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Avaust 21, 1885.] THE PHOTOGBAPHIC NEWS 533 precipitate by decantation, can an absolutely impalpable powder be obtained. This is a very long process, owing to the time taken by such a fine substance to settle; a little barium-sulphate of this fineness, say five to ten per cent, in weight of the gelatine and starch employed, can be added without giving a perceptible grain to the surface. Insufficient washing may be recognized by the paper being off colour, presenting a more or less darkened surface ; un less the precipitate has been most thoroughly washed, a white surface will not be obtained. The proportions of starch and gelatine we have found most suitable have ■ aried between the following :— Best arrowroot .. ... ... 8 grammes White hard gelatine ... ... 6 ,, Water ... 200 c. c's. aud— Best arrowroot 6 grammes White hard gelatine ... ... 10 „ Water 200 c. c’s. Prints produced on the first paper may be burnished satisfactorily, but those made in proportions similar to the second are better suited for stripping from waxed or talced glass. The best plan we find is to melt the gela tine first in about half the water according to proportions used, then heat the arrowroot in the remaining portion of water as in making ordinary starch paste for mounting purposes, and mix it with the melted gelatine, raising the temperature for a moment to boiling point after mixture has taken place. IC it is intended to employ barium sulphate, that sub stance should be soaked with the gelatine in the propor tion of cold water already named, and stirred in briskly during melting. A fine precipitate of another kind is easily obtained during the process of salting, and this lends a little useful opacity to the preparation. The substance in question is barium citrate, and is produced by employing both barium chloride and sodium citrate for salting. It is soluble only in excess of sodium citrate with heat, and from the solution silver nitrate precipitates a salt which blackens freely in the light. It is rather a matter of prolonged experiment to prove exactly what proportion of the two substances and of a soluble chloride should be added to the gelatino-starch compound as above. We give, however, the following as a practical formula by means of which we have obtained vigorous prints. Sodium chloride 2 grammes Barium chloride 1 2 „ Double tartrate of sodium and potassium 1'2 „ The above are dissolved together in about 20 c.c’s. of water, and well stirred into the starch compound; then the following is also dissolved in about 10 c.c.s of water Sodium citrate ... 2 5 grammes Citric acid 1 gramme This is stirred into the melted starch and gelatine as before, and when the whole is thoroughly mixed, it is well topass it through a filter composed of cambric or swansdown, and coat the paper before setting takes place. When larger quantities are dealt with it may be advantageous to effect a thorough mixture by a mechanical arrangement resem bling a churn. Carbon tissue is prepared in that manner, and the result appears to be quite satisfactory. Methods of coating paper are set forth on page 426, so do not require repetition here, since alt that has been said concerning the modes of coating paper with a sensitive emulsion applies equally in the present instance, with one exception only. We refer to the character of illumination employed. Ordinary daylight unprotected by coloured screens may be used throughout the stages of coating, drying, flattening, packing, &c., up to rendering the paper sensitive to light, for, until the paper has been in contact with nitrate of silver, it 18 as insensitive a? unsensitized albumen paper. Our suggestion for the use of auric-chloride to accele rate toning can be applied here if thought desirable. To the above quantities we add two cubic centimetres of the gold solution referred to in the previous article, page 467, and thoroughly mix the same just before coating. The following modification is suited, when, for obvious reasons, it is not desired to form a precipitite in the film, a clear coating rather than that of an enamel-like or egg shell surface being preferable. Under these circumstances barium, either soluble or insoluble, is not made use of. We take: — I. Sodium chloride ... ... ... 3 grammes Double tartrate of potassium and sodium 1 gramme Water 20 c.c.s II. Sodium citrato 2 grammes Citric acid ... ... ... ... 1 gramme Water ... 20 c.c.s No. I. is first incorporated with the melted gelatine aud starch mixture, after which No. II. is in like manner added. The tartrate salt can be omitted or the amount of citric acid varied at pleasure. Should potassium acetate be employed in conjunction with barium, the paper will not keep so well after sensitizing, and there is a basic barium acetate precipitated, which adds to the opalescence of the mixture, otherwise energy is slightly added by the use of an acetate salt. Sensitizing and finishing will be dealt with io our next article. HOW TO SUCCEED IN TAKING GOOD PICTURES ON GELATINE PLATES—No. IV. BY S. B. BOTTONE, By the means detailed in the last two papers, the amateur can form a very good idea of what exposure will give a good picture with his usual apparatus, plates, and conditions of working. But circumstances alter cases ; aud it is quite possible that he may not always time the exposure correctly—owing either to excess or deficiency in light, to having changed the make of his plates, or to having been driven by the restlessness of babies—and to give an exposure which he dreads may have been too short; or, lastly, by inadven ture, to having given one which he feels must have been too long. It may also sometimes happen that a friend, who has faith in his superior powers, sends him a plate to develop, without any details which may guide him as to length of exposure given, &c. We may therefore divide these developments which need control, into three great classes, viz. :—A, the under-ex posed ; B. the over-exposed ; and C, the unknown. Beginning with the under-exposed, we shall find that these fall naturally under two heads : those which are so very much under-exposed that the image is impressed only in the high lights ; aud those in which the image is im pressed in all but the very deepest shadows. The former never make <jood pictures ; the latter may, by a little coax ing, be made to give passable results. A. Having to deal with an under-exposed plate, we pour into a white porcelain dish (or a glass-bottomed one with a sheet of white paper below), a sufficient quantity of fresh developer made in the proportions given before, which I repeat here : — Saturated solution of ferrous sulp hate ... 1 part Saturated solution of potassium oxalate ... 3 parts N.B.—The solution must be saturate ; that is to say, the water used to dissolve the ferrous sulphate and the neutral
