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The photographic news
- Bandzählung
- 6.1862
- Erscheinungsdatum
- 1862
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- Englisch
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- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 220, November 21, 1862
- Digitalisat
- SLUB Dresden
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- Parlamentsperiode
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- Wahlperiode
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Zeitschrift
The photographic news
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Band 6.1862
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- Ausgabe Ausgabe 73
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- Register Index 619
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Band
Band 6.1862
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556 THE PHOTOGRAPHIC NEWS. [November 21,1862. Note the reason why the public does not like contrasts, because they instinctively feel that they are too strong, while the photographer, who believes that he has rendered nature to perfection, believes they are not strong enough. To remedy this radical imperfection in a general way, certain photographers provide permanent reflections by the arrangement of lights, or rather, what is to them much easier reflect upon-the sitter, by means of a white screen, a supple mentary light, which acts only during the sitting, without considering that the reflection from the screen upon the corner extinguishes the light of the eye, and invariably falsifies the model, since the result is a lighting greatly at variance with nature. This remark has been suggested to me by a portrait which presents a bizarre aspect, with an unnatural expression and false shadows. Upon enquiry, I ascertained that the pho tographer had employed a screen to lighten the shadows, and he succeeded so well that his portrait was—nowhere. Consequently, an indispensable condition consists in cover ing the walls of the operating room with white or blue sur faces, for any other coloured reflection gives not only a loss of sensibility but of harmony in the shadows. The back ground, when uniform, must be white, and never of a non- photogenic colour; no inconvenience arises from this, as the eye of the sitter cannot be affected by its brilliancy, while a non-actinic colour is always injurious as part of the picture by contrast with the model. By closing the shutter of the skylight, we can always diminish the quantity of light to any desired point. In a word, the pose and the lighting are the bases of fine portraits, and to succeed with them, we must, as much as possible, employ only actinic light in the reflections.—La Lumiere. • PHOTOGRAPHIC CHEMICALS: Their Manufacture, Adulteration and Analysis. Lime Salts (continued).—Amongst the plans suggested for preventing the incrustation of boilers by the precipi tation of the carbonate, or sulphate, of lime contained in the feed water, perhaps the best is due to the ingenuity of Mr. Spiller. He adds caustic soda, which is now prepared at a cheap rate, to the water in the boiler. This decomposes the lime salts forming carbonate and sulphate of soda, and, liberating caustic lime, rendering the accumulation of hard scale in the boiler an impossibility. Not only is this plan perfectly effectual in preventing incrustation, but it is equally valuable in removing the deposit when already formed. If a little caustic soda be added to the water with which a boiler is fed, it will gradually disintegrate the hard cake of carbonate and sulphate of lime lining it, and reduce it to a fine sand, which may easily be blown out at the lower taps, when the boiler is emptied. The plan has stood the test of many years' experience in some Government, as well as private, works. We have already explained that carbonate of lime, a per fectly insoluble body, becomes soluble in water in the pre sence of an extra quantity of carbonic acid. If any substance be added to this solution of carbonate of lime in carbonic acid, which will remove the excess of carbonic acid, will be at once precipitated, and it is by an application of this principle, that Dr. Clark has succeeded in softening hard waters by his liming process. A solution of caustic lime in water is prepared, and this is added to the solution of bicar bonate of lime in such proportion that the caustic lime shall just unite with the extra quantity of carbonic acid present. It thus becomes converted into carbonate of lime, which is precipitated in the soluble form, and, at the same time, by removing the solvent for the original carbonate of lime, causes it also to be precipitated. The following equation expresses the action :— CaO + Ca02C0, = 2CaOCO.. Solution. Solution. Precipitate. The process is equally successful on a large, as on a small scale, and is, or was very recently, practically carried out at the works of one of the metropolitan water companies. It affords a curious instance of a chemical paradox—the removal of lime from water 1 by the addition of a fresh quantity of lime. The same chemist (Dr. Clark) has also devised a very useful test for the hardness of water, by means of a standard solution of soap. This determination is one of the utmost importance in an economical point of view, and the indications which it affords are of such great value to all who have to use quantities of water, photogra phers especially, that we shall give a brief account of the pro cess, as at present practised. The test consists in ascertaining the quantity of a standard solution of soap, or spirit, required to produce a permanent lather, with a given quan tity of the water under examination, the result being ex pressed in degrees of hardness, each of which corresponds to one grain of carbonate of lime in a gallon (70,000 grains) of water. The soap test is made by dissolving curd soap in proof spirit, in the proportion of about 120 grains to a pint. This is graduated by comparing its action upon a carefully prepared solution of chloride of calcium of such a strength that it shall represent a water of 16 degrees of hardness. This, as well as the soap test, can be purchased ready pre pared at the operative chemist’s. To apply the test, the water to be examined is introduced into a stoppered bottle, which should be half filled with it, and violently agitated, in order to disengage any free carbonic acid, which would increase the quantity of soap required to form a lather; the air in the bottle is then sucked out through a glass tube, and these operations repeated two or three times, until it is judged that the free carbonic acid is entirely removed. A hundred measures of this water arc then introduced into a stoppered bottle of twice that capacity, and the soap test very gra dually added from a bourette (the stopper being replaced, and the solution violently agitated from time to time) until a lather is formed, which remains for five minutes over the whole surface of the liquid when the bottle is laid down on the table. The carbonic acid should bo sucked out at inter vals from the upper part of the bottle. The number of measures of soap solution used is then noticed, and the hard ness of the water is then inferred directly from them by' reference to a table. Thus 3'2 measures of soap test indicate 1° of hardness, 7’6 soap test show 3° hardness, 11’6 soaptest show 5° hardness, &c. The indications given by the soap test are very valuaba not so much for their chemical accuracy, for they only gire rough approximations to the analytical results; but 0,1 account of their showing in a perfect manner the fitness, or unfitness, of a water for domestic purposes. For this reason, no analysis of a water is considered complete, unless it con tains the degrees of hardness according to Clark's soap test-. Sulphate of Lime, known more commonly as plaster o Paris, is a compound of universal application. It is found in the natural state in combination with two equivalents of water, as gypsum, alabaster, &c. When this native variety is heated to a little above the boiling point it loses the com bined water, and becomes then converted into the compound known as plaster of Paris. When moderately burned gypsum is mixed with water, it forms a paste at first; bul this quickly hardens, heat being evolved, and the water passing into the solid condition of water of crystallization-. Sulphate of lime is slightly soluble in water, 460 parts ot liquid being required for one part of sulphate of lime. Heat does not increase the solvent power of the water, but rather seems to diminish it. It is to the presence of this salt in some mineral waters that their excessive hardness is owing- Many, in fact, being saturated solutions of gypsum, from the water having trickled through a stratum of this earth. Chloride of Lirnc.—This, although strictly speaking an incorrect form of expression, has now become the recognized term for the mixture of chloride of calcium with hypochlorit of lime in equivalent proportions prepared by the action oi chlorine upon lime. This is prepared on an enormous seal® for bleaching and disinfecting purproses. Well-slaked limo is place, until it < fectly ca portions, Chloride smelling in well-c. When ex moisture, {as, and treated w tients ; X itranspa f hypocl Its loses i 'Unge li specially invert c (cording C Hen ex tfected, b (ormed ox •mken uj flime ar me. If, neath is loride c Bance wh Nfecting Me of li id to si * is the a d a more Ahuric, or PRINT It ’■rn you Bharks i ‘mpariso Bper, bu •nk this ‘e first p "two res the g "ver reta "st havi ।'face eit has "uk you fther of I L. al bum( Btison is "betweer I in a pai '•tiled at hers, s Vybt that give a C’emedie equal, i 0mposi nterminc /Pared s< ,, solutic is ne oum "alone. Read at t
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