WKCTKUM ANALYSIS.] 



UNDULATORY FORCES. LIGHT 



55 



white light is produced by burning phosphorus in a jar 

 of oxygen gas ; which has a similarly optical effect to thai 

 produced from the two previously named sources. 



When certain colours, especially blue and green, are 

 viewed by means of a gas or candle flame, it is founc 

 that, in many instances, the actual tint is quite lost ; for 

 some blues when thus seen appear of a green hue. This 

 is owing to the predominance of the yMow rays in the 

 source of light ; which, by mixing with the colour re- 

 flected from the object, produce a green hue. We have 

 already explained the reason of this when speaking of the 

 prismatic spectrum, as a result of the overlapping of two 

 colours. In commerce, it is therefore impossible to judge 

 of natural colours by ordinary artificial light ; and this 

 is well appreciated by ladies in purchasing articles of any 

 intermediate spectral tint. This fact is another, though 

 j partial, illustration of the laws of mono-chromatic light. 

 In connection with this subject, and also with a dis- 

 ! covery made by Fraiinhofer, we shall introduce some 

 remarkable facts lately published by MM. Bunsen and 

 Kirchkoff. These gentlemen, by means of the prism, 

 have been enabled to identify the peculiar colour of the 

 vapour of each metal whilst the solid is undergoing 

 combustion. Most of our readers will be familiar with 

 the colours produced by some metals when undergoing 

 combustion as seen in the blue flame of zinc, the red 

 of strontium, and the green of barium. Instances of the 

 two latter we mentioned in the previous page. Now 

 these are easily distinguished from each other when 

 separately in combustion ; but the eye might fail to 

 detect their presence when they are burned in contact 

 with other substances. As, however, each coloured or 

 mono-chromatic light has a peculiar effect of its own in 

 producing a special spectrum when refracted by a prism, 

 MM. Bunsen and Kirchkoff have availed themselves of 

 the fact, and have thus made the prism an instrument 

 which may be employed as an agent in chemical analysis. 

 As the discovery has only been made public but recently, 

 we cannot, at present, afford more information than that 

 which we append, and which is a condensation of their 

 pnMUhed experiments, extracted from the London Re- 

 new, of January and March, 1861. 



" MM. Bunsen and Kirchkoff have, by a complicated 

 arrangement of prisms and telescopes, arrived at such 

 certainty and delicacy in their results, as to be able to 

 see, by a glance at the most complicated mixture (of 

 burning metals), through the eye-piece of their instru- 

 ment, its constituents ranged side by side, each indivi- 

 dual metal glowing with its own peculiar colour. By 

 thus removing the interfering action which coloured fires 

 would naturally have if all were united and burnt toge- 

 ther, they were at once placed in possession of a very 

 delicate method of chemical analysis. Not only could 

 they have visible proof of the existence, in any mixture, 

 of a body capable of giving a colour to flame ; but this 

 effect was produced by such an inconceivably minute 

 quantity of the element as the mind almost fails to 

 comprehend the one hundred millionth part of a grain 

 being thus capable of detection. During their examina- 

 tion of these phenomena, every mineral substance which 

 came to hand was optically interrogated in this searching 

 manner, and in all cases with remarkable results bodies 

 which had hitherto been looked upon as so rare as to be 

 merely chemical curiosities, being seen to form constitu- 

 ent parts of most rocks, soils, and waters. Some mineral 

 waters, however, gave results which could not be accounted 

 for by any of the previously observed facts ; a new colour 

 starting up before their eyes, and occupying a hitherto 

 blank space in the glowing row of metals. Evidently 

 this was a new element, which has thus revealed itself to 



;ral inspection. 



" The next point was to collect some of the unknown 

 body. * * * * After boiling down twenty tons of 

 ral water, and submitting the residue to various 

 chemical agents, the latest accounts report that they 

 have at thoir disposal upwards of one hundred grains of 

 crcsium ; this being the name under which the new metal 

 enters the domain of science, given to it to recall the 

 colour which it communicates to flame." 



In another number of the same periodical (March 30), 

 a further account has been published respecting these 

 interesting facts ; from which we make the following ex- 

 tracts : 



" Mr. Crookes, the editor of the Chemical News, has 

 just discovered that the sulphur group of elements con- 

 tains another member. The appearance shown by the 

 spectrum of this new body, when a little of it is burned 

 in a gas flame, is very remarkable ; and, as the dis- 

 coverer has repeated before us many of his experiments 

 in his laboratory, we are enabled to give a full account 

 of the grounds upon which he bases his claim for the 

 elementary character of this newly discovered body. 



"Nearly every elementary substance in nature (and 

 recent developments of this branch of science would 

 almost warrant us in saying every element) possesses the 

 property of imparting certain colours to a name, provided 

 that the latter is hot enough to volatilise the body ; and 

 this holds good not only with the elements in the free 

 state, but to all their compounds. When these colours 

 are examined by a prism, in an appropriate spectrum 

 apparatus, they are, in most cases, resolved into a series 

 of brilliantly luminous lines of every conceivable tint 

 and colour some sharp and denned as if ruled by hand, 

 others shaded off at the edges, and having a nebulous 

 appearance ; but each system of coloured bauds being 

 invariably associated with the same element, and thus 

 being as accurate a test of its presence as the most rigid 

 chemical analysis could devise : in fact, far better ; for 

 chemists, being human, may err; whilst the laws of 

 light and refraction can never fail. " ****** 

 " The now body which we have mentioned above 

 as having been just discovered, came to light in this 

 way. A new mineral from the Hartz mountains having 

 given some very discordant results in ordinary chemical 

 analysis, a portion was subjected to spectrum examina- 

 tion, when the disturbing cause was at once seen in the 

 shape of a beautiful green line, glowing on a pale back- 

 ground of the same colour. Our experimentalist having 

 an intimate knowledge of the appearances which the 

 bodies present ought to give, at once pronounced this 

 line to be an interloper, and set to work to isolate its 

 cause. After numerous trials and failures, constantly 

 appealing to the spectrum for information as to how the 

 stranger behaved under the various and unusual methods 

 of treatment to which it was being subjected, success at 

 last crowned his effort, and Mr. Crookes is now the 

 happy possessor of a whole grain of the new element. 

 The appearance of its spectrum is very striking. The 

 discoverer stationing himself at the gas-burner, placed 

 us in front of a powerful telescope, with which the in- 

 strument is fitted. At first, only the green and blue 

 bands due to the gases in the burner were visible ; but 

 suddenly a bright flash of green light illuminated the field 

 of view, and the new element was glowing before us as a 

 sharply-cut brilliant green line, on a jet-black ground. 

 The only elements wluch at all give a similarly striking 

 spectrum, are sodium and lithium ; and the equally beau- 

 ;iful lines of these, which were afterwards exhibited 

 simultaneously with the green line, are in entirely dif- 

 ferent portions of the spectrum, besides being of different 

 colours one being crimson, and the other yellow." 



These results are obtained by viewing the spectrum 

 produced from the flame by means of an achromatic 

 ttlescope, which is employed in a similar way to that 

 we shall presently describe as adopted for viewing 

 Frailnhofer's lines. We need scarcely add, that this dis- 

 covery, although still in a crude state, is of the highest 

 importance in experimental science, and promises to add 

 largely to our stock of knowledge of the chemical cha- 

 racters of elementary bodies, and also to add to their 

 number. 



FRAUNHOFER'S LINES. 



IT was discovered by Dr. Wollaston, and independently 

 by M. Fraunhofer, that there exists, throughout the 

 length of the spectrum obtained from any source of light 

 by means of a flint-glass prism, a number of black baiult 



