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General and Physical Chemistry.

Fluorescence of Manganese and Bismuth. By L. DE BOISBAUDRAN (Compt. rend., 104, 1680-1685; see also this vol., pp. 3, 4, and 189). The author has investigated the behaviour of (1) two solid solvents, and an active substance giving fluorescence with each of the solvents, as represented by mixtures of cadmium, zinc, and manganese sulphates, and calcium, magnesium, and manganese sulphates; (2) two solvents and an active substance giving fluorescence with only one of them, as represented by mixtures of barium, calcium, and manganese sulphates; (3) two solvents behaving towards each other as moderately active substances, and an active substance which fluoresces brilliantly with one only of the solvents, as represented by zinc, calcium, and bismuth sulphates, and calcium, cadmium, and bismuth sulphates.

In the first case, the effects of the two solvents are practically equal when they are mixed in the proportions of their molecular weights. In the other cases, the presence of the indifferent substance reduces the brilliancy of the fluorescence, but the latter is still distinctly recognisable, even when the proportion of the active substance is very small. C. H. B.

Variations in the Absorption-spectra of Didymium Salts. By H. BECQUEREL (Compt. rend., 104, 1691-1693).-The results previously obtained (this vol., p. 537) indicate either that ordinary didymium is a mixture of a large number of elements, or that its compounds as usually obtained are mixtures of the salts and sub-salts of a smaller number of elements in different states of oxidation.

The absorption-spectra of dilute solutions of different compounds of didymium differ but little, whilst the absorption-spectra of the same substance in a crystallised condition are very different. When the water and acid of a hydrated didymium salt are gradually driven off by heating, a series of products is obtained, some of which are transparent, whilst others are pulverulent or opaque, but all of them show absorption-spectra by reflection, and these spectra consist of very different bands. A table is given showing the principal absorption-bands of different didymium-compounds prepared in the way described.

In the spectra of the products obtained by successive alterations of the same compounds, there are series of bands, which are transferred from one spectra to another in such a way that the variations in the number of luminous vibrations or the inverse squares of the wavelengths of corresponding bands are sensibly constant for each series. Each spectrum shows several of these series of bands with constant differences. The bands characteristic of several of these compounds coincide with the variable bands in the spectra of the crystals (loc. cit.). C. H. B.

VOL. LII.

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Orthochromatic Photography. By C. H. BOTHAMLEY (J. Soc. Chem. Ind., 6, 423-433). It is well known that gelatino-bromide plates, that is, plates coated with a dry film of an emulsion of silver bromide in gelatin, fail to give photographic representations of coloured objects with correct gradations, or in other words, with their proper degrees of relative brightness. They show a maximum sensitiveness to blue and violet, are much less sensitive to green, and are only affected by yellow, orange, and red when prepared by special methods, and when the exposure to light is greatly prolonged, whereas the eye is most sensitive to yellow, less sensitive to green and orange, and only slightly sensitive to blue, violet, and red. The term "orthochromatic" photography denotes the photographic representation of coloured objects in monochrome with correct gradations, and it is obvious that in order to attain this end the character of the plates must be altered in such a way that the sensitiveness to yellow, orange, and green is greatly increased, whilst the sensitiveness to blue and violet is reduced.

Vogel (Ber., 6, 1302) discovered in 1873 that the addition of certain dyes, such as corallin, to sensitised collodion films, made them highly sensitive to yellow or greenish-yellow rays, and this observation was confirmed and applied in photographic practice. In 1883 Attout and Clayton patented the application of eosin to gelatin plates, and their observation has since been confirmed and greatly extended by other observers, and the application of dyes to photographic plates has become of great practical importance. Eder has investigated the effect of a large number of dyes (Sitzungsb. d. Kais. Akad. der Wissensch., Vienna, 1884, 1885, 1886; see also Abstr., 1885, 703; 1886, 405, 497, 958). Very few are found to exert any useful sensitising action. Cyanin and some of the eosin dyes produce the greatest effect, whilst naphthol-blue, the neutral blue of the Frankfort AnilinFarben-Fabrik, and coeruleïn S. are remarkable, because they render the plates sensitive without interruption to the entire length of the spectrum from λ 3600 to 7600. The dyes are either added to the melted emulsion before it is poured on the plate, or the dried plates are immersed in dilute solutions of the dyes and again dried. The curves representing the action of the prismatic spectrum on plates thus prepared show two maxima, one in the blue, corresponding with the ordinary sensitiveness of the gelatino-bromide, the other in the less refrangible half of the spectrum, representing the sensitiveness due to the presence of the dye. This second part of the curve corresponds but does not coincide with the absorption-band in the spectrum of the dye (Abstr., 1886, 958). Between the two maxima there is a region of minimum action, which is generally in the green, and when the exposure to light is short the effect of this part of the spectrum is inappreciable, and the action is represented by two detached curves, one in each half of the spectrum.

Different commercial samples of the dyes have been found to produce very different effects, and the author has compared a number of samples of dyes of the eosin-group by examining their absorptionspectra, &c. The chief eosin colours are eosin, phloxin, erythrosin, and rose Bengal, which are the sodium or potassium salts of tetra

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brom fluoresceïn, dichlorotetrabromfluoresceïn, tetraiodofluoresceïn, and dichlorotetraiodofluorescein respectively, the different commercial varieties being distinguished by letters. Eosin A, eosin JJ, eosin VE, and eosin SGF, were found to be practically identical. Erythrosin, erythrosin extra, erythrosin I, and erythrosin RE, are also identical, and rose Bengal is indistinguishable from rose Bengal B. samples of phloxin on the other hand were found to be variable in their properties. Erythrosin B (Casella and Co.) seems to be a mixture of eosin with either the true erythrosin or rose Bengal, and erythrosin BE is really rose Bengal. One sample of erythrosin lost 8.8 per cent. at 115-120°, whilst another lost 12.5 per cent. Attention is called to the relation between the optical properties of these dyes and their molecular weights. The absorption-band becomes narrower, more intense, and less refrangible, the fluorescence rapidly diminishes, and the tint of the dye becomes bluer as the molecular weight increases. Fluorescein and eosin are brilliantly fluorescent; erythrosin and rose Bengal do not fluoresce at all.

The effect of the prismatic spectrum was also investigated. Commercial dry plates were immersed for two or three minutes in water, and then for two minutes in a solution of 1 part of the dye in 10,000 parts of water, and allowed to dry in the dark; others were immersed for two or three minutes in strong ammonia diluted with 100 vols. of water, and then for two minutes in a solution of the dye (1: 10,000) to which 1 per cent. of ammonia solution had been added. Plates prepared in this way were exposed to the prismatic spectrum of light from burning magnesium, the same length of ribbon being burnt for each exposure. This source of light was selected because it contains a very high proportion of rays of high refrangibility which tend to correct the known defect of the prismatic spectrum, that is, the abnormal extension of the more refrangible rays. When a sufficient length of magnesium ribbon is burnt at a uniform rate, the effect of inequalities is eliminated and a practically constant unit of exposure is obtained. The exposed plates were developed with alkaline pyrcgallol in the usual way, and the results are expressed in the form of curves. The general character of Eder's results is confirmed, but important differences of degree were observed. Each curve has two maxima separated by a region of minimum action. Eosin in aqueous solution exerts a comparatively slight though distinct sensitising action, and with commercial phloxin the effect is slightly greater. Erythrosin produces by far the greatest effect of the dyes of this group, and rose Bengal is much superior as a sensitiser to eosin, although inferior to erythrosin. The maximum effect of erythrosin and rose Bengal is exerted somewhat on the more refrangible side of D. Cyanin differs from eosin, &c., in that it sensitises for yellow, orange, and orange-red, instead of for greenish-yellow and yellow, the maximum action being exerted between D and C. The results with this dye entirely confirm those previously obtained by Eder and by Schumann. When ammonia is applied with the dye the magnitude of the sensitising action is increased, and extends to a greater distance on either side of the maximum, thus tending to obliterate the region of minimum action. The difference between the effect of

aqueous and ammoniacal solutions is greatest in the case of eosin, but even in presence of ammonia this dye produces less effect than aqueous erythrosin. Erythrosin in presence of ammonia is the most efficient sensitiser that has yet been suggested. The most important result obtained, however, is that plates dyed with aqueous or ammoniacal erythrosin, ammoniacal rose Bengal, or ammoniacal cyanin, are more sensitive to the yellow or orange-yellow of the prismatic spectrum than to the blue and violet, and this even with a source of light which is extremely rich in rays of high refrangibility. When burning magnesium is the source of light, plates dyed with aqueous erythrosin or ammoniacal rose Bengal are about half as sensitive again to the yellow as to any part of the blue or violet; plates dyed with ammoniacal erythrosin are somewhat more than twice as sensitive to yellow as to blue or violet; plates dyed with ammoniacal cyanin are about half as sensitive again to orange as to blue or violet. The difference between these results and those of Eder is probably due to the fact that in most of the latter's experiments the dye was added to the melted emulsion, and the bath solutions used were weaker than those employed by the author.

At present it is not possible to explain why these dyes act as sensitisers whilst others do not. No connection can be traced between the physical and chemical properties of the dyes and their sensitising action. Eder has shown (loc. cit.) that the dyed silver gelatinobromide shows an absorption-band identical in position with the sensitising effect, this band being somewhat less refrangible than the absorption-band of the dye alone or mixed with gelatin, owing to the presence of the dense silver bromide. Eder has also found that when the dye is once added to the silver bromide it cannot be removed by prolonged washing, and he concludes that the dye forms a molecular compound with the silver salt, which is decomposed by the rays which it absorbs. Some experiments by Abney indicate, however, that the dye is first decomposed by light, and the products of its decomposition reduce the silver salt when the developing solution is applied. The author points out that the dyes which exert the greatest sensitising action contain a number of atoms of bromine or iodine, have a very high molecular weight, and a complex molecular constitution, and he suggests that the weight and complex structure of the molecules may confer upon them the power of entangling and arresting the ether waves to a greater extent than lighter and simpler molecules.

For practical purposes the results with the spectrum must not be interpreted too literally. The difference between the brightness of pigments is never so great as between the brightness of the corresponding colours of the spectrum, and, moreover, all coloured objects reflect more or less unaltered white light which tends to reduce the contrasts due to differences in colour. The curve representing the action of the spectrum on plates dyed with ammoniacal erythrosin approximates to the curve representing the action of the spectrum on the eye, and this dye is the most useful sensitiser for general purposes. In order to obtain correct gradations it is necessary to reduce the intensity of the blue and violet rays, to which the plates remain very

sensitive, and this is done by interposing a transparent yellow screen between the object and the lens. The depth of tint of the screen determines the proportion of blue and violet cut off, and thus affects the result. With gas or lamp light a yellow screen is unnecessary.

The author has photographed various coloured objects, such as pottery, flowers, paintings, and landscapes, with plates prepared with ammoniacal erythrosin by the method described above, and by a slightly different method previously recommended by Mallman and Scolik (Photo. Journal, 1886). With ordinary plates, even under the most favourable conditions, yellow, orange, and red are represented as almost black, whilst green is far too dark, and blue and violet are practically white. With the dyed plates without any yellow screen, yellow, green, and orange are more satisfactorily rendered, but blue and violet remain far too light. When, however, the dyed plates are used with a yellow screen before the lens the results are very satisfactory; yellow objects come next to white in brightness, and the other colours are represented in their proper gradations. The plates are, however, deficient in sensitiveness to red. In landscape work, in addition to the better rendering of foliage, &c., a great advantage is gained by the fact that the yellow screen cuts off the blue atmospheric haze. The interposition of the screen of course necessitates a longer

exposure.

Dyed plates have already been largely employed, with great advantage, in the reproduction of pictures, and are also found to be valuable in micro-photography and in stellar photography. Hasselberg, of Pulkowa, has used dyed plates with success in photographing the less refrangible end of the spectrum, and he recommends the following solutions, which are applied to the plates after the latter have been immersed in dilute ammonia:-from C to 5600; alcoholic cyanin solution 1: 400, 2 parts; ammonia 1 part; water 100 parts: from λ 5600 to F; chrysaniline solution (1: 1000) 3 parts; eosin solution (11000) 5 parts; ammonia 1 part; water 100 parts. Eosin sensitises for yellowish-green and chrysaniline for green. C. H. B.

Conductivity of Mixtures. By E. BOUTY (Compt. rend., 104, 1699-1702). The author explains certain methods of calculating the conductivity of dilute solutions of mixed salts from the known conductivity of its constituents. The methods and results will be more fully developed in a subsequent paper.

C. H. B.

Phosphates of the Alkaline Earths. By BERTHELOT (Compt. rend., 104, 1666—1667).—A note on a recent paper by Joly.

Trimetallic Phosphates. By A. JOLY (Compt. rend., 104, 1702— 1705). The formation of the colloidal modification of sodium strontium phosphate (this vol., p. 637) from trisodium phosphate and strontium chloride develops +153 Cal., whilst the formation of the same salt in the crystalline condition, NaSrPO, + 9H2O, develops +504 Cal. When free from sodium chloride, the crystals are very slightly decomposed by cold water. Decomposition takes place more

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