Sir David Brewster appears to have been the first who analysed coloured light with a prism; and in 1822 Sir John Herschel, besides having made a series of observations on coloured flames, had determined the spectra of the muriates of strontia and lime, the chlorides and nitrate of copper and boracic acid; and observes that ‘the colours thus communicated by different bases to flame afford, in many cases, a ready and neat way of detecting extremely minute quantities of them.’[[15]]
The same opinion was afterwards formed by Mr. Fox Talbot, who after many experiments on metallic salts, says in his paper,[[16]] that a glance at the prismatic spectrum of a flame may show it to contain substances which it would otherwise require a laborious chemical analysis to effect. In that paper this gentleman noticed that the glowing salts of lithium and strontium give a crimson or red colour to flame so exactly of the same tint that if these metals were in combination it would be impossible to decide to which metal the colour is due. But when he passed their respective lights through a prism, he found that the bright lines on their spectra are entirely different. ‘The strontia flame,’ he observes, ‘exhibits a great number of red rays well separated from each other by dark intervals, not to mention an orange, and a very definite bright blue ray. The lithia exhibits one single red ray,’ Whence Mr. Fox Talbot observes, ‘I hesitate not to say that optical analysis can distinguish the minutest portions of these two substances from each other with as much certainty, if not more, than any other known method.’ Thus Sir John Herschel and Mr. Fox Talbot laid the foundation of a spectrum analysis of unrivalled delicacy and beauty, since carried to perfection by Messrs. Bunsen, Kirchhoff and other experimenters, presently to be mentioned.
M. Bunsen detected the characteristic crimson lithium line in the spectra of numerous substances; in granite, in the earliest geological strata, in meteoric stones, in the ashes of most land plants, in blood and other animal matter; so that instead of being one of the rarest metals, it exists in all the three kingdoms of nature. In the year 1857 Mr. Swan gave an instance of the extreme minuteness of spectrum analysis, by detecting the 1⁄2,300,000th part of a grain of salt by its yellow light; but by the same reaction M. Bunsen not only recognised the 180 millionth part of a grain of sodium, but found that there is hardly any substance that does not contain it. It exists in the dust on our clothes and furniture, particles of it float in the air we breathe, so that while examining the spectra of other incandescent substances, flashes of yellow light appear as these atoms are volatilized and instantly burnt up, which shows that common salt is perhaps more universally diffused than any other kind of matter.
By spectrum analysis, M. Bunsen has discovered the two new metals, rubidium and cæsium. While examining with a prism the spectrum of the hundredth part of a grain of an alkaline substance separated from the residuum of the Durckheim mineral water, he saw coloured lines, which he had never seen before on the spectrum of any other alkali, and at once concluded that they belonged to a new metal; and having obtained about 200 grains of the substance by the evaporation of forty tons of the water, he found that they contained the chlorides of the two new metals in question. Moreover he perceived that these metallic chlorides resemble the chloride of potassium so nearly in spectrum and chemical character, that a refined prismatic analysis could alone determine the difference. He thus ascertained that the spectra of all the three have two red lines in the red part of their spectrum, and two violet lines in the indigo, while the middle part is occupied by a continuous diffused light. The only difference is that the two red lines in the rubidium spectrum are less refrangible than the red lines in the potassium spectrum, and that the cæsium spectrum is distinguished by two bright blue lines in the diffuse middle part. Rubidium received its name from rubidus, on account of the dark red of its lines, and cæsium from its sky-coloured blue lines.
M. Bunsen thinks that there can hardly be a doubt of rubidium having been mistaken for potassium, but he has shown that they may be distinguished by the difference in the solubility of the double salts which the chlorides of these two metals form with the chloride of platinum. An aqueous solution of the bichloride of platinum and potassium gives an insoluble yellow precipitate, consisting of the bichlorides of platinum and potassium. An aqueous solution of the bichlorides of platinum and rubidium gives an insoluble yellow precipitate of the bichlorides of platinum and rubidium. These two precipitates are undistinguishable to the eye. Now if a solution of platinum be added to the first, no further precipitate can take place, but if a solution of rubidium be added to it, a yellow precipitate is formed consisting of the bichloride of rubidium and potassium, because the chloride of rubidium resolves the precipitate, combines with the chloride of potassium, and sets the chloride of platinum free. Thus the precipitate of the bichloride of rubidium and potassium is the least soluble of the two. The yellow colour is evidently due to the potassium. Cæsium may be distinguished from potassium by the same process. The carbonates, hydrates, and other salts of the two metals were determined; their carbonates were shown to be readily separated, because the carbonate of cæsium is soluble in alcohol, which the carbonate of rubidium is not, and finally the metal rubidium was separated. It has an extreme avidity for oxygen, and burns in water like potassium, and possesses many other analogous qualities. It melts at the temperature of 38·5° Cent., and has a specific gravity of 1·516. Rubidium is abundant in the mineral lepidolite in many parts of Europe and North America, and M. Grandeau has detected it in the ashes of beetroot, tobacco, coffee, tea, and grapes by spectrum analysis. It exists in various mineral waters, and in fact is very general. Traces of various metals are met with in the same vegetable; thus the spectrum of tobacco gives lines characteristic of lithium, potassium, rubidium, and lime.
Mr. W. Crookes discovered the new metal thallium by means of its spectrum, which differs from every other in having one bright green line upon a dark ground. He obtained its various salts, and the metal itself, which he describes as being heavy, dense, and very like lead, but of greater specific gravity. Its fresh surface has a bright metallic lustre, not so blue as that of lead, but it tarnishes more easily. It is so soft that it can be indented by the nail, yet it can be drawn into wire, and in chemical properties it resembles mercury, lead, and bismuth. Altogether it is more like a metal than a metalloid, perhaps something between the two. Thallium is completely volatilized at a temperature below red heat, whether single or in composition. If the quantity be small, the green line appears in a sudden flash, lasting but the fraction of a second. If a larger quantity of the metal be gradually put into the flame, it lasts a little longer, appearing as a single green line of extraordinary purity and intensity, sharply defined on a black ground. With respect to volatility, thallium is analogous to the non-metallic element selenium, which is so volatile that its beautiful blue light only lasts a few seconds. The green light of thallium comes out more rapidly, and with less of the substance, than the blue light of selenium, a quantitative distinction which accords with Dr. Miller’s observation that the rapidity with which a result is obtained, and the minuteness of the quantity required for the examination, gives this method a superiority over every other for the qualitative analysis of the alkalies and alkaline earths. Thallium has been detected in mineral waters, wine, treacle, tobacco, and chicory.
Drs. Reich and Richter discovered a fourth new metal in the zinc-blende at Freiberg in Saxony, which has been called indium, from two beautiful indigo-blue lines in its spectrum, which have a greater refrangibility than the blue lines in strontium. The chemical relations of indium resemble those of zinc, with which it is associated in nature. The metal can be reduced before the blowpipe into a bead, which marks paper and has the colour of tin.
The practical importance of spectrum science has been beautifully illustrated by Professor Roscoe by its application to overcome a difficulty in Bessemer’s process for the manufacture of steel. According to that process, steel is made by sending a blast of air through a quantity of melted iron; the difficulty was when to stop the blast, for if stopped too soon, the metal retains so much carbon that it crumbles under the hammer; if continued a few minutes too long, the molten metal is so viscid that it cannot be poured into the moulds. Experience had hitherto enabled the manufacturer to judge of the right time from the appearance of the flame which issued from the mouth of the converting vessel, but now Professor Roscoe has determined the exact moment for cutting off the blast by a spectral examination of the flame, the light of which is most intense. The flame spectrum in its various phases revealed complicated masses of dark absorption bands and bright lines, showing that a variety of substances were present in the flame in a state of incandescent gas; and by a simultaneous comparison of these with well-known spectra of certain elementary bodies, Mr. Roscoe ascertained the presence of sodium, potassium, lithium, iron, carbon, phosphorus, hydrogen, and nitrogen in the flame.
Both Dr. Wollaston and Fraunhofer noticed that the spectrum of the electric spark was crossed by bright-coloured lines; and in the year 1835, Professor Wheatstone determined the spectra of the electric spark taken from fused zinc, cadmium, tin, bismuth, lead, and from mercury, and found that each is crossed by bright lines differing in number, position, and colour, but which are the same whether the electric spark be from a static, voltaic, or magneto-electric machine. Having given a plate showing the colours of these bright lines on the respective spectra, he proved that they are not owing to the electricity, but to the incandescent atoms of the metals, for by using different metals as terminals to the conducting wires, he determined the spectra of these metals in vacuo, which proved that they were due alone to the volatilization of the metallic terminals, and concluded that any one metal may be distinguished from another by the appearance of the spark.
Wheatstone discontinued his spectrum researches, for he had invented the electric telegraph, and was busy in extending the first telegraphic wire that ever carried the thought of man to man between London and Manchester. Soon after he laid the first aquatic line across the Thames, and he has lived to see his telegraphic lines spread over the surface of the earth and the bottom of the ocean.