42 



SIX LECTURES ON LIGHT, 



would yield a few bands of color with spaces 

 of darkness between them. 



What is true of the carbon is true in a still 

 mo r e striking degree of the metals, the most 

 refractory of which can be fused, boiled, ind 

 reduced to vapor by the electric current. 

 From the incandescent vapor the light, as a 

 general rule, flashes in groups of rays of 

 definite degrees of refrangibility, spaces ex- 

 isting between group and group, which are 

 unfilled by rays of any kind. But the con- 

 templation of the facts will render this sub- 

 ject more intelligible than words can make it. 

 Within the camera is now placed a cylinder 

 of carbon hollowed out at the t<"p to receive 

 a bit of metal; in the hollow is placed a frag- 

 ment of the m;tal thallium, and now you see 

 the arc of incandescent thallium-vapor upon 

 the screen. It is of a beautiful green color. 

 What is the meaning of that green? We 

 answer the question by subjecting the light 

 to prismatic analysis. Here you have its 

 spectrum, consisting of a single refracted 

 band. Light of one degree of icfrangibility, 

 and that corresponding to green, is emitted 

 by the thallium-vapor. 



We will now remove the thallium and put 

 a bit of silver in its place. The arc of silver 

 is not to be distinguished from that of thal- 

 lium; it is not only green, like the thallium- 

 vapor, but the same shade cf green. Are 

 they, then, alike? Prismatic analysis en- 

 ables us to answer the question. It is per- 

 fectly impossible to confound the spectrum 

 < f incandescent silver vapor with that of 

 thallium. Here are two green bands instead 

 of one. Adding to the silver in our camera 

 a bit of thallium, we obtain the light of both 

 metals, and you see that the green of the thal- 

 lium lies midway between the two greens of 

 the silver. Hence this similarity of color. 



But you observe another interesting fact. 

 The thallium band is now far brighter than 

 the silver bands ; indeed, the latter have won- 

 derfully degenerated since the bit of thallium 

 was put in. They are not at all so bright as 

 they were at first, and for a reason worth 

 knowing. It is the resistance offered to the 

 passage of the electric current from carbon 

 to carbon that calls forth the power of the 

 current to produce heat. If the resistance 

 were materially lessened, the heat would be 

 materially lessened ; and, if all resistance 

 were abolished, there would be no heat at 

 all. Now, thallium is a much more fusible 

 and vaporizable metal than silver; and its 

 vapor facilitates the passage of the current to 

 such a degree as to render it almost incom- 

 petent to vaporize the silver. But the thal- 

 lium is gradually consumed ; its vapor di- 

 ininishes, the resistance rises, until finally 

 you see the two silver bands as brilliant as 

 they were at first. The three bands cf the 

 two metals are now of the same sensible 

 brightness. 



We have in these bands a perfectly unal- 

 terable characteristic of these two metals. 



You never get other bands than these two 

 green ones from the silver, never other than 

 the single green band from the thallium, 

 never olher than the three green bands from 

 the mixture of both metals. Every known 

 metal has its bands, and in no known case 

 are the bands of two different metals al;ke. 

 Hence these spectra may be made a test for 

 the presence or absence of any particular 

 metal. If we pass from the metals to their 

 alloys, we find no confusion. Copper gives 

 us green bands, zinc gives us blue and red 

 bands ; brass, an alloy of copper and zinc, 

 gives us the bands of both metals, perfectly 

 unaltered in position or character. 



But we are not confined to the metals ; the 

 salts of these metals yield the bands of the 

 metals. Chemical union is ruptured by a 

 sufficiently high heat, the vapor of the metal 

 is set free and yields its characteristic bands. 

 The chlorides of the metals arc particularly 

 suitable for experiments of this character. 

 Common salt, for example, is a compound of 

 chlor.nc and sodium ; in the electric lamp, it 

 yields the spectrum of the metal sodium. 

 The chlorides of lithium and cf strontium 

 yield in like manner the bands of these 

 metals. When, therefore, Bunsen and Kirch- 

 hoff, the celebrated founders of spectrum 

 analysis, after having established by an ex- 

 haustive examination the spectra of all known 

 substances, discovered a spectrum containing 

 bands different from any known bands, they 

 immediately inferred the existence of a new 

 metal. They were operating at the time 

 upon a residue obtained by evaporating one 

 of the mineral waters of Germany. In that 

 water they knew the new metal was con- 

 cealed, but vast quantities of it had to be 

 evaporated before a residue could be obtained 

 sufficiently large to enable ordinary chemistry 

 to grapple with the metal. But they hunted 

 it down, and it now stands among chemical 

 substances as the metal Rubidium* They 

 subsequently discovered a second metal, 

 whic:i they called Casium. Thus, having 

 first placed spectrum analysis on a safe foun- 

 dation, they demonstrated its capacity as an 

 agent of discovery. Soon afterwards Mr. 

 Crookes, pursuing this same method, ob- 

 tained the salts of the thallium which yielded 

 that bright monoc- romatic green band. The 

 metal itself was first isolated by a French 

 chemist. 



All this relates to chemical discovery upon 

 earth, where the materials are in our own 

 hands. But Kirchhoff showed how spectrum 

 analysis might be applied to the investigation 

 cf the sun and stars, and on 1 is way to this 

 result he solved a problem which had been 

 long an enigma to natural philosophers. A 

 spectrum is pun in which the colors do not 

 overlap each other. We purify the spectrum 

 by making our slit narrow and by augment- 

 ing the number of our prisms. When a pure 

 spectrum of the sun has been obtained in 

 this way it is found to be furrowed by in- 



