WHAT TEE SEW IS MADE OF. 



315 



(as I hope it some day will be, though certainly 

 not in our time), from the ultra-violet, already 

 mapped by Mascart and Corou, to the ultra-red, 

 which has quite recently for the first time been 

 brought under our ken by Captain Abney, will 

 be 315 feet long. This is a considerable scale to 

 apply to the investigation of these problems ; but 

 recent work has shown that, gigantic as the scale 

 is, it is really not beyond what is required for 

 honest, patient work. 



So much for one of the new tools. There 

 are others of recent application which are of 

 very considerable importance. 



Suppose, for example, instead of inquiring 

 into the coincidence of the lines of the various 

 metals with the dark lines in the sun's spectrum 

 with a view of seeing whether any particular 

 metal exists in the sun, we wish to determine the 

 coincidence of the lines due to various gases. 

 The method hitherto employed has been to in- 

 close the gases in Geissler tubes, to reduce their 

 pressure, and in that way to fine down the lines. 

 The importance of this apparently small matter 

 can be very well demonstrated by an easy ex- 

 periment, the point of which is that, if we vary 

 the density of any vapor, we vary sometimes to 

 a very considerable extent the thickness and in- 

 tensity of the lines observed in the spectrum of 

 that vapor. 



In an article which has recently appeared in 

 Nature, 1 I have shown how a Bunsen burner 

 which produces a very hot flame can be con- 

 structed with two pieces of glass tubing. If a 

 piece of sodium be held in this in an old spoon, 

 and the flame be looked at with a small spectro- 

 scope, the variation in the thickness of the line 

 of sodium will be readily seen. Every change in 

 the quantity of the vapor in the flame varies to 

 a very considerable extent the thickness of the 

 line. 



When we make the sodium-vapor as dense as 

 possible, then the line is very thick. When we 

 make it much less dense, the line becomes thinner. 

 If the spectrum had been a gas-spectrum, the 

 exact equivalent of that experiment would have 

 been this, that the gaseous spectrum at atmos- 

 pheric pressure would have given us some of 

 the lines as thick as the sodium-line was at its 

 thickest ; while, on the pressure being reduced, 

 the lines would thin. 



In practice there are very great objections to 

 the using of Geissler tubes. One very valid ob- 

 jection is that the gas becomes much less lumi- 

 nous as its pressure is reduced. The new method 

 1 " Physical Science for Artists." 



which here comes in and helps us is excellent in 

 this way : it enables much of the work connected 

 with gaseous spectra to be done at atmospheric 

 pressure, and we get the line down as we choose, 

 not by reducing the pressure, but by reducing the 

 quantity of any particular gas in a mixture. 



If we take, for instance, a spark in air and 

 observe its spectrum, we find the lines of the 

 constituents of atmospheric air considerably 

 thick ; but, if we wish to reduce the lines, say of 

 oxygen, down to a considerable fineness so that 

 we can photograph them, these should be fine, in 

 order to enable us to determine their absolute 

 position. To accomplish this, the spark is taken 

 in a glass vessel with two adits and one exit tube. 

 If we wish to observe the oxygen-lines fine, the 

 vessel is flooded with nitrogen so that there is 

 only, say, one per cent, of oxygen present, and 

 pass the current between the inclosed electrodes. 

 If we wish to observe nitrogen-lines fine, it is 

 flooded with oxygen, so that there is only, say, 

 one per cent, of nitrogen present. 



In this way, by merely making an admixture 

 in which the gas to be observed is quantitatively 

 reduced, so that the lines which we wish to in- 

 vestigate are just visible in their thinnest state, 

 we have a perfect means of doing it without any 

 apparatus depending on the use of low pressures. 

 A very great simplicity of work is thus intro- 

 duced. 



A few years ago, taking the work of Kirch- 

 hoff, Bunsen, Angstrom, and Thalen, into consid- 

 eration, and connecting it, so far as one could con- 

 nect it, with those ideas in which recent eclipses 

 have been so fruitful, our chemical view of the 

 sun's atmosphere was one something like this : 

 We had, let us say, first of all an enormous shell 

 of some gas, probably lighter than hydrogen, 

 about which we know absolutely nothing, be- 

 cause at present none of it has been found here. 

 Inside this we had another shell of hydrogen ; 

 inside this we had another shell of calcium, an- 

 other of magnesium, another of sodium, and then 

 a complex shell which has been called the re- 

 versing layer, in which we got all the metals of 

 the iron group plus such other metals as cadmium, 

 titanium, barium, and so on. The solar atmos- 

 phere, then, from top to bottom, consisted, it 

 was imagined, of a series of shells, the shells 

 being due not to the outside substance existing 

 only outside, but to the outside substance extend- 

 ing to the bottom of the sun's atmosphere, and en- 

 countering in it, at a certain height, another 

 shell which again found another shell inside it, 

 and so on ; so that the composition of the solar 



