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242 THE PHOTOGRAPHIC NEWS. [May 22,1863. of operating, which places little reliance upon, and has little need for, after “dodging " of any kind. William Cobbett, in advising the young literary student to acquire a certain and exact habit of writing, recommends him never to erase or alter a word, but to compel himself to send forth his matter as first written, with all its imperfections, so that the mortifications he must suffer, for awkward blunders, may so conduce to caution that he will gradually acquire a habit of precision and exactitude which requires no correction. We do not exactly give this counsel to young photographers ; we recommend them rather to destroy with ruthless hand their first crudities. But we do recommend them to acquire habits of certain manipulation, and to remember that in placing in their hands expedients of any kind, they are to be regarded as reserve power to be used in emergency, and not as encouragements to carelessness in one operation be cause the result can be modified in another. THE ENAMELLED PHOTOGRAPHIC PAPERS. Wb have already stated that some of the enamelled papers are prepared with oxide of zinc. We are now in a position to state that our original conjecture as to the nature of the enamel in some other specimens is correct. A sample we placed in the hands of an eminent analytical chemist proves to be prepared with sulphate of baryta in combination with al bumen. The material used is probably the ordinary mineral white (powdered heavy spar, or native Ba 0 SO 3 ), which is in this country largely employed in adulterating white lead. Mr. Cooper’s experience that it is insoluble in dilute mineral acids would be quite consistent with this material. We see no reason to apprehend any injurious influence from it, either to the silvered bath or to the finished photograph. PHOTOGRAPHIC CHEMICALS. Their Manufacture, Adulteration, and Analysis. Dismissino silicic acid for the present, we will pass over a series of metals which are either too rare or not sufficiently important, to require treatment in these chapters. The next body which claims attention from the importance of many of its compounds to the photographer, is chromium. This metal forms two or three oxides of interest to the photographer, the brown oxide, sesquioxide, and chromic acid. We will, in this case, reverse our usual order in de scribing metallic compounds, and treat of the acid first, as it is this body, in combination with potash, which is mostly used for photographic purposes, the lower oxides being generally the result of the action of light. Chromic acid forms carmine-coloured needle-shaped crystals, rapidly de liquescing in the air, and dissolving very easily in water, forming a dark reddish brown, or lemon yellow solution. The concentrated acid corrodes paper like oil of vitriol, and when in dilute solution, it likewise attacks paper and other kinds of organic matter, especially when aided by light or heat, becoming converted into the brown oxide or the green sesquioxide, according to the extent of the decomposition. Chromic acid is too energetic and unmanageable an agent to be used in the pure state for photographic purposes. It can, however, be converted into an acid salt with potash, in which form it possesses very valuable properties. Bichro mate of potash is the name given to this compound. It is composed of one part of potash and two parts of chromic acid. In most places, bichromate of potash can be purchased ready made, and then nothing more is necessary than purifi cation, before it can be employed for photographic purposes. It may, however, sometimes happen that bichromate of potash may be wanted when only the materials for making it can be obtained, and as its preparation is not difficult when the ordinary resources of a laboratory are available, we shall now give the most approved method of manufacturing this salt direct from the ores. Chrome iron ore (a mixture of sesquioxide of chromium with protoxide of iron) is finely powdered and ignited with an equal weight of nitre. The mass is raised to a bright red heat in an earthen or iron crucible, and kept at that temperature for a considerable time. It is then boiled out in water and filtered from the insoluble portion. The aqueous solution contains chromate, silicate, and albuminate of potash, together with nitrate and nitrate of potash. To this add nitric acid, until the alkali in the liquid is exactly neutralized, the acid being added in small portions at a time, until the lemon yellow colour of the liquid begins to change to reddish yellow. The alumina and silica will be by this treatment precipitated and can be filtered off. Evaporate the solution to the crystallizing point, and separate the resulting yellow crystals of chromate of potash. This salt is liable to contain as impurities, silica, alumina, chlorine, and sulphuric acid. From the first two it is separated by repeated crystallization, and the sulphuric acid is removed by adding a little nitric acid and adding nitrate of baryta until there is no further precipitation of sulphuric acid. To the filtrate nitrate of silver is added as long as chloride of silver is formed ; afterwhich the solution is filtered, evaporated to dryness, and the residue ignited in a platinum crucible. The residue is dissolved in water, and the solution left to crystallize. Yellow chromate of potash has an alkaline reaction and a cooling persistently bitter and metallic taste ; it is permanent in the air, it is tolerably soluble in water, dissolving in twice its weight at the common temperature, and in about 1} times its weight of boiling water. It is insoluble in alcohol. It possesses remarkable colouring properties: one part of the salt imparting a dis tinct yellow colour to 40,000 parts of water. The mono-chromate of potash is very little used in photo graphy in comparison to the bichromate; it is, however, the salt from which the latter compound is always prepared. Take a saturated solution of yellow chromate of potash, add nitric acid to it, and evaporate the liquid at a gentle heat to half its volume : bichromate of potash crystallizes out on cooling, together with nitre. The two classes of crystals are so dissimilar to one another that there will be no difficulty in separating the two salts by mechanical means. The bichromate forms bright-red rectangular, four-sided prisms, whilst the nitre occurs in smaller colourless crystals. The red salt must be redissolved in water and purified again by crystallization. It crystallizes very readily in large prisms which have a cooling, bitter, and metallic taste, redden litmus, and are permanent in the air. When three parts of this salt are heated with four parts of oil of vitriol oxygen gas is evolved. According to Balmain, this method gif® perfectly pure oxygen, and at a cheaper rate than when chlorate of potash is used. The great use of bichromate of potash in photographic operations is due to the ready way in which it parts with some of its oxygen to organic matter under the influence of light. Bichromate of potash may be looked upon as con taining one part of chromic acid in the free state. Now chromic acid has a large quantity of oxygen locked up in 14 (three equivalents of oxygen to one equivalent of chromium) and it is somewhat of an unstable compound, having a ten dency to give up oxygen and pass to a lower state of oxida- tion at the slightest opportunity. When placed in contact with an organic substance, such as paper, gelatine, leather, horn, parchment, the skin, &c., the oxygen of the chromic acid is ready to unite with the carbon and hydrogen in these bodies. The change goes on, however, very slowly in the dark, requiring the stimulating action of light to set it up, but under this influence it speedily passes to the state of brown oxide Cr O 2 , and then to the sesquioxide Cr 2 0, By some chemists the brown oxide is looked up as a com pound of chromic acid with sesquioxide of chromium. This appears a very reasonable view of the question. Chromic acid forms salts by union with most, if not all, bases; and on the other hand chromic oxide (sesquioxide of chromium) forms salts by union with most acids. Why, therefore, should not chromic acid form a salt by union with chromic