Page:Popular Science Monthly Volume 14.djvu/623

Rh 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 intensely 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 invariably 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