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FEBRUARY 16, 1883.] THE PHOTOGRAPHIC NEWS. 107 bells attaching themselves to the sheet, but should there be any, they may be easily removed with the glass rod or blown off; the sheet is again replaced on the bath and allowed to remain long enough to become fully sensitized. The length of time will be found to vary. For instance, if we are using paper which has been albumenized for a few months, three minutes will be found to be long enough; but if the paper is freshly prepared, it repels the solution, and will take as long as five minutes in warm weather ; also in cold weather a longer time will be required than is neces sary when the temperature ranges from 60° to 75°F. When the sheet is down on the bath it will be seen that it does not lie perfectly flat. After the space of a little while it becomes absorbed with moisture and will present an even surface. When that period arrives, whether it requires three minutes or ten, the sheet is ready for removal. Raise the farthest corner of the sheet to the left, off the solution, by means of the glass rod. Seize it by the left hand, raising the sheet slowly ; now take the nearest corner with the right hand and draw the sheet steadily and slowly over a glass rod fixed at the end of the dish ; if this is carefully done, all excess will be removed, and the sheet will be dry in a few minutes ; suspend it from one corner (albumenized side outward) by means of one of the American clips in the drying room. By the time ten sheets have been floated, about 290 grains of silver nitrate will have been converted into chloride or absorbed. This should be replaced by means of 4 fluid ounces of a strong stock solution of nitrate of silver, made by adding 36 fluid ounces of water to 6 ounces of re crystallized nitrate of silver. If a less quantity than ten sheets are floated the end may be gained by making a dia mond scratch on the bottle at 80 ounces, and whenever the bath is poured back, add strong stock solution to bring the bulk up to that mark ; a mercury testing bulb, called an argentometer, is sold for the purpose of ascertaining the strength of silver contained in a solution with water, but it is not reliable. A more satisfactory method will be given at the end of these lessons. In the last lesson, we stated that other nitrates are some times used in addition to the nitrate of silver. We have also said that the strength or number of grains per fluid ounce of nitrate of silver should be in proportion to the quantity of soluble chloride contained in the paper. This may be considered a safe rule to work by ; but as we have mentioned the addition of other nitrates, our student will naturally wonder why they are used. The answer to this is, they are used as a substitute of the silver. Our previous remarks will now be supplemented with a formula, wherein a paper heavily salted—that is, containing twelve to twenty grains of a soluble chloride per ounce of albumen—may be made to yield rich prints with a small proportion of nitrate of silver. Nitrate of soda 40 grains Nitrate of silver 20 „ Ordinary water 1 ounce The time required for floating will be from six to eight minutes, according to temperature. Another— Nitrate of potash 35 grains Nitrate of silver ... 30 ,, Ordinary water ... ... ... 1 ounce Float three to five minutes, according to temperature. Paper so sensitized will require ammonia-fuming for a quarter of an hour and upwards. W e shall enter fully into the method of fuming io our next lesson. NOTES ON PHOTOGRAPH!. BY E. HOWARD FARMER. Lecture XI.—The Gelatine Process—Continued. Thiosulphate of Soda (hypo.') Na,S,03, 5H,0.- Having developed the latent image by either of the methods shown, the next operation is to dissolve out the unreduced silver bromide and iodide from the film, which answers the double purpose of preventing any further action of light, and rendering the film transparent, except at those parts contain ing the reduced silver; this operation is technically called fxing. As we already know, there are several solvents of these substances, but the one universally employed with gelatine plates is thiosulphate of soda (hypo). This substance is manufactured in large quantities from pentasulphide of calcium, a substance obtained as refuse in the manufacture of common washing soda, and in the purification of coal gas ; it may be prepared by passing the fumes of burning sul phur (sulphurous anhydride) through a solution of soda, and subsequently digesting the solution thus obtained with sulphur. It occurs in commerce as large translucent crystals, which contain 5 equivalents or 42 per cent, of water. The crystals can be fused in this water of crystallization, or are soluble in water to almost any extent, in the act of dissolving absorbing a considerable amount of heat, so that the solution becomes very cold ; they have a bitter, disagreeable taste. The solution gradually undergoes oxydation by exposure to the air, the hypo becoming converted into bisulphate of soda— Nag (S:0,)+40+H:0=2NaH(SO:) When nitrate, bromide, or other salt of silver is added fo a solution of hypo in equivalent proportions, a white substance is formed, which consists of thiosulphate of silver; this thiosulphate of silver rapidly darkens in colour until it becomes black, at the same time splitting up into silver sulphide and sulphuric acid ; if, however, a silver salt is added to hypo, so that the latter remains in considerable excess of the equivalent proportions, the silver thiosulphate, as soon as it is formed, combines with two more equivalents of hypo, forming what is called a double salt, which sub stance is readily soluble in water thus : — Silver Bromide Thiosulphate Double Thiosulphate of of Soda Silver and Soda 2AgEr+3Na,(S,0a)-Ag(S.03),2Na,(S:02)+2NaBr It is this property of forming a soluble double salt with silver compounds which renders hypo so valuable as a fixing agent. Its use for this purpose was first pointed out by Sir John Herschel. If an acid be added to hypo, the latter is immediately decomposed, giving off an odour of burning brimstone, and a yellow substance (sulphur) separating out; this fact may be used as a test for the substance, and obviously, the addition of acids or substances of an acid character to a hypo solution should be carefully avoided. Fixing.—As soon as the image is fully developed, the plate is first rinsed, and immersed in a saturated solution of common alum— Alum 1 part Water 15 parts the object of which is to harden the film and prevent its frilling in the fixing bath ; it also, in the case of oxalate development, dissolves out any oxalate of lime which has been precipitated in the film by double decomposition between the lime salts usually contained in ordinary hard water, and the oxalate of potash in the developer. After remaining in the alum for five minutes, it is again rinsed, and then placed in the fixing bath, which should consist of— Hypo ... 1 part Water ... 5 parts until the whole of the unreduced silver salts are dissolved out. The time taken for this may vary from one to fifteen minutes, or more, according to the hardness and thickness of the film, and whether it contains silver iodide, in which case the fixing always takes longer ; it also depends on the temperature of the bath, and whether it be at all saturated through fixing previous plates. As soon as the fixing is complete, which is known by the total disappearance of the ; yellow silver salts from the back of the plate, it is taken , out, and thoroughly washed either in running or frequent