May 28, 1874] 



NATURE 



71 



instead of bringing the spectroscope close to the poles, in which 

 case, in every part of the spectrum, we should get light 

 from every part of the spark, I prefer to use a lens, by means 

 cf which 1 throw an image of tlie spark on the slit ; then in each 

 strip of the total visible spectrum is the spectram of some particu- 

 lar part of the vapour. Think the matter over a little for your- 

 selves. Tliese poles are perpetually giving off vapour, which is 

 constantly going away ; some of it is being oxidised, some of it 

 is travellinj away along the currents of air set up. What follows? 

 There must be more vapour close to the pole than in the interval 

 between the poles ; that will be still more true if I make the 

 interval between tlie two pcles longer. In the part between the 

 two poles, if they consist of two different elements, we have 

 three distinct spectra. In the upper part, a region rich in the upper 

 vapour ; in the lower, one rich in the lower vapour ; between 

 them one which is rich in neither. We hive then at least three 

 distinct layers, so to speak, in the spectrum, the spectrum of the 

 vapour of the upper pole the spectrum of the vapour of the lower 

 one, and also of the central region. The number of particles of each 

 vapotir will decrease from each pole. Vou will see in a moment 

 that mucli the same condition of affjirs will be brought about, if, 

 instead of using a spaik, I use an electric arc, in which the pure 

 vapour of the substance which is being rendered incandescent 



Fig. I — C, ?patk : D, lens ; A, collimator ; B, obser\'ing telescope. 



fills the whole interval between the poles, the number of particles 

 being smaller at the sides of the arc. Now I can throw an 

 image of such 3. /lorizoiilal arc on a vertical slit; the slit will 

 give then the spectrum of a section of tlie are at right angles to 

 its length. You have a photograph of such a spectrum of 

 iron now before you. I wish to draw your attention to the 

 long and the short lines. The vapour which exists furthest 

 from the core of the arc has a much more simple spectrum 

 than that of the core of the arc itself. The spectrum of the 

 centre consists of a lar;;e number of lines ; that furthest from the 

 centre consists of one line. If you pictm'e to yourselves the 

 particles getting nearer to each other, as you get nearer the 

 source of supply, you seethat the nearer the particles are together 

 the more they bang about and the more lines we get in the 

 spectrum. It is important to notice that vibration once begun 

 always goes on ; it never gives flucc to others, although it may 

 give rise to others ; so that you get the largest number of lines 

 in the centre, where the particles arc closest together. Now I have 

 specially to refer to the fact that the way in which the continuous 



spectrum is built up varies in different substances. Here I 

 have a photograph giving tlie spectrum of aluminium and calciuin 

 compared with that of the Lenarto meteorite. The spectra of 

 calcium and aluminium differ generically from that of the 

 meteorite. I want to draw attention to the thick or winged 

 lines you get in the case of aluminium and calcium. These spectra 

 are good specimens of those which give a continuous spectrum by 

 thickening the lines, while the elements in the meteorite are as 

 good specimens as I could put before you of those which produce 

 a continuous spectrum by increasing the number of their lines. 

 There is another remarkable fact connected with this. 



Fig. 2. — Long and short lines of zinc and cadmium. 



You see a thin dark line in the centre of the thick bright 

 lines ; this is due to the absorption by the rarer cooler 

 vapour lying outside this vapour. This is almost invariably 

 observed in the substances giving us the lines thickening 

 as the continuous spectrum is approached, while iron does not 

 give us any such reversal. It is well to see if one can group 

 facts together. That is the first business of a man of Science. 

 It is extraordinary that in all the substances I have yet examined 

 the question of specific gravity decides whether the substance 

 should have its spectrum complicated by thickening or increasing 



I. 



its lines. You know the specific gravity of iron is high. In the 

 case of aluminium, magnesium, sodium, and others where this is 

 low you have the widening of tlie lines and the easy reversal. 



So much for the continuity of the spectroscopic record of the 

 continually increasing distance of particles from particles. We 

 began with a solid and a continuous spectrum, we end with a 

 tenuous gas and a spectrum of a single line, and we partly bridge 

 over the gap between these states in two different ways. 



J. Norman I.oukyer 



{To be continued.) 



