5S6 



NA TURE 



[April 20, 1899 



used in laboratories is equally inoperative. We must 

 have the induced current, and with different tensions 

 different spectra are produced. 



We have then arrived so far. Heat-energy, which 

 does give us line-spectra in some cases when metals are 

 concerned, fails us in the case of the permanent gases 

 and many metals. A voltaic current gives us spectra 

 when metals are in question, but, like heat-energy, it will 

 not set the particles of the permanent gases vibrating. 



But when both metals and the permanent gases are 

 subjected to the action of a strong induced current — that 

 is, a current of high tension when an induction coil with 

 leyden jars and an air break are employed, we get this 

 vibration ; gases now become luminous, a distmct change 

 in the spectra of the metals is observed, a change as well 

 marked, or perhaps better marked, than any of the previous 

 lower temperature changes to which I have already 

 drawn attention. 



When the tension is still further increased, the differ- 

 ences in the spectra are most marked in the case of 

 gases, for the reason that, being enclosed in tubes, they 

 cannot escape from the action of the current ; all the 

 molecules are equally affected. The spectrum is some- 

 limes NOT a mixed one. In the case of the metals the 

 spark is made to pass between two small pointed poles, 

 and the region of most intense action is a very limited 

 one ; we get from the particles outside this region the 

 spectrum obtained with a lower degree of electrical 

 energy. The specln/m IS a mixed one. Even when we 

 take the precaution of throwing an image of the spark on 

 ■the slit of the spectroscope, the outer cooler layers 

 pierced by the line of sight add their lines to the spectrum 

 of the centre. 



Not only so, but the individuality of the various 

 chemical elements comes out in a remarkable manner. 



To take one or two instances. I will begin with the 

 gases with a weak and strong induced current. Hydrogen 

 gives us what is termed a structure spectrum, a spectrum 

 full of lines ; this changes to a series. Oxygen gives us 

 series which change into a complicated line-spectrum 

 in which no series has been traced. Nitrogen gives us a 

 fluted spectrum which changes into a complicated line- 

 spectrum. 



I next pass to the metals, and again, for brevity's sake, 

 I will deal with three substances only. In the case of 

 magnesium, iron and calcium, the changes observed on 

 passing from the temperature of the arc to that of the 

 spark have been minutely observed. In each, new lines 

 are added or old ones are intensified at the higher 

 temperature. Such lines have been termed enhanced 

 lines. 



These enhanced lines are not seen alone : as in the 

 case of the spark, so in the arc outside the region of high 

 temperature in which they are produced, the cooling 

 vapours give us the lines visible at a lower temperature. 



Bearing in mind what happens in the case of the 

 gases, we can conceive the enhanced lines to be seen 

 alone at the highest temperature in a space sufficiently 

 shielded from the action of all lower temperatures, but 

 such a shielding is beyond our laboratory expedients ; 

 still, as I shall show, in the atmospheres of the stars 

 we have probably the closest approximation open to our 

 observation of that equally heated space condition to 

 which I have referred. 



The enhanced lines are very few in number as com- 

 pared with those seen at the temperature of the arc. 

 In the case of iron thousands are reduced to tens. 



The above statements are only general : if we include 

 the non-metals, more stages of temperature are required, 

 and it then becomes evident that different kinds of 

 spectra are produced at the same temperature in the 

 ■case of different elements ; in other words, at many 

 different heat-levels changes occur, always in one direc- 

 tion but differmg widely for different substances at the 



NO. 1538, VOL. 59] 



lower temperatures. At the highest temperatures — at 

 the limit — there is mt»ch greater constancy in the 

 phenomena observed if we disregard the question of 

 series. If considered from the series point of view, there 

 is no constancy at all. 



It is obvious that with all these temperature effects 

 observid in a large number of elements, very many 

 comparisons are rendered possible. .MI these suggest 

 that if dissociation is really in question, in some cases, 

 at least more than two simplifications in the line stage 

 are necessary to explain the facts. It is possible that the 

 effects at first ascribed to quantity may be due to the 

 presence of a series of molecules of different complexities, 

 and that this is the true reason why " the more there is to 

 dissociate, the more time is required to run throuijh the 

 series, and the better the first stages are seen."' 



After this general statement of the changes in spectra 

 observed to accompany change in the quantity and kind 

 of energy used in the experiments, I propose to refer 

 briefly to the most recent work on this subject, touching 

 the changes ol^scrved on passing from the arc to the 

 spark in the case of many of the metallic elements. By 

 the kindness of Mr. Hugh Spoltiswoode, the photographs 

 of the enhanced lines have been obtained by the use of 

 the large induction coil, giving a 40-inch spark, formerly 

 belonging to Dr. Spottiswoode, P.R.S. I am anxious to 

 express here my deep obligation to Mr. Hugh Spottis- 

 woode for the loan of such a magnificent addition to my 

 instrumental stock-in-trade. 



The spark obtamed by means of the .Spottiswoode coil 

 is so luminous that higher dispersions than those formerly 

 employed can be effectively used, and in consequence of 

 this, the detection of the enhanced lines becomes more 

 easy ; their number therefore has been considerably 

 increased. 



.^t the higher temperature enhanced lines have been 

 found to make their appearance in the spectra of nearly 

 all the melals already e.xamined. Lithium is one ex- 

 ception. 



Neglecting then all changes at the lowest tempera- 

 tures, but including the flame spectrum, four distinct 

 temperature stages are indicated by the varying spectra 

 of the metals ; for simplicity I limit myself to iron as an 

 example. These are : — 



(i) The flame spectrum, consisting of a few lines and 

 flutings only, including several well-marked lines, some of 

 them arranged in triplets. 



(2) The arc spectrum consisting, according to Row- 

 land, of 2000 lines or more. 



(3) The spark spectrum, differing from the arc spec- 

 trum in the enhancement of some of the short lines and 

 the reduced relative brightness of others. 



(4) A spectrum consisting of a relatively very small 

 number of lines which are intensified in the spark. This, 

 as stated above, we can conceive to be visible alone 

 at the highest temperature in a space efticiently shielded 

 from the action of all lower ones, since the enhanced 

 lines behave like those of a metal when a compound of 

 a metal is broken up by the action of heat. 



Klach line of each element at whatever temperature it 

 is produced, can at once be compared in relation to 

 position in the spectrum with the lines visible in celestial 

 bodies with a view of determining whether the element 

 exists in it. 



At the time at which the earlier inquiries of this kind 

 were made it was only possible for the most part to deal 

 with eye observations of the heavenly bodies. The results 

 were, therefore, limited to the visible spectrum. 



During the last few years photographs of the spectra 

 of the brighter stars and of the sun's chromosphere dur- 

 ing eclipses have been obtained ; it became of import- 

 ance, therefore, to extend the observations of terrestrial 

 spectra into the photographic regions for the purpose of 

 ' I'roc. Roy. Soc, 1879. No. joo. 



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