322 



NATURE 



\Aiigust 4, 1 88 1 



"Now if into tlie furnace A we tlirow some of this doubly 

 compounded body 7, we shall get at first an integration of the 

 three spectra to which I have drawn attention ; the lines of 7 

 will first be thicliest, then those of ^ ; finally o will exist alone, 

 and the spectrum will be reduced to one of the utmost simplicity. 



" This is not the only conclusion to be drawn from these con- 

 siderations. Although we have by hypothesis 3, 7, and 5 all 

 higher, that is, more compound forms of a, and although the 

 strong lines in the diagram may represent the true spectra of 

 these substances in the furnaces B, C, and D, respectively, yet, 

 in consequence of incomplete dissociation, the strong lines of /3 



will be seen in furnace C, and the strong lines of 7 will be seen 

 in furnace D, all as thin lines. Thus, although in C we have no 

 line which is not represented in D, the intensities of the lines in 

 C and D are entirely changed. 



" The same reasoning therefore which shows how variation in 

 intensity can most naturally explain the short line coincidences 

 — lines «hich I have termed basic, for the line of o strong in A 

 is basic in B, C, and D, the lines of /3 strong in B are basic in 

 C and D, and so on. 



" I have prepared another diagram which represents the facts on 

 the supposition that the furnace A, instead of having a tempera- 



Kio. 34. — Spectrum of sur.-spot observed at Greenw.^h. 



tare sufficient to dissociate 0, 7, and S into a, is far below that 

 stage, although higher than H. 



" It will be seen from this diagram (Fig. 33) that then the only 

 difference in the spectra of the bodies existing in the four furnaces 

 would consist in the relative thicknesses of the lines. The -pec- 

 trum of the substances as they exist in A would contain as many 

 lines as would the spectrum of the substances as they exist in 

 D ; cacli line would in turn be basic in the whole series 0/ furnaces 

 instead of in one or two only." 



We are therefore completely ju-tified in asking whether these 

 are not the differences in intensities of lines to which Kirchhoff 



and Angstrom have referred, and it is quite easy to see that if 

 we change the temperature of the furnaces in such a manner as 

 to produce the strongest lines, owing to the greatest quantity of 

 the vapour given oft' at any temperature, that the long lines 

 produced at these different temperatures would vary, and the 

 longest line ]roduced in furnace D would not be the same there- 

 fore as the longest line produced in furnace A, so that in that 

 way we can imagine a transcendental temperature giving a very 

 long line to a particular substance, and that substance may exist 

 highly compounded in another substance, and yet at a lower 

 tem; erature it may only appear as an exceedingly short feeble 



Fir.. 5.— Portion of a large r 



•Ing the 



line. The result of this reasoning was, in short, to explain at 

 once variations of intensity of the short feeble lines which were 

 common to so many of the so-called element.ary bodies. 



I am particularly anxious to point out that there is absolutely 

 nothing new in these views. We have simply taken as our 

 exemplar the behaviour of a known compound body, and then 

 pushed the reasoning three or four st.iges further. We have gone 

 just the safest possible "ay, by the easiest possible stages, from 

 the known to the unknown. 



affected in 100 sun-spots observed .^t South Kensington. 



I have now to refer, one by one, to the various tests which 

 have been applied to these considerations, and I should now like 

 to bring the first considerable test under notice. I ^hall show on 

 a subsequent occasion the various 1 boratory methods that we 

 possess of determining whether short lines are really the product 

 of high temperature. I shall at once draw your attention to the 

 fact that ihe short lines may be due, not merely to the work of 

 high temperature, being thus truly produced by the tem- 

 Derature which we are employing, but they may be also the 



