Let us now turn to another part of this subject. By a special arrangement of apparatus it is possible to observe the spectrum of the light emitted by a glowing vapour, parts of which are hotter than other parts, and to compare the lines in the spectrum of the light coming from the hottest parts with the lines in the spectrum of the light coming from the cooler parts of the vapour. If this is done for sodium vapour, certain lines are apparent in all the spectra, others only in the spectrum of the light coming from the hottest parts of the sodium vapour: the former lines are called "long lines," the latter "short lines." A rough representation of the long and short lines of sodium is given in Fig. 7. [Illustration: Fig. 7.--Long and short lines of sodium.] Now, suppose that the lines in the spectrum of the light emitted by glowing manganese vapour have been carefully mapped, and classed as long and short lines: suppose that the same thing has been done for the iron lines: now let a little manganese be mixed with much iron, let the mixture be vaporized, and let the light which is emitted be decomposed by the prism of a spectroscope, it will be found that the long lines of manganese alone make their appearance; let a little more manganese be added to the mixture, and now some of the shorter lines due to manganese begin to appear in the spectrum. Hence it has been concluded by Lockyer that if the spectrum of the light emitted by the glowing vapour of any element--call it A--is free from the long lines of any other element--say element B--this second element is not present as an impurity in the specimen of element A which is being examined. Lockyer has applied this conclusion to "purify" various elementary spectra. The spectrum of element A is carefully mapped, and the lines are divided into long and short lines, according as they are noticed in the spectrum of the light coming from all parts of the glowing vapour of A, or only in the spectrum of the light which comes from the hotter parts of that vapour. The spectra of elements B and C are similarly mapped and classified: then the three spectra are compared; the longest line in the spectrum of B is noted, if this line is found in the spectrum of A, it is marked with a negative sign--this means that so far as the evidence of this line goes B is present as an impurity in A; the next longest B line is searched for in the spectrum of A--if present it also is marked with a negative sign; a similar process of comparison and elimination is conducted with the spectra of A and C. In this way a "purified" spectrum of the light from A is obtained--a spectrum, that is, from which, according to Lockyer, all lines due to the presence of small quantities of B and C as impurities in A have been eliminated. [Illustration: Fig. 8.]