solution assuming a blue colour when mixed with an excess of ammonia. To detect nitrate of potash, hydrochloric acid should be added to the solution in sufficient quantity to precipitate the whole of the silver. The liquid should then be freed from the precipitate by filtration, and evaporated; if nitrate of potash is present, a fixed residue will remain after evaporation. The presence of a salt of lead is detected by adding a few drops of sulphuric acid to the solution of nitrate of silver, which precipitates the lead as sulphate if present. It is, however, necessary to dilute the acid with a considerable quantity of water, and, if any precipitate forms, to allow it to subside previous to using it as a test for lead, as ordinary sulphuric acid is frequently contaminated with sulphate of lead, which is soluble in the strong, but not in dilute, acid.
Any free nitric acid in the nitrate of silver can be detected by the smell. The crystals can be freed from
it, should they contain any, by fusing them in a porcelain crucible over a spirit-lamp. The ordinary fused lunar caustic of the surgeon is unfit for general use as a photographic agent.
J. Leachman.
Professor Hunt's Photographic Studies.—My attention has just been directed to a "Practical Photographic Query" in your Journal, Vol. ix., p. 41., which appears to require a reply from me. It is quite evident that your correspondent, notwithstanding the personal respect which he professes to entertain, cannot have any intimate knowledge of either my works or my studies. Allow me to make my position clear to him and other of your readers. My first photographic experiment dates from January 28, 1839, and since that period the investigation of the chemical phenomena of the solar rays has been the constant employment of all the leisure which a busy life has afforded me. The production of photographic pictures has never been the ultimate object at which I have aimed, although my researches have caused me to obtain thousands. My object has been, and is, to endeavour to obtain some light into the mysteries of the radiant force with which the photographic artist works, being quite content to leave the production of beautiful images to other manipulators.
As I write on the subject, it appears, of course, necessary that I should be familiar with all the details of manipulation in each process which I may describe. Whenever I have mentioned, in either of my works, a process with which I have not been entirely familiar, I have given the name of the authority upon whom I have depended. But there will not be found in either my Photography, or my Researches on Light (of which a greatly enlarged edition will soon be submitted to the public), any one process upon which I have not made such experiments as appeared to me necessary to my understanding the rationale of the chemical changes involved, and of the physical phenomena which arise.
Now, since it is not necessary to select a picturesque object to instruct me in these points, the same buildings, trees, and plaster casts have been copied times beyond number; and when the problem under examination has been solved, these pictures have been destroyed.
There are twenty exhibitors of pictures in the Photographic Gallery who would certainly leave my productions far behind, as it concerns their pictorial character; but I am confident there is not one who has made the philosophy of Photography so entirely his study as I have done.
I have been engaged for the last two years in studying the chemical action of the prismatic spectrum. I inclose you my report on this subject to the British Association for 1852 (that for 1853 is now in the hands of the printer), from which you will perceive that I am employing myself to greater advantage to photography, as science under art, than I should be did I enter the lists with those who catch the beauties of external nature on their sensitive tablets, and secure for themselves and others pictures drawn by the solar pencil, in which no one can more deeply delight than your humble servant.
Robert Hunt.
Waxed-paper Pictures.—Will your correspondents or yourself do me the favour to say, how such beautiful pictures have been produced and exhibited by Mr. Fenton and others by the waxed-paper medium, if that process be so bad and defective? When I have followed it, and exercised consistent patience, I have ever produced pleasing and faithful results. That when parties do not themselves prepare, it becomes expensive, I am willing to admit; but I am inclined to attribute many failures to the uncertain heat of hot irons, which must vary; and I make this fact known to you as the result of my own observation on many sheets: added to which, defective manipulation, or impure chemicals, must not be allowed to do away with its having much merit.
Harley Lane.
The Double Iodide Solution.—In a note appended to Dr. Mansell's communication on the calotype (Vol. ix., p. 134.), you state that having lately prepared the double iodide solution according to the formula given by Dr. Diamond, in which it required 650 grains of iodide of potassium to dissolve a 60-grain precipitate, you were inclined to believe, until you made the experiment yourself, that Dr. Mansell must have made a wrong calculation as to the quantity of iodide of potassium (680 grains) which he stated was sufficient to dissolve a 100-grain precipitate, as the difference appeared so small for a solution more than one-third stronger.
The small difference referred to with respect to the quantity of iodide of potassium required, is owing to the amount of water used being in both cases the same. A slight difference in the strength of a solution of iodide of potassium makes a great difference with respect to the quantity of iodide of silver it is capable of dissolving. Thus, if you remove a small proportion of the water from a solution of the double iodide of silver by evaporation, the slight increase of strength which the solution will thereby acquire, will enable it to take up a much larger proportion of iodide of silver than it already contains; and if, on the other hand, you dilute it with a small proportion of water, its diminished strength (unless the solution contains a great excess of iodide of potassium) will cause the precipitation of a large proportion of the iodide of silver. And hence the great variation in the amount of iodide of potassium which is found requisite to form a solution of the double iodide of silver, under the same apparent conditions with regard to the proportions of the other ingredients employed, may be accounted for by the impossibility of measuring off with sufficient accuracy the proper proportion of water.
Whenever exact quantities of liquids are required, recourse should always be had to the balance, for no great accuracy can be depended upon by measurement with our ordinary glass measures, even supposing them to be correctly graduated, which is not always the case.
J. Leachman.
Dr. Mansell's Process.—Dr. Mansell's lucid and very practical paper on the calotype process in "N. & Q." must, I am sure, be of the greatest service to photographers in general; and as one of the many I am irresistibly tempted to offer my sincere and hearty
thanks to him for the truly valuable hints it contains. If Dr. Mansell will give the rationale of the necessity of not allowing a longer time than absolutely required for the soaking out the now injurious iodide of potassium, set free by the deposit of the iodide of silver; and also, an explanation of the cause of that part of the iodized papers which takes the longest time in drying being weaker than that part which had been more hastily dried, the learned Doctor will still be adding to our present account of obligation to him.
Henry Hele.