_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 {182} 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.