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540 THE PHOTOGRAPHIC NEWS. [July 31, 1891. uncertain in their action, and possess very poor keeping qualities. They are made without the ammonia salt, and are dependent on soda and potash for their action, take a long time to work, and are too uncertain for practical use. There is no doubt whatever but that the white acid compounded with fluoride of ammonia is the best. In using other white acids, spots and streaks often form in the glass, and these cannot always be removed by repeat ing the etching. With ammonia acids, however, any streaks which may appear, either from applying the acid unevenly or from imperfections in the glass, may be removed by repeated etchings. The following recipe is one which is used by several practical glass etchers, and is said to give good results. It is of German origin, and the only objection to it is that it is too complicated, which objection may also be raised to other recipes from the same source. In a container of lead the following mixture is made :— Distilled water ... ... ... ... 500 parts Fluoride of ammonia (strong) ... ... 500 „ Sulphate of ammonia ... ... ... 50 ,, Sulphuric acid ... ... ... ... 100 ,, This solution is ready for use within two hours, and may be tested by immersing a piece of clean glass, which should get a nice, fine matt surface after five or six minutes. In practical experience the writer has found that a simpler method of preparing the acid than the foregoing is capable of giving good results. Besides being cheaper, it is possible to recover the materials in it, should it for any reason get out of order. A container of sufficient size is filled one-third full of ordinary commercial hydrofluoric acid. Carbonate of ammonia is then added. About equal parts by weight may be used. When effervescence has ceased, a small slip of clean glass is immersed in the mixture and permitted to remain six or eight minutes. Upon with drawing, it is rinsed in clean water, wiped, and dried. If examination shows that it has become evenly translucent over its entire surface, the mixture is all right and may be used for regular work. If, however, it is deeply and irregularly etched, with some parts clear and some parts ground, the acid is in excess and carbonate should be added. If, on the other hand, the glass seems to be only partially affected by the acid, and, while being slightly ground all over, is transparent, too great an amount of ammonia has been used, and acid must be added. With a little experience, it is possible to keep the balance between the alkali and the acid, so that good results can be obtained. All white acids are subject to change in their action from day to day, but in none of the recipes the writer has used can it be so easily regulated as in the foregoing. Before trusting any important work to the action of white acid, the acid should be tested with a clean piece of glass, and by following the hints given the acid can be corrected to give the proper action. In preparing glass for etching, any of the ordinary resists may be used. The drawing may be either put on glass by means of a ruling pen dipped in asphaltum properly diluted, by means of a brush, or by means of the somewhat antiquated process of covering the entire plate with Brunswick black, and scraping away the parts which it is desired to grind. The best method, however, is that in which tin-foil is used, a description of which must be deferred to some future time. The design can also be transferred or photographed on glass if desired.—Scientific American, THE PHOTOGRAPHIC WORK OF HERSCHEL AND FOX TALBOT* BY WILLIAM LANG, JUN., F.C.S. A communication from Talbot to the Philosophical Maga zine on “A New Property of the Iodide of Silver” may be here referred to. It appears at p. 258, vol. xii., and is a note of the appearance that silver iodide presents when heat is applied. A pale primrose yellow, when cold, this salt, when exposed to the heat of a fire, turns to a rich gaudy yellow. Before passing on to consider Talbot’s later researches, I may be allowed to refer as briefly as possible to some of his earlier work, giving the original source of publication. In 1826, “Some Experiments on Coloured Flame,” Edinburgh Journal of Science, v. In 1827, on “Monochromatic Light,” Quarterly Journal of Science, vol. xxii. In 1833, “Remarks on Chemical Changes of Colour,” Philosophical Magazine, vol. ii. In 1834, “ Experiments on Light,” Philosophical Maga zine, vol. v., p. 321—334, and “Royal Society Proceed ings,” p. 298, vol. iii. In 1835, on “Nature of Light,” Philosophical Maga zine, vol. vii. The foregoing by no means exhaust the scientific con tributions of Talbot. His mathematical memoirs were considerable. A Royal Society’s medal was awarded him in 1838 for “ Researches in Integral Calculus.” Dealing again with the year 1839, we find Talbot mak ing a short communication to the Royal Society on March 21st, the title of which is “ Note respecting a New Kind of Sensitive Paper.” In using bromide of potassium instead of sodium chloride, the author finds that greater sensitiveness is manifested. He also refers to a mode of making a negative design on glass by blackening the same by means of the smoke of a candle, and etching out the lines by means of a needle point. The ninth meeting of the British Association was held at Birmingham, in 1839. It opened its proceedings on the 29th of August, ten days after the public announce ment of Daguerre’s method. Accordingly, we find Tal bot contributing a paper to the section of Physics and Mathematics, dealing with the Frenchman’s discovery, “ Remarks on M. Daguerre’s Photogenic Process.” Tal bot’s paper is, for the most part, taken up with the con sideration of the optical phenomena presented when a particle of iodine is laid on a silver plate and afterwards gently treated. He seems, moreover, to doubt the superior sensibility of Daguerre’s plate over his photogenic paper. The discussion that ensued is reported in No. 618 of the Athenieum, August 31st, 1339. Up to this point, as we have seen, Talbot had been busying himself with the obtaining of actual images, “printed out” subjects, as we would now call them. Talbot’s own words, referring to his calotype process, may be here quoted; they are taken from the appendix already alluded to. “ The discovery of the latent image and the mode of its development was made rather suddenly on September 20th and 21st, 1840. This immediately changed my whole system of work in photography. The acceleration obtained was so great, amounting to fully one hundred times, that, whereas it took me an hour to take a pretty large camera view of a building, the same now only took about half a minute, so that instead of having to * Concluded from page 524.
