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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1275, February 9, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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FEBRUARY 9, 1883.] THE PHOTOGRAPHIC NEWS. 93 slit to fall on such an arrangement, a spectrum will be formed on a screen placed behind the prisms in the neighbourhood of the point in which the incident light would, if produced, meet the screen. Light of one definite refrangibility passes through the combination without deviation ; the other rays are slightly deviated, and form a spectrum. The figure shows the path of the light through such a spectroscope with five prisms, three of crown and two of flint glass. “ To see the spectrum formed by a given source of light, we have only to hold the apparatus between our eye and the light at such a distance that, if the prism were removed, we could see the slit distinctly. The spectrum formed may be magnified by means of a convex lens placed between the observer’s eye and the slit, and focussed on the slit. Small pocket instruments of this kind, which show distinctly the dark lines in the solar spectrum, are made by Hilger, of Tottenham Court Road, London, and other opticians. “ Larger instruments of the kind are fitted with a collimating lens and a telescope to view the spectrum formed, and are frequently used for observations which do not require a very high dispersive power. “ Let us now turn the slit of the spectroscope towards the non- luminous flame of a Bunsen gas-burner, and introduce in turn some of the salts of the various metallic elements into the hottest portion of the flame. The salt becomes volatilised by the heat, and the vapour rises and colours the flame. Thus, if we place on a small platinum spoon a little common salt or chloride of sodium, the flame glows with a brilliant yellow light, and on looking through the spectroscope we see the spectrum consists of a narrow line of yellow light. If the instrument be sufficiently powerful we should find that this narrow line is really double—that there are, in fact, two narrow lines very close together. Some other faint lines which occur in the sodium spectrum would not be visible. If for the chloride of sodium we substitute that of some other metal which can be volatilised at the temperature of the Bunsen burner, the colour of our flame alters, and the spectrum changes too. Thus, strontium colours the flame red, and its spectrum consists of a number of lines in the red, with an orange line somewhat less refrangible than the sodium line, and a line in the blue part of the spectrum. Lithium, too, colours the flame red, and with the naked eye it would be difficult to say whether the red colour of a given flame was due to the presence of strontium or of lithium ; but the spectroscope tells us at once, for the spec trum of lithium consists of one brilliant red line with three fainter lines respectively in the orange, green, and greenish blue. A potassium salt will give us two lines in the red and one in the violet, while the flame would appear to be of a violet tinge. Barium will colour it green, and calcium a yellowish red. The spectrum of calcium, seen in this way, consists of two lines in the red, four in the yellow, one in the green, and one in the violet; while in that of barium we have a series of brilliant lines in the green and greenish blue, with others in the red and orange. “ Again, it is immaterial what salt of these metals we use—we always get the same spectrum. We may put into our flame either chloride, carbonate, or nitrate of sodium—we shall still have the two yellow lines close together—and we may use these two yellow lines as a mark of the presence of incaudescent sodium vapour. Whenever we see them we know that sodium is present in the flame, and so delicate is the test that Bunsen and Kirchhoff have calculated that the eighteen-millionth part of a grain can be detected by it. In fact, it is difficult to obtain a flame free from some trace of sodium vapour. “ The presence of any other of these metals may with equal certainty be detected by its spectrum. We wish to determine if a certain salt contains lithium. Volatilise it in the Bunsen burner, and examine the spectrum. A little practice enables the observer to detect the distinctive lines of each metal at once, and a glance is almost sufficient to decide—if, for example, the bright red line lithium is present. If there be a doubt as to whether the light seen is really the lithium line, introduce into the field the spectrum of lithium by means of the reflecting prism at the n of the spectroscope, and notice if the red line observed, and that of lithium, really coincide.” Corxesponence. COLONEL DAWSON’S QUERIES, AND A REPLY TO “CALX.” Sib,—May I ask the favour of the insertion of this letter in answer to the questions raised by Colonel Dawson in a recent number of the News. With regard to the first question, I may state that in my experience the addition ofzinc-bromide previous to emulsifi cation accelerates the formation of the blue condition, for in one experiment I found that by heating the mixture to 200° F. for ten minutes, the violet stage was arrived at; but one objection to using this salt is, that when its solution is boiled, the substance is decomposed into hydrated oxide of zinc and hydrobromic acid, thus accounting for the Colonel’s observation that the acidity of the solution in creases as the boiling progressrs. The curious effect observed during the development of plates prepared by this method is not peculiar to the bro mide of zinc emulsion, but may be noticed with other sensitive films. Presuming that the phrase, “on adding silver to a solu tion of bromide of zinc,” should be, “ on adding silver nitrate to a solution of bromide of zinc,” the first action that takes place would be that the electro-positive metal potassium would seize upon the stronger acid (nitric), form ing potassium nitrate and leaving the silver and bromine to form silver bromide, thus :— KBr ZnBr,+AgNOa =AgBr-ZnBr,+ KNO, If sufficient silver nitrate be added to decompose both the bromine compounds, the following equation represents the reaction— KBr ZnBr,++3AgNO,=3AgBr+KNO,=Zn(NO3), When ammonia is added to an aqueous solution of zinc bromide, hydrated oxide of zinc (not metallic zinc) is formed, thus— ZnBr++2NHHO=Zn(OH)2++2NHBr The concluding obseivation may be explained, I think, by saying that several saturated saline solutions have the property of precipitating gelatine from an aqueous solution, and one can well understand that when the dry salt is added to the solution of gelatine, it withdraws the water, and precipitates that which is immediately in contact; and when a substance is thus precipitated, it is not easily re-dissolved. Passing from “ Colonel Dawson’s Queries,” I must add a few words on the criticisms offered by “ Calx ’’ regarding my communication on “ The Phosphorescent Tablet as a Standard Light.” Your correspondent supposes that because 1 described only the experiments performed with one sample of a luminous substance, I have, therefore, never experi mented with other samples of this and other phosporescent substances. Now it so happens that not only have I ex perimented with different samples of Balmain’s paint and with phosphorescent substances prepared by heating sulphur with calcined oyster-shells, and again with calcined calcite, and by reducing calcium sulphate with organic matter, but the experiments of Seebeck and Becquerel forty years ago, and later, the results of Heaton, Cathcart, and others with Balmain’s luminous powder, show that the light given off from a phosphorescent body at the ordinary temperature (say 15° 0.) increases when it is heated with hot water, and decreases when cooled with ice. Taking into consideration the above results, and my experiments with the four test tubes coated with luminous paint (see PuOTOaRAPMIC News, page 59), 1 think I was quite justified in stating that “a phosporescent surface is capable of absorbing varying amounts of light at different temperatures; the lower the tempera ture, the greater the amount of absorption," at any rate between the limits of 0° and 80° C. If “Calx” has found a phosphorescent substance which does not follow this rule, he would do a good service to science by publishing his discovery in the usual way.
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