THE CHEMICAL ELEMENTS. 605 



much more simple spectrum than that of the core of the arc itself. The 

 spectrum of the core consists of a large number of lines, all of which 

 die out until the part of it farthest from the center gives but one line. 



In this way the spectrum of each substance furnishes us with long 

 and short lines, the long lines being common to the more and less in- 

 tensely heated parts of the arc, and the short lines special to the more 

 heated one. This is the first step. 



It has been necessary to enter thus at length into the origin of the 

 terms long and short lines, because almost all the subsequent work 

 which need be referred to now has had for its object the investigation 

 of the phenomena presented by them under different conditions. The 

 first results obtained were as follows : 



1. When a metallic vapor was subjected to admixture with another 

 gas or vapor, or to reduced pressure, I found that its spectrum became 

 simplified by the abstraction of the shortest lines and by the thinning 

 of many of the remaining ones. To obtain reduction of pressure, the 

 metals were inclosed in tubes in which a partial vacuum was produced. 

 In all these experiments it was found that the longest lines hivariably 

 remained visible longest.^ 



2. When we use metals chemically combined with a metalloid — in 

 other words, when we pass from a metal to one of its salts (I used 

 chlorine) — only the longest lines of the metal remain. The number is 

 large in the case of elements of low atomic weight, and small In the 

 case of elements of high atomic weight, and of twice the atom-fixing 

 power of hydrogen. 



3. When we use metals mechanically mixed, only the longest lines 

 of the smallest constituent remain. On this point I must enlarge 

 somewhat by referring to a series of experiments recorded in the 

 *' Philosophical Transactions " (1873). 



A quantity of the larger constituent, generally from five to ten 

 grammes, was weighed out, the weighing being accurate to the fraction 

 of a milligramme ; and the requisite quantity of the smaller constituent 



' In the case of zinc the effect of these circumstances was very marked, and they may 

 be given as a sample of the phenomena generally observed. When the pressure-gauge 

 connected with a Sprengel pump stood at from 35 to 40 millimetres, the spectrum at the 

 part observed was normal, except that the two lines 4924 and 4911 (both of which, when 

 the spectrum is observed under the normal pressure, are lines with thick wings) were 

 considerably reduced in width. On the pump being started these lines rapidly decreased 

 in length, as did the line at 4679 — 4810 and 4721 being almost unaffected ; at last the 

 two at 4924 and 4911 vanished, as did 4679, and appeared only at intervals as spots on 

 the poles, the two 4810 and 4721 remaining little changed in length, though much in 

 brilliancy. This experiment was repeated four times, and on each occasion the gauge 

 was found to be almost at the same point, viz. : 



1st observation, when the lines 4924 and 4911 were gone, the gauge 



stood at 30 millimetres. 

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